Outside the rainbow there was part of a secondary bow. The Ochil Hills were north of his point of observation. These hills were covered with snow, and the setting sun was glowing with rosy light. Never had he seen such a depth of colour as was on them on this occasion. It was a deep, furnacy red. The sun's light was shorn of all the rays of short-wave length on its pa.s.sage through the atmosphere, and only the red rays reached the earth. The reason why the Ochils glowed with so deep a red was owing to their being overhung by a dense curtain of clouds, which screened off the light of the sky. The illumination was thus princ.i.p.ally that of the direct softer light of the sun.

CHAPTER XXI

THE AURORA BOREALIS

He must be a very careless observer who has not been struck with the appearance of the streamers which occasionally light up the northern heavens, and which farmers consider to be indicators of strong wind or broken weather.

The time was when the phenomenon was considered to be supernatural and portentous, as the chroniclers of spectral battles, when "fierce, fiery warriors fought upon the clouds, in ranks and squadrons, and right form of war." And even in the rural districts of Britain, the blood-coloured aurora, of October 24th, 1870, was considered to be the reflection of an enormous Prussian bonfire, fed by the beleaguered French capital.

In joyful spirit, the Shetlanders call the beautiful natural phenomenon, "Merry Dancers." Burns a.s.sociated their evanescence with the transitoriness of sensuous gratification:--"they flit ere you can point their place." And Tennyson spoke of his cousin's face lit up with the colour and light of love, "as I have seen the rosy red flushing in the northern night."

Yet this phenomenon is to a great extent under the control of cosmical laws. One of the most difficult problems of our day has been to disentangle the irregular webwork of aurorae, and bring them under a law of periodicity, which depends upon the fluctuations of the sun's photosphere and the variations on the earth's magnetism, and which have such an important influence upon the fluctuations of the weather.

The name "Aurora Borealis" was given to it by Ga.s.sendi in 1621.

Afterwards, the old almanacs described it as the "Great Amazing Light in the North." In the Lowlands of Scotland, the name it long went by, of "Lord Derwent.w.a.ter's Lights," was given because it suddenly appeared on the night before the execution of the rebel lord. In Ceylon aurorae were called "Buddha Lights."

The first symptom of an aurora borealis is commonly a low arch of pale, greenish-yellow light, placed at right angles to the magnetic meridian.

Sometimes rays cover the whole sky, frequently showing tremulous motion from end to end; and sometimes they appear to hang from the sky like the fringes of a mantle. They are among the most capricious of natural phenomena, so full of individualities and vagaries. To the glitter of rapid movement they add the charm of vivid colouring. It is strongly a.s.serted that aurorae are preceded by the same general phenomena as thunder-storms. This was borne out by Piazzi Smith (late Astronomer-Royal for Scotland), who observed that their monthly frequency varies inversely with that of thunder-storms--both being safety-valves for the discharge of surplus electricity.

Careful observers have, moreover, noticed a remarkable coincidence between the display of aurorae and the maxima of the sun's spots and of the earth's magnetic disturbances. Some have supposed that the light of the aurora is caused by clouds of meteoric dust, composed of iron, which is ignited by friction with the atmosphere. But there is this difficulty in the way, shooting stars are more frequent in the morning, while the reverse is the case with the aurora. The highest authorities have concluded, pretty uniformly, that aurorae are electric discharges through highly rarefied air, taking place in a magnetic field, and under the sway of the earth's magnetic induction. They are not inappropriately called "Polar lightnings," for when electricity misses the one channel it must traverse the other.

The natives of the Arctic regions of North America pretend to foretell wind by the rapidity of the motions of the streamers. When they spread over the whole sky, in a uniform sheet of light, fine weather ensues.

Fitzroy believed that aurorae in northern lat.i.tudes indicated and accompanied stormy weather at a distance. The same idea is still current among many farmers and fishermen in Scotland.

Is there any audible accompaniment to the brilliant spectacle? The natives of some parts, with subtle hearing-power, speak of the "whizzing" sound which is often heard during auroral displays. Burns tells of their "hissing, eerie din," as echoes of the far-off songs of the Valkyries.

Perhaps the most striking incident which corroborates this opinion occurred during the Franco-Prussian War. Rolier, a practised aeronaut, left Paris in a balloon, on his mission of city defence, and fourteen hours afterwards landed in Norway. He had reached the height of two and a half miles. When descending, he pa.s.sed through a peculiar cloud of sulphurous odour, which emitted flashed light and a slight scratching or rustling noise. On landing, he witnessed a splendid aurora borealis. He must, therefore, have pa.s.sed through a cloud in which an electrical discharge of an auroral nature was proceeding, accompanied with an audible sound. There is, moreover, no improbability of such sounds being occasionally heard, since a somewhat similar phenomenon accompanies the brush discharge of the electric machinery, to which the aurora bears considerable resemblance.

Though no fixed conclusions are yet established about the causes of the brilliant auroral display, yet, as the results of laborious observations, we are a.s.sured that the stabler centre of our solar system holds in its powerful sway the several planets at their respective distances, supplying them all with their seasonable light and heat, vibrating sympathetic chords in all, and even controlling under certain--though to us still unknown--laws the electric streamers that flit, apparently lawlessly, in the distant earth's atmosphere.

CHAPTER XXII

THE BLUE SKY

If we look at the sky overhead, when cloudless in the sunshine, we wonder what gives the air such a deep-blue colour. We are not looking, as children seem to do, into vacancy, away into the far unknown. And even, if that were the case, would not the s.p.a.ce be quite colourless? What, then, produces the blueness?

Some of the very fine dust-particles, even when clothed with an exceedingly thin coating of water-vapour, are carried very high; and, looking through a vast acc.u.mulation of these, we find the effect of a deep-blue colour.

Why so? Because these particles are so small that they can only reflect the rays of the blue end of the spectrum; and the higher we ascend, the smaller are the particles and the deeper is the blue. But it is also because water, even in its very finest and purest form, is blue in colour.

For long this was disputed. Even Sir Robert Christison concluded, after years of experimenting on Highland streams, that water was colourless.

Of course, he admitted that the water in the Indian and Pacific Oceans has frequent patches of red, brown, or white colour, from the myriads of animalcules suspended in the water. Ehrenberg found that it was vegetable matter which gave to the Red Sea its characteristic name. But these, and similar waters, are not pure.

It is to Dr. Aitken that the final discovery of the real colour of water is due. When on a visit to several towns on the sh.o.r.es of the Mediterranean, he set about making some very interesting experiments, which the reader will follow with pleasure.

It is a well-known fact that colour transmitted through different bodies differs considerably from colour reflected by them. In his first experiment he took a long empty metal tube, open at one end, and closed at the other end by a clear-gla.s.s plate. This was let down vertically into the water, near to a fixed object, which appeared of most beautiful deep and delicate blue at a depth of 20 feet. Scientific men know that, if the colour of water is due to the light reflected by extremely small particles of matter suspended in the water, then the object looked at through it would have been illuminated with yellow (the complementary colour of blue). A blackened tube was then filled with water (which had a clear-gla.s.s plate fixed to the bottom), and white, red, yellow, and purple objects were sunk in the water, and these colours were found to change in the same way as if they were looked at through a piece of pale-blue gla.s.s.

The white object appeared blue, the red darkened very rapidly as it sank, and soon lost its colour; at the depth of seven feet a very brilliant red was so darkened as to appear dark brick-red. The yellow object changed to green, and the purple to dark blue.

But, still further to satisfy himself that water is really blue in itself, even without any particles suspended in it, he tested the colour of _distilled_ water. He filled a darkened tube with this water (clear-gla.s.s plates being at the ends of the tube), and looked through it at a white surface. The effect was the same as before, the colour was blue, almost exactly of the same hue as a solution of Prussian blue.

This is corroborated by the fact that, the purer the water is in nature, the bluer is the tint when a large quant.i.ty is looked through. Some Highland lochs have crystal waters of the most extraordinary blue. Of course, some cling to the old idea that this is accounted for by the reflected blue of the clear heavens above. No doubt, if the sky be deep blue, then this blue light, when reflected by the surface of the water, will enrich and deepen the hue. But the water itself is _really_ blue.

At the same time, the dust-particles suspended in the water have a great effect in making the water appear more beautiful, brilliant, and varied in its colouring; because little or no light is reflected by the interior of a ma.s.s of water itself. If a dark metal vessel be filled with a weak solution of Prussian blue, the liquid will appear quite dark and void of colour. But throw in some fine white powder, and the liquid will at once become of a brilliant blue colour. This accounts for the change of depth and brilliancy of colour in the several sh.o.r.es of the Mediterranean.

When, then, you look at the face of a deep-blue lake on a summer evening--the heavens all aglow with the unrivalled display of colour from the zenith, stretching in lighter hues of glory to the horizon--though to you the calm water appears like a lake of molten metal glowing with sky-reflected light, so powerful and brilliant as entirely to overpower the light which is internally reflected, yet blue is the normal colour of the water: _blueness is its inherent hue_.

Looking upwards, we observe three distinct kinds of blue in the sky from the horizon to the zenith. All painters in water-colours know that. Newton thought that the colour of the sky was produced in the same way as the colours in thin plates, the order of succession of the colours gradually increasing in intensity.

CHAPTER XXIII

A SANITARY DETECTIVE

The impure state of the air in the rooms of a house can now be determined by means of colour alone. Dr. Aitken has invented a very simple instrument for that purpose; and this ought to be of great service to sanitary officers. It is called the koniscope--or dust-detective.

The instrument consists of an air-pump and a metal tube with gla.s.s ends.

Near one end of the test-tube is a pa.s.sage by which it communicates with the air-pump, and near the other end is attached a stop-c.o.c.k for admitting the air to be tested. It is not nearly so accurate as the dust-counter; but it is cheaper, more easily wrought, and more handy for quick work. All the grades of blue, from what is scarcely visible to deep, dark blue, may be attached alongside the tube on pieces of coloured gla.s.s; and opposite these colours are the numbers of dust-particles in the cubic inch of the similar air, as determined by the dust-counter.

While the number of particles was counted by means of the dust-counter, the depth of blue given by the koniscope was noted; and the piece of gla.s.s of that exact depth of blue attached. A metal tube was fitted up vertically in the room, in such a way that it could be raised to any desired height into the impure air near the ceiling, so that supplies of air of different degrees of impurity might be obtained. To produce the impurity, the gas was lit and kept burning during the experiments. The air was drawn down through the pipe by means of the air-pump of the koniscope, and it pa.s.sed through the measuring apparatus of the dust-counter on its way to the koniscope. It may be remarked that, by a stroke of the air-pump, the air within the test-tube is rarefied and the dust-particles seize the moisture in the super-saturated air to form fog-particles; through this fog the colour is observed, and the shade of colour determines the number of dust-particles in the air. These colours are named "just visible," "very pale blue," "pale blue," "fine blue," "deep blue," and "very deep blue."

When making a sanitary inspection, the pure air should be examined first, and the colour corresponding to that should be considered as the normal health colour. Any increase from the depth would indicate that the air was being gradually contaminated; and the amount of increase in the depth of colour would indicate the amount of increase of pollution.

As an ill.u.s.tration of what this instrument can detect, a room of 24 by 17 by 13 feet was selected. The air was examined before the gas was lighted, and the colour in the test-tube was very faint, indicating a clear atmosphere. In all parts of the room this was found the same. A small tube was attached to the test-tube, open at the other end, for taking air from different parts of the room. Three jets of gas were then lit in the centre of the room, and observations at once begun with the koniscope.

Within thirty-five seconds of striking the match to light the gas, the products of combustion had extended near the ceiling to the end of the room; this was indicated by the colour in the koniscope suddenly becoming a deep blue. In four minutes the deep-blue-producing air was got at a distance of two feet from the ceiling. In ten minutes there was strong evidence of the pollution all through the room. In half-an-hour the impurity at nine feet from the floor was very great, the colour being an intensely deep blue.

The wide range of the indications of the instrument, from pure clearness to nearly black blue, makes the estimate of the impurity very easily taken with it; and, as there are few parts to get out of order, it is hoped it may come into general use for sanitary work.

CHAPTER XXIV

FOG AND SMOKE

Just two hundred and forty years ago, Mr. John Evelyn, F.R.S., a well-known writer on meteorology, sent a curious tract to King Charles II., which was ordered to be printed by his Majesty. It was ent.i.tled "Fumifugium," and dealt with the great smoke nuisance in London. I find from the thesis that he had a very hazy idea of the connection between fog and smoke; and no wonder, for it is only lately that the connection has been fully explained.