I have read your ma.n.u.script with some attention. By the argument it contains against a particular Providence, though you allow a general Providence, you strike at the foundations of all religion. For without the belief of a Providence that takes cognizance of, guards, and guides, and may favour particular persons, there is no motive to worship a Deity, to fear His displeasure, or to pray for His protection. I will not enter into any discussion of your principles, though you seem to desire it. At present I shall only give you my opinion, that, though your reasonings are subtle, and may prevail with some readers, you will not succeed so as to change the general sentiments of mankind on that subject, and the consequence of printing this piece will be a great deal of odium drawn upon yourself, mischief to you, and no benefit to others. He that spits against the wind spits in his own face.

But were you to succeed, do you imagine any good would be done by it? You yourself may find it easy to live a virtuous life without the a.s.sistance afforded by religion; you having a clear perception of the advantages of virtue and the disadvantages of vice, and possessing strength of resolution sufficient to enable you to resist common temptations. But think how great a portion of mankind consists of weak and ignorant men and women, and of inexperienced, inconsiderate youth of both s.e.xes, who have need of the motives of religion to restrain them from vice, to support their virtue, and retain them in the practice of it till it becomes _habitual_, which is the great point for its security. And perhaps you are indebted to her originally, that is, to your religious education, for the habits of virtue upon which you now justly value yourself. You might easily display your excellent talents of reasoning upon a less hazardous subject, and thereby obtain a rank with our most distinguished authors. For among us it is not necessary, as among the Hottentots, that a youth, to be raised into the company of men, should prove his manhood by beating his mother.

I would advise you, therefore, not to attempt unchaining the tiger, but to burn this piece before it is seen by any other person; whereby you will save yourself a great deal of mortification by the enemies it may raise against you, and perhaps a good deal of regret and repentance. If men are so wicked _with religion_, what would they be _if without_ it? I intend this letter itself as a _proof_ of my friendship, and therefore add no _professions_ to it; but subscribe simply yours.

During the last few years of his life Franklin suffered from a painful disease, which confined him to his bed and seriously interfered with his literary work, preventing him from completing his biography.

During this time he was cared for by his daughter, Mrs. Bache, who resided in the same house with him. He died on April 17, 1790, the immediate cause of death being an affection of the lungs. He was buried beside his wife in the cemetery of Christ Church, Philadelphia, the marble slab upon the grave bearing no other inscription than the name and date of death. In his early days (1728) he had written the following epitaph for himself:--

THE BODY

OF

BENJAMIN FRANKLIN,

PRINTER,

(LIKE THE COVER OF AN OLD BOOK, ITS CONTENTS TORN OUT AND STRIPT OF ITS LETTERING AND GILDING,) LIES HERE, FOOD FOR WORMS.

BUT THE WORK SHALL NOT BE LOST, FOR IT WILL (AS HE BELIEVED) APPEAR ONCE MORE IN A NEW AND MORE ELEGANT EDITION, REVISED AND CORRECTED BY

THE AUTHOR.

When the news of his death reached the National a.s.sembly of France, Mirabeau rose and said:--

"Franklin is dead!

"The genius, which gave freedom to America, and scattered torrents of light upon Europe, is returned to the bosom of the Divinity.

"The sage, whom two worlds claim; the man, disputed by the history of the sciences and the history of empires, holds, most undoubtedly, an elevated rank among the human species.

"Political cabinets have but too long notified the death of those who were never great but in their funeral orations; the etiquette of courts has but too long sanctioned hypocritical grief. Nations ought only to mourn for their benefactors; the representatives of free men ought never to recommend any other than the heroes of humanity to their homage.

"The Congress hath ordered a general mourning for one month throughout the fourteen confederated States on account of the death of Franklin; and America hath thus acquitted her tribute of admiration in behalf of one of the fathers of her const.i.tution.

"Would it not be worthy of you, fellow-legislators, to unite yourselves in this religious act, to partic.i.p.ate in this homage rendered in the face of the universe to the rights of man, and to the philosopher who has so eminently propagated the conquest of them throughout the world?

"Antiquity would have elevated altars to that mortal who, for the advantage of the human race, embracing both heaven and earth in his vast and extensive mind, knew how to subdue thunder and tyranny.

"Enlightened and free, Europe at least owes its remembrance and its regret to one of the greatest men who has ever served the cause of philosophy and liberty.

"I propose, therefore, that a decree do now pa.s.s, enacting that the National a.s.sembly shall wear mourning during three days for Benjamin Franklin."

HENRY CAVENDISH.

It would not be easy to mention two men between whom there was a greater contrast, both in respect of their characters and lives, than that which existed between Benjamin Franklin and the Honourable Henry Cavendish. The former of humble birth, but of great public spirit, possessed social qualities which were on a par with his scientific attainments, and toward the close of his life was more renowned as a statesman than as a philosopher; the latter, a member of one of the most n.o.ble families of England, and possessed of wealth far exceeding his own capacity for the enjoyment of it, was known to very few, was intimate with no one, and devoted himself to scientific pursuits rather for the sake of the satisfaction which his results afforded to himself than from any hope that they might be useful to mankind, or from any desire to secure a reputation by making them known, and pa.s.sed a long life, the most uneventful that can be imagined.

Though the records of his family may be traced to the Norman Conquest, the famous Elizabeth Hardwicke, the foundress of two ducal families and the builder of Hardwicke Hall and of Chatsworth as it was before the erection of the present mansion, was the most remarkable person in the genealogy. Her second son, William, was raised to the peerage by James I., thus becoming Baron Cavendish, and was subsequently created first Earl of Devonshire by the same monarch. His great-grandson, the fourth earl, was created first Duke of Devonshire by William III., to whom he had rendered valuable services. He was succeeded by his eldest son in 1707, and the third son of the second duke was Lord Charles Cavendish, the father of Henry and Frederick, of whom Henry was the elder, having been born at Nice, October 13, 1731.

His mother died when he was two years old, and very little indeed is known respecting his early life. In 1742 he entered Dr. Newcome's school at Hackney, where he remained until he entered Peterhouse, in 1749. He remained at Cambridge until February, 1753, when he left the university without taking his degree, objecting, most probably, to the religious tests which were then required of all graduates. In this respect his brother Frederick followed his example. On leaving Cambridge Cavendish appears to have resided with his father in Marlborough Street, and to have occasionally a.s.sisted him in his scientific experiments, but the investigations of the son soon eclipsed those of the father. It is said that the rooms allotted to Henry Cavendish "were a set of stables, fitted up for his accommodation," and here he carried out many of his experiments, including all those electrical investigations in which he forestalled so much of the work of the present century.

During his father's life, or, at any rate, till within a few years of its close, Henry Cavendish appears to have enjoyed a very narrow income. He frequently dined at the Royal Society Club, and on these occasions would come provided with the five shillings to be paid for the dinner, but no more. Upon his father's death, which took place in 1783, when Henry was more than fifty years of age, his circ.u.mstances were very much changed, but it seems that the greater part of his wealth was left him by an uncle who had been an Indian officer, and this legacy may have come into his possession before his father's death. He appears to have been very liberal when it was suggested to him that his a.s.sistance would be of service, but it never occurred to him to offer a contribution towards any scientific or public undertaking, and though at the time of his death he is said to have had more money in the funds than any other person in the country, besides a balance of 50,000 on his current account at his bank, and various other property, he bequeathed none to scientific societies or similar inst.i.tutions. Throughout the latter part of his life he seems to have been quite careless about money, and to have been satisfied if he could only avoid the trouble of attending to his own financial affairs. Hence he would allow enormous sums to acc.u.mulate at his banker's, and on one occasion, being present at a christening, and hearing that it was customary for guests to give something to the nurse, he drew from his pocket a handful of guineas, and handed them to her without counting them. After his father's death, Cavendish resided in his own house on Clapham Common. Here a few rooms at the top of the house were made habitable; the rest were filled with apparatus of all descriptions, among which the most numerous examples were thermometers of every kind. He seldom entertained visitors, but when, on rare occasions, a guest had to be entertained, the repast invariably consisted of a leg of mutton. His extreme shyness caused him to dislike all kinds of company, and he had a special aversion to being addressed by a stranger. On one occasion, at a reception given by Sir Joseph Banks, Dr. Ingenhousz introduced to him a distinguished Austrian philosopher, who professed that his main object in coming to England was to obtain a sight of so distinguished a man. Cavendish listened with his gaze fixed on the floor; then, observing a gap in the crowd, he made a rush to the door, nor did he pause till he had reached his carriage. His aversion to women was still greater; his orders for the day he would write out and leave at a stated time on the hall-table, where his house-keeper, at another stated time, would find them. Servants were allowed access to the portion of the house which he occupied only at fixed times when he was away; and having once met a servant on the stairs, a back staircase was immediately erected. His regular walk was down Nightingale Lane to Wandsworth Common, and home by another route. On one occasion, as he was crossing a stile, he saw that he was watched, and thenceforth he took his walks in the evening, but never along the same road. There were only two occasions on which it is recorded that scientific men were admitted to Cavendish's laboratory. The first was in 1775, when Hunter, Priestley, Romayne, Lane, and Nairne were invited to see the experiments with the artificial torpedo. The second was when his experiment on the formation of nitric acid by electric sparks in air had been unsuccessfully attempted by Van Marum, Lavoisier, and Monge, and he "thought it right to take some measures to authenticate the truth of it."

Besides his house at Clapham, Cavendish occupied (by his instruments) a house in Bloomsbury, near the British Museum, while a "mansion" in Dean Street, Soho, was set apart as a library. To this library a number of persons were admitted, who could take out the books on depositing a receipt for them. Cavendish was perfectly methodical in all his actions, and whenever he borrowed one of his own books he duly left the receipt in its place. The only relief to his solitary life was afforded by the meetings of the Royal Society, of which he was elected a Fellow in 1760; by the occasional receptions at the residence of Sir Joseph Banks, P.R.S.; and by his not infrequent dinners with the Royal Society Club at the Crown and Anchor; and he may sometimes have joined the social gatherings of another club which met at the Cat and Bagpipes, in Downing Street. It was to his visits to the Royal Society Club that we are indebted for the only portrait that exists of him. Alexander, the draughtsman to the China Emba.s.sy, was bent upon procuring a portrait of Cavendish, and induced a friend to invite him to the club dinner, "where he could easily succeed, by taking his seat near the end of the table, from whence he could sketch the peculiar great-coat of a greyish-green colour, and the remarkable three-cornered hat, invariably worn by Cavendish, and obtain, un.o.bserved, such an outline of the face as, when inserted between the hat and coat, would make, he was quite sure, a full-length portrait that no one could mistake. It was so contrived, and every one who saw it recognized it at once." Another incident is recorded of the Royal Society Club which, perhaps, reflects as much credit upon Cavendish as upon the Society. "One evening we observed a very pretty girl looking out from an upper window on the opposite side of the street, watching the philosophers at dinner. She attracted notice, and one by one we got up and mustered round the window to admire the fair one.

Cavendish, who thought we were looking at the moon, hustled up to us in his odd way, and when he saw the real object of our study, turned away with intense disgust, and grunted out, 'Pshaw!'"

In the spring and autumn of 1785, 1786, 1787, and 1793, Cavendish made tours through most of the southern, midland, and western counties, and reached as far north as Whitby. The most memorable of these journeys was that undertaken in 1785, since during its course he visited James Watt at the Soho Works, and manifested great interest in Watt's inventions. This was only two years after the great controversy as to the discovery of the composition of water, but the meeting of the philosophers was of the most friendly character. On all these journeys considerable attention was paid to the geology of the country.

Allusion has already been made to the two committees of the Royal Society to which the questions of the lightning-conductors at Purfleet, and of points _versus_ k.n.o.bs for the terminals of conductors, were referred. Cavendish served on each of these committees, and supported Franklin's view against the recommendation of Mr. Wilson. On the first committee he probably came into personal communication with Franklin himself.

Cavendish's life consisted almost entirely of his philosophical experiments. In other respects it was nearly without incident. He appears to have been so const.i.tuted that he must subject everything to accurate measurement. He rarely made experiments which were not _quant.i.tative_; and he may be regarded as the founder of "quant.i.tative philosophy." The labour which he expended over some of his measurements must have been very great, and the accuracy of many of his results is marvellous considering the appliances he had at disposal. When he had satisfied himself with the result of an experiment, he wrote out a full account and preserved it, but very seldom gave it to the public, and when he did publish accounts of any of his investigations it was usually a long time after the experiments had been completed. One of the consequences of his reluctance to publish anything was the long controversy on the discovery of the composition of water, which was revived many years afterwards by Arago's _eloge_ on James Watt; but a much more serious result was the loss to the world for so many years of discoveries and measurements which had to be made over again by Faraday, Kohlrausch, and others.

The papers he published appeared in the _Philosophical Transactions of the Royal Society_, to which he began to communicate them in 1766. On March 25, 1803, he was elected one of the eight Foreign a.s.sociates of the Inst.i.tute of France. His _eloge_ was p.r.o.nounced by Cuvier, in 1812, who said, "His demeanour and the modest tone of his writings procured him the uncommon distinction of never having his repose disturbed either by jealousy or by criticism." Dr. Wilson says, "He was almost pa.s.sionless. All that needed for its apprehension more than the pure intellect, or required the exercise of fancy, imagination, affection, or faith, was distasteful to Cavendish. An intellectual head thinking, a pair of wonderfully acute eyes observing, and a pair of very skilful hands experimenting or recording, are all that I realize in reading his memorials." He appeared to have no eye for beauty; he cared nothing for natural scenery, and his apparatus, provided it were efficient, might be clumsy in appearance and of the cheapest materials; but he was extremely particular about accuracy of construction in all essential details. He reminds us of one of our foremost men of science, who, when his attention was directed to the beautiful lantern tower of a cathedral, behind which the full moon was shining, remarked, "I see form and colour, but I don't know what you mean by beauty."

The accounts of Cavendish's death differ to some extent in their details, but otherwise are very similar. It appears that he requested his servant, "as he had something particular to engage his thoughts, and did not wish to be disturbed by any one," to leave him and not to return until a certain hour. When the servant came back, at the time appointed, he found his master dead. This was on February 24, 1810, after an illness of only two or three days.

It is mainly on account of his researches in electricity that the biography of Cavendish finds a place in this volume. These investigations took place between the years 1760 and 1783, and, as already stated, were all conducted in the stables attached to his father's house in Marlborough Street. It was by these experiments that electricity was first brought within the domain of measurement, and many of the numerical results obtained far exceeded in accuracy those of any other observer until the instruments of Sir W. Thomson rendered many electrical measurements a comparatively easy matter. The near agreement of Cavendish's results with those of the best modern electricians has made them a perpetual monument to the genius of their author. It was at the request of Sir W. Thomson, Mr. Charles Tomlinson, and others, that Cavendish's electrical researches might be given to the public, that the Duke of Devonshire, in 1874, entrusted the ma.n.u.scripts to the care of the late Professor Clerk Maxwell. They had previously been in the hands of Sir William Snow Harris, who reported upon them, but after his death, in 1867, the report could not be found. The papers, with an introduction and a number of very valuable notes by the editor, were published by the Cambridge University Press, just before the death of Clerk Maxwell, in 1879. Sir W. Thomson quotes the following ill.u.s.tration of the accuracy of Cavendish's work:--"I find already that the capacity of a disc was determined experimentally by Cavendish as 1/157 of that of a sphere of the same radius. Now we have capacity of disc = (2/[pi])_a_ = _a_/1571!"

Cavendish adopted Franklin's theory of electricity, treating it as an incompressible fluid pervading all bodies, and admitting of displacement only in a closed circuit, unless, indeed, the disturbance might extend to infinity. This fluid he supposed, with Franklin, to be self-repulsive, but to attract matter, while matter devoid of electricity, and therefore in the highest possible condition of negative electrification, he supposed, with aepinus, to be, like electricity, self-repulsive. One of Cavendish's earliest experiments was the determination of the precise law according to which electrical action varies with the distance between the charges. Franklin had shown that there was no sensible amount of electricity on the interior of a deep hollow vessel, however its exterior surface might be charged. Cavendish mounted a sphere of 121 inches in diameter, so that it could be completely enclosed (except where its insulating support pa.s.sed through) within two hemispheres of 133 inches diameter, which were carried by hinged frames, and could thus be allowed to close completely over the sphere, or opened and removed altogether from its neighbourhood. A piece of wire pa.s.sed through one of the hemispheres so as to touch the inner sphere, but could be removed at pleasure by means of a silk string. The hemispheres being closed with the globe within them, and the wire inserted so as to make communication between the inner and outer spheres, the whole apparatus was electrified by a wire from a charged Leyden jar. This wire was then removed by means of a silken string and "the same motion of the hand which drew away the wire by which the hemispheres were electrified, immediately after that was done, drew out the wire which made the communication between the hemispheres and the inner globe, and, immediately after that was drawn out, separated the hemispheres from each other," and applied the electrometer to the inner globe. "It was also contrived so that the electricity of the hemispheres and of the wire by which they were electrified was discharged as soon as they were separated from each other.... The inner globe and hemispheres were also both coated with tinfoil to make them the more perfect conductors of electricity." The electrometer consisted of a pair of pith-b.a.l.l.s; but, though the experiment was several times repeated, they shewed no signs of electrification. From this it was clear that, as there could have been no communication between the globe and hemispheres when the connecting wire was withdrawn, there must have been no electrification on the globe while the hemispheres, though themselves highly charged, surrounded it. To test the delicacy of the experiment, a charge was given to the globe less than one-sixtieth of that previously given to the hemispheres, and this was readily detected by the electrometer. From the result Cavendish inferred that there is no reason to think the inner globe to be at all charged during the experiment. "Hence it follows that the electric attraction and repulsion must be inversely as the square of the distance, and that, when a globe is positively electrified, the redundant fluid in it is lodged entirely on its surface." This conclusion Cavendish showed to be a mathematical consequence of the absence of electrification from the inner sphere; for, were the law otherwise, the inner sphere must be electrified positively or negatively, according as the inverse power were higher or lower than the second, and that the accuracy of the experiment showed the law must lie between the 2-1/50 and the 1-49/50 power of the distance. With his torsion-balance, Coulomb obtained the same law, but Cavendish's method is much easier to carry out, and admits of much greater accuracy than that of Coulomb. Cavendish's experiment was repeated by Dr.

MacAlister, under the superintendence of Clerk Maxwell, in the Cavendish Laboratory, the absence of electrification being tested by Thomson's quadrant electrometer, and it was shown that the deviation from the law of inverse squares could not exceed one in 72,000.

The distinction between _electrical charge_ or _quant.i.ty of electricity_ and "_degree of electrification_" was first clearly made by Cavendish. The latter phrase was subsequently replaced by _intensity_, but _electric intensity_ is now used in another sense.

Cavendish's phrase, _degree of electrification_, corresponds precisely with our notion of electric _potential_, and is measured by the work done on a unit of electricity by the electric forces in removing it from the point in question to the earth or to infinity. Along with this notion Cavendish introduced the further conception of the amount of electricity required to raise a conductor to a given degree of electrification, that is, the capacity of the conductor. In modern language, the _capacity_ of a conductor is defined as "the number of units of electricity required to raise it to unit potential;" and this definition is in precise accordance with the notion of Cavendish, who may be regarded as the founder of the mathematical theory of electricity. Finding that the capacities of similar conductors are proportional to their linear dimensions, he adopted a sphere of one inch diameter as the unit of capacity, and when he speaks of a capacity of so many "inches of electricity," he means a capacity so many times that of his one-inch sphere, or equal to that of a sphere whose diameter is so many inches. The modern unit of capacity in the electro-static system is that of a sphere of _one centimetre radius_, and the capacity of any sphere is numerically equal to its radius expressed in centimetres. Cavendish determined the capacities of nearly all the pieces of apparatus he employed. For this purpose he prepared plates of gla.s.s, coated on each side with circles of tinfoil, and arranged in three sets of three, each plate of a set having the same capacity, but each set having three times the capacity of the preceding. There was also a tenth plate, having a capacity equal to the whole of the largest set. The capacity of the ten plates was thus sixty-six times that of one of the smallest set. With these as standards of comparison, he measured the capacities of his other apparatus, and, when possible, modified his conductors so as to make them equal to one of his standards. His large Leyden battery he found to have a capacity of about 321,000 "inches of electricity," so that it was equivalent to a sphere more than five miles in diameter. One of his instruments employed in the measurement of capacities was a "trial plate," consisting of a sheet of metal, with a second sheet which could be made to slide upon it and to lie entirely on the top of the larger plate, or to rest with any portion of its area extending over the edge of the former. This was a conductor whose capacity could be varied at will within certain limits. Finding the capacity of two plates of tinfoil on gla.s.s much greater than his calculations led him to expect, Cavendish compared them with two equal plates having air between, and found their capacity very much to exceed that of the air condenser. The same was the case, though in a less degree, with condensers having sh.e.l.lac or bee's-wax for their dielectrics, and thus Cavendish not only discovered the property to which Faraday afterwards gave the name of "specific inductive capacity," but determined its measure in these dielectrics. He also discovered that the apparent capacity of a Leyden jar increases at first for some time after it has been charged--a phenomenon connected with the so-called residual charge of the Leyden jar. Another feature on which he laid some stress, and which was brought to his notice by the comparison of his coated panes, was the creeping of electricity over the surface of the gla.s.s beyond the edge of the tinfoil, which had the same effect on the capacity as an increase in the dimensions of the tinfoil. The electricity appeared to spread to a distance of 007 inch all round the tinfoil on gla.s.s plates whose thickness was 021 inch, and 009 inch in the case of plates 008 inch thick.

His paper on the torpedo was read before the Royal Society in 1776.

The experiments were undertaken in order to determine whether the phenomena observed by Mr. John Walsh in connection with the torpedo could be so far imitated by electricity as to justify the conclusion that the shock of the torpedo is an electric discharge. For this purpose Cavendish constructed a wooden torpedo with electrical organs, consisting of a pewter plate on each side, covered with leather. The plates were connected with a charged Leyden battery, by means of wires carried in gla.s.s tubes, and thus the battery was discharged through the water in which the torpedo was immersed, and which was rendered of about the same degree of saltness as the sea. Cavendish compared the shock given through the water with that given by the model fish in air, and found the difference much greater than in the case of the real torpedo, but, by increasing the capacity of the battery and diminishing the potential to which it was charged, this discrepancy was diminished, and it was found to be very much less in the case of a second model having a leather, instead of a wooden, body, so that the body of the fish itself offered less resistance to the discharge. One of the chief difficulties lay in the fact that no one had succeeded in obtaining a visible spark from the discharge of the torpedo, which will not pa.s.s through the smallest thickness of air. Cavendish accounted for this by supposing the quant.i.ty of electricity discharged to be very great, and its potential very small, and showed that the more the charge was increased and the potential diminished in his model, the more closely did it imitate the behaviour of the torpedo.

But the main interest in this paper lies in the indications which it gives that Cavendish was aware of the laws which regulate the flow of electricity through multiple conductors, and in the comparisons of electrical resistance which are introduced. It had been formerly believed that electricity would always select the shortest or best path, and that the whole of the discharge would take place along that route. Franklin seems to have a.s.sumed this in the pa.s.sage quoted[4]

respecting the discharge of the lightning down the uninsulated conductor instead of through the building. The truth, however, is that, when a number of paths are open to an electric current, it will divide itself between them in the inverse ratios of their resistances, or directly as their conductivities, so that, however great the resistance of one of the conductors, some portion, though it may be a very small fraction, of the discharge will take place through it. But this law does not hold in the case of insulators like the air, through which electricity pa.s.ses only by disruptive discharges, and which completely prevent its pa.s.sage unless the electro-motive force is sufficient to break through their substance. In the case of the lightning-conductor, however, its resistance is generally so small in comparison with that of the building it is used to protect, that Franklin's conclusion is practically correct.

[Footnote 4: Page 96.]

In his paper on the torpedo Cavendish stated that some experiments had shown that iron wire conducted 400,000,000 times better than rain or distilled water, sea-water 100 times, and saturated solution of sea-salt about 720 times, better than rain-water. Maxwell pointed out that this comparison of iron wire with sea-water would agree almost precisely with the measurements of Matthiesen and Kohlrausch at 11C.

The records of the experiments which led to these results were found among Cavendish's unpublished papers, and the experiments also showed that the conductivity of saline solutions was very nearly proportional to the percentage of salt contained, when this was not very large--a result also obtained long afterwards by Kohlrausch. In making these measurements Cavendish was his own galvanometer. The solutions were contained in gla.s.s tubes more than three feet long, and a wire inserted to different distances into the solution; thus the discharge could be made to pa.s.s through any length of the liquid column less than that of the tube itself. From the Leyden battery of forty-nine jars, six jars of nearly equal capacity were selected and charged together, and the charge of one jar only was employed for each shock.

The discharge pa.s.sed through the column of liquid contained in the tube, from a wire inserted at the further end, until it reached the sliding wire, when nearly the whole current betook itself to the wire on account of its smaller resistance, and thence pa.s.sed through the galvanometer, which was Cavendish himself. Two tubes were generally compared together, and the jars discharged alternately through the tubes, and the tube which gave the greatest shock was a.s.sumed to possess the least resistance. The wires were then adjusted till the shocks were nearly equal, and positions determined which made the first tube possess a greater and then a less resistance than the second. From these positions the length of the column of liquid was estimated which would make the resistances of the two tubes exactly equal. But the result which has the greatest theoretical interest was obtained by discharging the Leyden jars through wide and narrow tubes containing the same solutions. By these experiments Cavendish found that the resistances of the conductors were independent of the strengths of the currents flowing in them; that is to say, he established Ohm's law for electrolytes in a form which carried with it its full explanation. This was in January, 1781. Ohm's law was first formally stated in 1827. The physical fact which is expressed by it is that the ratio of the electro-motive force to the current produced is the same for the same conductor, otherwise under the same physical conditions, however great or small that electro-motive force may be.

Cavendish devoted considerable attention to the subject of heat, especially thermometry. In many of his investigations on latent and specific heat he worked on the same lines as Black, and at about the same time; but it is difficult to determine the exact date of some of Cavendish's work, as he frequently did not publish it for a long time after its completion, and most of Black's results were made public only to his lecture audience. Cavendish, however, improved upon Black in his mode of stating some of his results. The heat, for instance, which is absorbed by a body in pa.s.sing from the solid to the liquid, or from the liquid to the gaseous, condition, Black called "latent heat," and supposed it to become latent within the substance, ready to reveal itself when the body returned to its original condition. This heat Cavendish spoke of as being _destroyed_ or _generated_, and this is in accordance with what we now know respecting the nature of heat, for when a body pa.s.ses from the solid to the liquid, or from the liquid or solid to the gaseous, condition, a certain amount of work has to be done, and a corresponding amount of heat is used up in the doing of it. When the body returns to its original condition, the heat is restored, as when a heavy body falls to the ground, or a bent spring returns to its original form. Cavendish's determination of the so-called latent heat of steam was very slightly in error.

About 1760 very extraordinary beliefs were current respecting the excessive degree of cold and the rapid variations of temperature which take place in the Arctic regions. Braun, of St. Petersburg, had observed that mercury, in solidifying in the tube of a thermometer, descended through more than four hundred degrees, and it was a.s.sumed that the melting point of mercury was about 400 below Fahrenheit's zero. It then became necessary to suppose that, while the mercury in a thermometer was freezing, there was a variation of temperature to this extent, and thus these wild reports became current. Cavendish and Black independently explained the anomaly, and each suggested the same method of determining the freezing point of mercury. Cavendish, however, had a piece of apparatus prepared which he sent to Governor Hutchins, at Albany Fort, Hudson's Bay. It consisted of an outer vessel, in which the mercury was allowed to freeze, but not throughout the whole of its ma.s.s, and the bulb of the thermometer was kept immersed in the liquid metal in the interior. In this way the mercury in the thermometer was cooled down to the melting point without commencing to solidify, and the temperature was found to be between 39 and 40 below Fahrenheit's zero.

As a chemist, Cavendish is renowned for his eudiometric a.n.a.lysis, whereby he determined the percentage of oxygen in air with an amount of accuracy that would be creditable to a chemist of to-day, and for his discovery of the composition of water; but to the world generally he is perhaps best known by the famous "Cavendish experiment" for determining the ma.s.s, and hence the mean density, of the earth. The apparatus was originally suggested by the Rev. John Mich.e.l.l, but was first employed by Cavendish, who thereby determined the mean density of the earth to be 545. At the request of the Astronomical Society, the investigation was afterwards taken up by Mr. Francis Baily, who, after much labour, discovered that the princ.i.p.al sources of error were due to radiation of heat, and consequent variation of temperature of parts of the apparatus during the experiment. To minimize the radiation and absorption, he gilded the princ.i.p.al portions of the apparatus and the interior of the case in which it was contained, and his results then became consistent. Cavendish had himself suggested the cause of the discrepancy, but the gilding was proposed by Princ.i.p.al Forbes. As a mean of many hundreds of experiments, Mr. Baily deduced for the mean density of the earth 56604. Cavendish's apparatus was a delicate torsion-balance, whereby two leaden b.a.l.l.s were supported upon the extremities of a wooden rod, which was suspended by a thin wire. These b.a.l.l.s were about two inches in diameter, and the experiment consisted in determining the deflection of the wooden arm by the attraction of two large solid spheres of lead brought very near the b.a.l.l.s, and so situated that the attraction of each tended to twist the rod horizontally in the same direction. The force required to produce the observed deflection was calculated from the time of swing of the rod and b.a.l.l.s when left to themselves. The force exerted upon either ball by a known spherical ma.s.s of metal, with its centre at a known distance, being thus determined, it was easy to calculate what ma.s.s, having its centre at the centre of the earth, would be required to attract one of the b.a.l.l.s with the force with which the earth was known to attract it.

Dr. Wilson sums up Cavendish's view of life in these words:--

His theory of the universe seems to have been that it consisted _solely_ of a mult.i.tude of objects which could be weighed, numbered, and measured; and the vocation to which he considered himself called was to weigh, number, and measure as many of these objects as his allotted three score years and ten would permit. This conviction biased all his doings--alike his great scientific enterprises and the petty details of his daily life.

[Greek: _Panta metro, kai arithmo, kai stathmo_], was his motto; and in the microcosm of his own nature he tried to reflect and repeat the subjection to inflexible rule and the necessitated harmony which are the appointed conditions of the macrocosm of G.o.d's universe.