ELEMENTARY SUBSTANCES. 55 



ibis 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 in 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 VOGEL, in blood. The latter che- 

 mist " put a quantity of fresh urine 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 



