140 RESPONSE IN THE LIVING AND NON-LIVING 
which may affect one kind of tissue and not others. 
Poisons in general may be regarded as extreme cases of 
depressants. As an example of those which produce 
moderate physiological depression, potassium bromide 
may be mentioned, and this also diminishes electric 
response. There are other chemical reagents, on the 
other hand, which produce the opposite effect of increas- 
ing the excitability and causing a corresponding exalta- 
tion of electric response. 
We shall now proceed to inquire whether the re- 
sponse of inorganic bodies is affected by chemical 
reagents, so that their excitability is exalted by some, 
and depressed or abolished by others. Should it prove 
to be so, the last test will have been fulfilled, and 
that parallelism which has been already demonstrated 
throughout a wide range of phenomena, between the 
electric response of animal tissues on the one hand, and 
that of plants and metals on the other, will be com- 
pletely established. 
Action of stimulants on metals.—We shall first study 
the stimulating action of various chemical reagents. 
The method of procedure is to take a series of normal 
responses to uniform stimuh, the electrolyte being water. 
The chemical reagent whose effect is to be observed is 
now added in small quantity to the water in the cell, 
and a second series of responses taken, using the same 
stimulus as before. Generally speaking, the influence 
of the reagent is manifested in a short period, but there 
may be occasional instances where the eflect takes some 
time to develop fully. We must remember that by the 
introduction of the chemical reagent some change may 
