Page:Collected Physical Papers.djvu/274

254 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 (Fe3O4) 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