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 init. 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 
