56 PHILOSOPHY OF ZOOLOGY. 
being put under the receiver of an air-pump, the air was 
slowly exhausted. A great quantity of air bubbles issued 
from the urme, and the lime-water became milky, indica- 
ting the extraction of carbonic acid gas*.” By a similar 
process, it was obtained from blood. When proper precau- 
tions were used, Darwin could not detect any air either in 
blood, urine, or bile, by the aid of the air-pump. The car- 
bonic acid appeared, however, when these fluids had been 
exposed to the air, and seemed to be generated in conse- 
quence of the removal of atmospheric pressure +. Carbo- 
nic acid gas is likewise found in the intestines of quadru- 
peds; but it chiefly exists in animal bodies, in combination 
with the alkalies or earths. It is likewise emitted by all 
animals in the act of respiration. 
4 Axzote.—This gas is very widely distributed as a com- 
ponent part of animal substances. It occurs in almost all 
the fluids, and in those solid parts which have carbon as 
a base. The almost universal prevalence of this princi- 
ple in animal substances, constitutes one of the most cer- 
tain marks by which they may be distinguished from vege- 
tables. Azote likewise occurs, in an uncombined state, in 
the air-bag of some fishes. It was first detected as a secre-" 
tion, by Dr Prrestiey in the air-bag of the roach. 
5. Phosphorus.—This inflammable body exists, in union 
with oxygen, in the state of phosphoric acid, in many of 
the solids and fluids of animals. Its existence, however, in 
an uncombined state, has not been satisfactorily determined, 
although there appears a tendency to refer the luminousness 
of several animals to the slow combustion of this substance. 
Even phosphoric acid can scarcely be said to exist in a se- 
parate state, bemg found in combination with potash, soda, 
ammonia, lime, or magnesia. 

* Annals of Philosophy, vii. p. 56. + Phil. Trans. 1774, p. 345. 
