# Collected Physical Papers/On the Similarity of Effect of Electrical Stimulus on Inorganic and Living Substances

XVIII
ON THE SIMILARITY OF EFFECT OF ELECTRICAL STIMULUS ON INORGANIC AND LIVING SUBSTANCES
In working with receivers for electric waves, I found that under continuous stimulation by the oncoming message, the sensitiveness of the metallic detector disappeared. But after a sufficient period of rest it regained once more its normal sensitiveness. In taking records of successive responses, I was surprised to find that they were very similar to those exhibiting fatigue in the animal muscle. And just as animal tissue, after a period of rest, recovers its activity, so did the inorganic receiver recover after an interval of rest.
Thinking that prolonged rest would make the receiver even more sensitive, I laid it aside for several days and was astonished to find that it had become inert. A strong electric shock now stirred it up into readiness for response. Two opposite treatments are thus indicated for fatigue from overwork, and for inertness from long passivity.
A muscle-curve registers the history of the fundamental molecular change produced by excitation in a living tissue, exactly as the curve of molecular reaction registers an analogous change in an inorganic substance. The two represent the same thing; in the latter the molecular upset is evidenced by the change of conductivity, while in the former it is manifested by the change of form. We have thus means of studying molecular reaction produced by stimulus of varying frequency, intensity and duration. An abyss separates ​the phenomena of living matter from those of inanimate matter. But if we are ever to understand the hidden mechanism of the animal machine it is necessary to face numerous difficulties which at present seem formidable.
I shall now describe the results of comparative study of the curves of molecular reactions of inorganic and living substances. For the former I will take the response of magnetic oxide of iron (Fe$_3$O$_4$) to the action of stimulus of electric radiation. Suppose we start with this substance in its normal condition with moderate conductivity, the galvanometer deflection being 50 div., under a definite electromotive force. The deflection of 50 will, therefore, indicate the normal conductivity. Next, under the stimulus of electric waves the induced molecular change causes an increase of conductivity represented by the larger deflection of 100. On the cessation of the stimulus there is a recovery, the galvanometer deflection returning from 100 to the original value 50. The suspected coil of the galvanometer thus moves in response to the varying molecular change induced in the sensitive substance by the action of stimulus. The invisible molecular change is thus revealed by the visible deflection of the galvanometer coil. Curves of molecular change due to electrical stimulation may thus be obtained with galvanometer deflections as ordinates and the periods of stimulation and recovery as abscissæ.
Response to a Single Stimulus
The curve given in fig. 59 represents the effect of instantaneous stimulus on the inorganic receiver. There is a latent period, the response taking place a short time after the incidence of the stimulus; the response ​continues for a short period even after the cessation of stimulation; it attains a maximum after which
Fig. 59. Curve for Fe$_3$O$_4$.
the substance begins to recover, at first quickly, then more slowly. In all this, an analogy is found with the response curve of muscles, in which also there is a short phase of latent period, a phase of increasing action and a phase of recovery.
Superposition of Stimuli
Three sets of experiments were carried out (1) on the effect of succession of maximum stimuli; (2) on the summation-effects of medium stimuli with slow intermittence; and (3) on the summation-effects of rapid intermittence.
Maximum stimulation.—By bringing the radiator sufficiently near the inorganic receiver, a maximum effect was observed, the deflection being 230 divisions. Before the substance could recover to any extent, a second stimulus was superposed. This produced no further deflection, but when the second stimulus was applied after recovery, then the second deflection was the same as the first, i.e., 230 divisions.
​Medium stimulus, slow intermittence.—In this set of experiments, the radiator was moved further away so that the intensity of the stimulus was moderate; the successive flashes of radiation were applied at intervals of two seconds. These moderate excitations are found to be summated and when in slow succession, the effect of each shock can be distinguished as the steps in the ascending curve, as in fig. 60 (a).
Fig. 60. Superposition of Effects. (a) Effect due to Slow Intermittence. (b) Tetanic Effect due to High Frequency Intermittence.
Rapid intermittence.—When the stimuli follow each other with great rapidity, the intermittent effects are fused together; the rising curve is found to be unbroken and the effect may be described as 'tetanic' as in fig. 60 (b).
The response curves of muscles under the above conditions are similar to the above.
Opposite Effects of Strong and Feeble Stimulus
The response of many inorganic receivers was found to exhibit the peculiarity that while moderate intensity of stimulus produced the normal response of a given sign, a feeble stimulus elicited the opposite reaction, the sign of response being reversed (cf. p. 137). I succeeded later in demonstrating the occurrence of similar ​reactions in living tissues, as for example in the physiological action of drugs, which may be regarded as chemical stimuli. A small 'dose' in such cases is often found to give rise to an effect precisely opposite to that produced by a large dose.
Effect of Variation of Temperature
Variation of temperature produces a marked effect on the response of muscle. At a low temperature the response is very sluggish and the amplitude of response is very much reduced. With rise of temperature the size of response increases and the period of recovery becomes quickened (fig. 61 a, b). Above a certain optimum the response becomes diminished.
Fig. 61. Effect of Variation of Temperature on Response. Response of Muscle, left; Response of Inorganic receiver, right. (a) response at low temperature (b) at a higher temperature.
Parallel effects are observed in the response of the inorganic receiver. The curve a to the right represents the response of the magnetic oxide of iron receiver when cold; the deflection was moderate and the recovery was extremely slow. On raising the temperature of the receiver to about 15° C. above that of the room, the amplitude of the response was found to be very much increased—nearly threefold. ​The recovery became rapid specially in the first part, complete recovery taking place in the course of 75 secs. .When the temperature was raised to 100° C., the response became greatly reduced. It is thus seen that a rise of temperature up to an optimum increases the amplitude of response and hastens the recovery in both inorganic and living substances.
Effect of Chemical Substances
Traces of certain substances are found to produce an extraordinarily great increase in the sensibility of the inorganic receivers; these act like stimulants. There are other substances which abolish the sensibility acting like "poisons."
In all the phenomena described above there is no break of continuity. It is difficult to draw a line and say 'here the physical phenomenon ends and the physiological begins' or "that is a phenomenon of dead matter and this a vital phenomenon peculiar to the living"; such lines of demarcation do not exist.
I shall in a future occasion describe a different method namely that of electromotive variation for obtaining the response of inorganic matter, and demonstrate the continuity of response in the living and the non-living.
(International Congress of Science, Paris 1900.)