NITROGEN GIVEN OUT. 109 



enclosing air in the shape of froth, in a far higher degree 

 than even soap suds. This air, by means of the saliva, 

 reaches the stomach with the food, and there its oxygen 

 enters into combination, while its nitrogen is given out 

 through the skin and lungs. The longer digestion con- 

 tinues, that is, the greater the resistance offered to the 

 solvent action by the food, the more saliva, and conse- 

 quently the more air enters the stomach. Rumination, 

 in certain graminivorous animals, has plainly for one 

 object a renewed and repeated introduction of oxygen ; 

 for a more minute mechanical division of the food, only 

 shortens the time required for solution. 



The unequal quantities of air which reach the stom- 

 ach with the saliva in different classes of animals, ex- 

 plain the accurate observations made by physiologists, 

 who have established beyond all doubt, the fact, that 

 animals give out pure nitrogen through the skin and 

 lungs, in variable quantity. This fact is so much the 

 more important, as it furnishes the most decisive proof, 

 that the nitrogen of the air is applied to no use in the 

 animal economy. 



The fact, that nitrogen is given out by the skin 

 and lungs, is explained by the property which animal 

 membranes possess of allowing all gases to permeate 

 them, a property which can be shown to exist by the 

 most simple experiments. A bladder, filled with car- 

 bonic acid, nitrogen, or hydrogen gas, if tightly closed 

 and suspended in the air, loses in twenty-four hours the 

 whole of the enclosed gas ; by a kind of exchange, it 

 passes outwards into the atmosphere, while its place is 

 10 



