4 A CHEMICAL SIGN OF LIFE 
part they are invisible and have to be detected by special 
apparatus. The visible changes consist in some reaction 
of the organism when its surroundings change. If it 
moves when it is touched, the degree of movement not 
being related to the impulse given it; if it breathes; if 
it changes its rate of growth under changing conditions, 
we say that it is alive. It is irritable, and it has the 
property of irritability. Another thing noticed in a 
living thing is that the impulse which arouses it to 
action may cause reaction in a part of the organism 
distant from the point of stimulation. In other words, 
the change, whatever its nature, set up in the organism 
by the stimulus is propagated to a distance from the 
point of irritation. We can see the results of this propa- 
gation. All things which show this conduction and 
response we say are living things. These are physical 
processes which apparently always accompany the 
psychic process. But sometimes these changes, although 
they occur, produce no visible result; consequently we 
must have methods which will detect conduction and 
irritability, even though there are no visible signs of 
them. One cannot see, for example, that anything has 
happened to a seed when it is pricked by a pin; it 
does not say “ouch!” loud enough for us to hear it, or 
in a language we understand; but nevertheless it jumps 
when it is pricked as if it did say “ouch!” as we can 
show by appropriate methods. 
There are two signs, or tests, which all living things 
show and which are an index of life. One of these is an 
electrical disturbance. This was discovered a very long 
time—a hundred years—ago, and its discovery was the 
basis of the development of knowledge of electricity. The 
