ELEMENTARY SUBSTANCES. 55 
this substance as derived from the sooty matter from coal, 
mixed with the air inspired. If this change to blackness, 
occurs only in the lungs of the aged inhabitants of cities, the 
explanation here offered must appear plausible; but if the 
same change takes place in the lungs of those who live chiefly 
in the open air, and in the country, we should be disposed 
to consider the charcoal not as a foreign body, but as a 
morbid secretion of the organs of respiration. 
When animal substances are exposed to a high temper- 
ature in closed vessels, the charcoal which is produced, 
differs considerably from that which is obtained by the same 
means from vegetables. It is more glossy m appearance, 
and is incinerated with much greater difficulty. 
2. Hydrogen.—This gaseous element is universally dis- 
tributed in the Animal Kingdom. It occurs as a constitu- 
ent ingredient of all the fluids, and of many of the solids. 
It is invariably in a state of combination with charcoal ; 
for, as far as we know, it has never been detected in:an un- 
combined or separate state. It has been found in the hu- 
man intestines, in the form of carburetted hydrogen. 
3. Oxygen.—This principle is equally widely distributed 
with the preceding, in the fluids and solids of animals. A 
constant supply of it from the atmosphere is indispensably 
necessary to the continuance of animal life. It occurs, not 
only in combination with other bodies, but probably like- 
wise in a separate state, in the air-bag of fishes, in which it 
is found varying in quantity, according to the species, and 
the depth at which the fishes have been caught. It is 
common, in union with charcoal, forming carbonic acid. 
This acid was first detected by Proust, in an uncombined 
state, in urine, and by Vocet, in blood. The latter che- 
mist “ put a quantity of fresh urme into a glass flask, to 
which was held a bent glass tube, the mouth of which dip- 
ped into a vessel containing lime-water. This apparatus 
