CHAPTER X

RESPONSE IN METALS

Is response found in inorganic substances?--Experiment on tin, block method--Anomalies of existing terminology--Response by method of depression--Response by method of exaltation.

We have now seen that the electrical sign of life is not confined to animals, but is also found in plants. And we have seen how electrical response serves as an index to the vital activity of the plant, how with the arrest of this vital activity electrical response is also arrested temporarily, as in the case amongst others of anaesthetic action, and permanently, for instance under the action of poisons. Thus living tissues--both animal and vegetable--may pa.s.s from a responsive to an irresponsive condition, from which latter there may or may not be subsequent revival.

Hitherto, as already said, electrical response in animals has been regarded as a purely physiological phenomenon. We have proved by various tests that response in plants is of the same character. And we have seen that by physiological phenomena are generally understood those of which no physical explanation can be offered, they being supposed to be due to the play of some unknown vital force existing in living substances and giving rise to electric response to stimulation as one of its manifestations.

#Is response found in inorganic substances?#[14]--It is now for us, however, to examine into the alleged super-physical character of these phenomena by stimulating inorganic substances and discovering whether they do or do not give rise to the same electrical mode of response which was supposed to be the special characteristic of living substances. _We shall use the same apparatus and the same mode of stimulation as those employed in obtaining plant response, merely subst.i.tuting, for the stalk of a plant, a metallic wire, say 'tin'_ (fig. 50). Any other metal could be used instead of tin.

#Experiment on tin, block method.#--Let us then take a piece of tin wire[15] from which all strains have been previously removed by annealing, and hold it clamped in the middle at C. If the strains have been successfully removed A and B will be found iso-electric, and no current will pa.s.s through the galvanometer. If A and B are not exactly similar, there will be a slight current. But this will not materially affect the results to be described presently, the slight existing current merely adding itself algebraically to the current of response.

If we now stimulate the end A by taps, or better still by torsional vibration, a transitory 'current of action' will be found to flow in the wire from B to A, from the unstimulated to the stimulated, and in the galvanometer from the stimulated to the unstimulated. Stimulation of B will give rise to a current in an opposite direction.

[Ill.u.s.tration: FIG. 50.--ELECTRIC RESPONSE IN METALS (_a_) Method of block; (_b_) Equal and opposite responses when the ends A and B are stimulated; the dotted portions of the curves show recovery; (_c_) Balancing effect when both the ends are stimulated simultaneously.]

#Experiment to exhibit the balancing effect.#--If the wire has been carefully annealed, the molecular condition of its different portions is found to be approximately the same. If such a wire be held at the 'balancing point' (which is at or near the middle) by the clamp, and a quick vibration, say, of 90 be given to A, an upward deflection will be produced; if a vibration of 90 be given to B, there will be an equal downward deflection. If now both the ends A and B are vibrated simultaneously, the responsive E.M. variation at the two ends will continuously balance each other and the galvanometer spot will remain quiescent (fig. 30, A, B, R). This balance will be still maintained when the block is removed and the wire is vibrated as a whole. It is to be remembered that with the length of wire constant, the intensity of stimulus increases with the amplitude of vibration. Again, keeping the amplitude constant, the intensity of stimulus is increased by shortening the wire. Hence it will be seen that if the clamp be shifted from the balancing point towards A, simultaneous vibration of A and B through 90 will now give a resultant upward deflection, showing that the A response is now relatively stronger. Thus keeping the rest of the circuit untouched, merely moving the clamp from the left, past the balancing point to the right, we get either a positive, or zero, or negative, resultant effect.

In tin the current of response is from the less to the more excited point. In the retina also, we found the current of action flowing from the less stimulated to the more stimulated, and as that is known as a positive response, we shall consider the normal response of tin to be in like manner positive.

Just as the response of retina or nerve, under certain molecular conditions, undergoes reversal, the positive being then converted into negative, and negative into positive, so it will be shown that the response in metallic wires under certain conditions is found to undergo reversal.

#Anomalies of present terminology.#--When there is no current of injury, a particular current of response can hardly be called a negative, or positive, _variation_. Such nomenclature is purely arbitrary, and leads, as will be shown, to much confusion. A more definite terminology, free from misunderstanding, would be, as already said, to regard the current towards the more stimulated as positive, and that towards the less stimulated, in tissue or wire, as negative.

The stimulated end of tin, say the end A, thus becomes zincoid, i.e.

the current through the electrolyte (non-polarisable electrodes with interposed galvanometer) is from A to B, and _through the wire_, from the less stimulated B to the more stimulated A. Conversely, when B is stimulated, the action current flows round the circuit in an opposite direction. This positive is the most usual form of response, but there are cases where the response is negative.

In order to show that normally speaking a stimulated wire becomes zincoid, and also to show once more the anomalies into which we may fall by adopting no more definite terminology than that of negative variation, I have devised the following experiment (fig. 51). Let us take a bar, one half of which is zinc and the other half copper, clamped in the middle, so that a disturbance produced at one end may not reach the other; the two ends are connected to a galvanometer through non-polarisable electrodes. The current through the electrolyte (non-polarisable electrodes and interposed galvanometer) will then flow from left to right. We must remember that metals under stimulation generally become, in an electrical sense, more zinc-like.

On vibrating the copper end (inasmuch as copper would then become more zinc-like) the difference of potential between zinc and copper ought to be diminished, and the current flowing in the circuit would therefore be lessened. But vibration of the zinc end ought to increase the potential difference, and there ought to be then an increase of current during stimulation of zinc.

[Ill.u.s.tration: FIG. 51.--CURRENT OF RESPONSE TOWARDS THE STIMULATED END Hence when Cu stimulated: action current -->, normal E.M.F. diminished (85-009) V.

When Zn stimulated: action current

In the particular experiment of fig. 51, the E.M.F. between the zinc and copper ends was found to be 85 volt. This was balanced by a potentiometer arrangement, so that the galvanometer spot came to zero.

On vibrating the zinc wire, a deflection of 33 dns. was obtained, in a direction which showed an _increase_ of E.M.F. On stopping the vibration, the spot of light came back to zero. On now vibrating the copper wire, a deflection of 23 dns. was obtained in an opposite direction, showing a _diminution_ of E.M.F. This transitory responsive variation disappeared on the cessation of disturbance.

By disturbing the balance of the potentiometer, the galvanometer deflection due to a known increase of E.M.F. was found from which the absolute E.M. variation caused by disturbance of copper or zinc was determined.

It was thus found that stimulation of zinc had increased the P.D. by fifteen parts in 1,000, whereas stimulation of copper had decreased it by eleven parts in 1,000. According to the old terminology, the response due to stimulation of zinc would have been regarded as positive variation, that of copper negative. The responses however are not essentially opposite in character, the action current in the bar being in both cases towards the more excited. For this reason it would be preferable, as already said, to employ the terms positive and negative in the sense I have suggested, i.e. positive, when the current in the acted substance is towards the more excited, and negative, when towards the less excited. The method of block is, as I have already shown, the most perfect for the study of these responses.

In the experiment fig. 50, if the block is abolished and the wire is struck in the middle, a wave of molecular disturbance will reach A and B. The mechanical and the attendant electrical disturbance will at these points reach a maximum and then gradually subside. The resultant effect in the galvanometer will be due to E_A-E_B when E_A and E_B are the electrical variations produced at A and B by the stimulus. The electric changes at A and B will continuously balance each other, and the resultant effect on the galvanometer will be zero: (_a_) if the exciting disturbance reaches A and B at the same time and with the same intensity; (_b_) if the molecular condition is similar at the two points; and (_c_) if the rate of rise and subsidence of excitation is the same at the two points. In order that a resultant effect may be exhibited in the galvanometer, matters have to be so arranged that the disturbance may reach one point, say A, and not B, and _vice versa_.

This was accomplished by means of a clamp, in the method of block. Again a resultant differential action may be obtained even when the disturbance reaches both A and B, if the electrical excitability of one point is exalted or depressed by physical or chemical means. We shall in Chap. XVI study in detail the effect of chemical reagents in producing the enhancement or depression of excitability. There are thus two other means of obtaining a resultant effect--(2) by the method of relative depression, (3) by the method of relative exaltation.

#Electric response by method of depression.#--We may thus by reducing or abolishing the excitability of one end by means of suitable chemical reagents (so-called method of injury) obtain response in metals without a block. The entire length of the wire may then be stimulated and a resultant response will be produced, owing to the difference between the excitability of the two ends. A piece of tin wire is taken, and one normal contact is made at A (strip of cloth moistened with water, or very dilute salt solution). The excitability of B is depressed by a few drops of strong potash or oxalic acid. By the application of the latter there will be a small P.D. between A and B; this will simply produce a displacement of zero. By means of a potentiometer the galvanometer spot may be brought back to the original position. The shifting of the zero will not affect the general result. The effect of mechanical stimulus is to produce a transient electro-motive response, which will be superposed algebraically on the existing P.D. The deflection will take place from the modified zero to which the spot returns during recovery. On now stimulating the wire as a whole by, say, torsional vibration, the current of response will be found towards the more excitable, i.e. from B to A (fig. 52, _a_).

[Ill.u.s.tration: FIG. 52.--RESPONSE BY METHOD OF DEPRESSION (WITHOUT BLOCK) When the wire is stimulated as a whole the current of response is towards the more excitable.

In (_a_) A is a normal contact, B has been depressed by oxalic acid; current of response is towards the more excitable A.

In (_b_) the same wire is used, only A is depressed by oxalic acid and a normal contact is made at a fresh point B', a little to the left of B in (_a_). Current of response is now from A towards the more excitable B'.]

A corroborative reversal experiment may next be made on the same piece of wire. The normal contact, through water or salt solution, is now made at B', a little to the left of B. The excitability of A is now depressed by oxalic acid. On stimulation of the whole wire, the current of response will now be found to flow in an opposite direction--i.e. from A to B'--but still from the relatively less to the relatively more excitable (fig. 52, _b_).

From these experiments it will be seen how in one identical piece of wire the responsive current flows now in one direction and then in the other, in absolute conformity with theoretical considerations.

#Method of exaltation.#--A still more striking corroboration of these results may, however, be obtained by the converse process of relative exaltation of the responsiveness of one contact. This may be accomplished by touching one contact, say B, with a reagent which like Na_2CO_3 exalts the electric excitability. On stimulation of the wire, the current of response is towards the more excitable B (fig. 53).

[Ill.u.s.tration: FIG. 53.--METHOD OF EXALTATION The contact B is made more excitable by chemical stimulant (Na_2CO_3). The current of response is towards the more excitable B.]

I give four records (fig. 54) which will clearly exhibit the responses as obtained by the methods of relative depression or exaltation. In (_a_) B is touched with the excitant Na_2CO_3, a permanent current flows from A to B, response to stimulus is in the same direction as the permanent current (positive variation). In (_b_) B is touched with a trace of the depressant oxalic acid, the permanent current is in the same direction as before, but the current of response is in the opposite direction (negative variation). In (_c_) B is touched with dilute KHO, the response is exhibited by a positive variation. In (_d_) B is touched with strong KHO, the response is now exhibited by a negative variation.

The last two results, apparently anomalous, are due to the fact, which will be demonstrated later, that KHO in minute quant.i.ties is an excitant, while in large quant.i.ties it is a depressant.

[Ill.u.s.tration: FIG. 54

+-------------------+-----------+----------+

Current

Permanent

of

Current

Response

+-------------------+-----------+----------+

B treated with

sodium carbonate.

-->

-->

+-------------------+-----------+----------+

B treated with

oxalic acid

-->

-->

+-------------------+-----------+----------+

B treated with

strong potash

-->

Current of response is always towards the more excitable point.

(_a_) Response when B is treated with sodium carbonate.--An apparent positive variation.

(_b_) Response when B is treated with oxalic acid.--An apparent negative variation.

(_c_) Response when B is treated with very dilute potash.--Positive variation.

(_d_) Response when B is treated with strong potash.--Negative variation.

The response is up when B is more excitable, and down when A is more excitable.

Lines thus ------ indicate deflection due to permanent current.]

We have thus seen that we may obtain response (1) by block method, (2) by the method of injury, or relative depression of responsiveness of one contact, and (3) by the method of relative exaltation of responsiveness of one contact. In all these cases alike we obtain a consistent action current, which in tin is normally positive, or towards the relatively more excited.

FOOTNOTES:

[14] Following another line of inquiry I obtained response to electric stimulus in inorganic substances using the method of conductivity variation (see 'De la Generalite des Phenomenes Moleculaires Produits par l'Electricite sur la Matiere Inorganique et sur la Matiere Vivante,'

_Travaux du Congres International de Physique, Paris, 1900_; and also 'On Similarities of Effect of Electric Stimulus on Inorganic and Living Substances,' _British a.s.sociation 1900_. See _Electrician_). To bring out the parallelism in all details between the inorganic and living response, I have in the following chapters used the method of electro-motive variation employed by physiologists.

[15] By 'tin' is meant an alloy of tin and lead used as electric fuse.

CHAPTER XI

INORGANIC RESPONSE--MODIFIED APPARATUS TO EXHIBIT RESPONSE IN METALS

Conditions of obtaining quant.i.tative measurements--Modification of the block method--Vibration cell--Application of stimulus--Graduation of the intensity of stimulus--Considerations showing that electric response is due to molecular disturbance--Test experiment--Molecular voltaic cell.

We have already seen that metals respond to stimulus by E.M. variation, just as do animal and vegetable tissues. We have yet to see whether the similarity extends to this point only, or goes still further, whether the response-curves of living and in organic are alike, and whether the inorganic response-curve is modified, as living response was found to be, by the influence of external agencies. If so, are the modifications similar? What are the effects of superposition of stimuli? Is there fatigue? If there be, in what way does it affect the curves? And lastly, is the response of metals exalted or depressed by the action of chemical reagents?

#Conditions of obtaining quant.i.tative measurements.#--In order to carry out these investigations, it is necessary to remove all sources of uncertainty, and obtain quant.i.tative measurements. Many difficulties at first presented themselves in the course of this attempt, but they were completely removed by the adoption of the following experimental modification. In the simple arrangement for qualitative demonstration of response in metals previously described, successive experiments will not give results which are strictly comparable (1) unless the resistance of the circuit be maintained constant. This would necessitate the adoption of some plan for keeping the electrolytic contacts at A and B absolutely invariable. There should then be no chance of any shifting or variation of contact. (2) There must also be some means of applying successive stimuli of equal intensity. (3) And for certain further experiments it will be necessary to have some way of gradually increasing or decreasing the stimuli in a definite manner.