IRRITABILITY AS A SIGN OF LIFE 5 
other is a chemical sign, which has just been discovered 
and which will be discussed in this book. The electrical 
sign of life was discovered by Galvani when he found that 
animal tissues are a source of electricity. He discovered 
animal electricity. It is now certain that whenever 
the response to a stimulus takes place in animals or 
plants—the response which is the sign of life—an elec- 
trical change accompanies it. By placing a galvanome- 
ter on the animal or plant we can study this electrical 
response. Life. and electricity are thus shown to be 
related. Electricity and psychism have something in 
common, although just what the connection is cannot at 
present be said. The English physiologist Waller has 
recently introduced as a measure of life a particular kind 
of electrical response which he has discovered and which 
he calls the “blaze’’ current, because it is as if the 
electrical display suddenly blazed up when the living 
matter was disturbed; this he calls an electrical sign of 
life. By it he can tell whether a dry seed is alive or not 
without putting it in the ground and letting it sprout. 
It is very hard to know whether this electrical dis- 
turbance which living things show is due to physical or 
to chemical changes in their substances. 
It is therefore a matter of very great interest that 
I have recently found that there is always and every- 
where an accompanying chemical change of a particular 
kind which is as sure a sign of life and as invariable an 
accompaniment of the vital reaction as the electrical 
change. This chemical sign is the sudden outburst of 
carbon dioxide which all living things show—plants as 
well as animals, dry seeds as well as the nerve tissues of 
the highest mammals—when they are stimulated in any 
