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 



