CHEMICAL SIGNS OF LIFE 93 



Thus we extended our experiments to the best-known 

 tissues in the plant and animal kingdoms, and found 

 no exception to the general rule cited. These results 

 surely justify the generalization that all living tissues 

 differ from all dead tissues in that they respond to 

 injury, producing more carbon dioxide than the normal 

 tissues; and that by measuring this output of the gas 

 in comparison with the uninjured we can detect the 

 vitality of the tissue. 



Chemical sign of life. We have now come to a con- 

 clusion on all the facts that we have presented so far. 

 Of all the signs of living processes irritability is one of 

 the most universal. This phenomenon of irritability is 

 expressed in the power of feeling the external world. It 

 is the inherent power of the living to react against a 

 stimulation. The necessary condition for this irritability 

 of tissues is metabolic activity. Although this chemical 

 condition is necessary for all tissue in order that it shall 

 be irritable, yet it is not a sufficient criterion for the 

 detection of vitality in it. We must inaugurate a further 

 test of whether or not it reacts chemically to a stimula- 

 tion. In order to test this power, we injure the tissue 

 and watch the response. If the tissue is alive, me- 

 chanical crushing will produce a metabolic response; 

 if it is not alive, there is no response. 



The detail of testing the vitality of a tissue is as 

 follows : 



In order to test that of a seed, take two or more 

 kernels of the seed in question having about equal 

 weights. One is placed in the right chamber of the 

 biometer, and the other is crushed, or is cut to pieces, 

 and placed in the left. The apparatus is filled with 



