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 



