NITROGEN IN FOOD. 223 



rally. But it was exceedingly difficult to 

 ascertain its true source. There exists now 

 little doubt that its chief source, when not 

 supplied artificially, is in the ammonia of the 

 atmosphere. Minute, therefore, although the 

 quantity of this ingredient be in the air, it has 

 an importance which can scarcely be exag- 

 gerated, when we consider the uses it fulfils. 

 " The quantity of food required by animals," 

 writes Liebig, " for their nourishment, increases 

 or diminishes in the same proportion as it con- 

 tains more or less nitrogen." In other words, 

 that kind of food, as a general rule, is the most 

 nutritious, which contains the greatest propor- 

 tion of nitrogen in its composition.* This 

 element consequently becomes most essential 

 to the existence of animals ; and it is sup- 

 plied to herbivorous and graminivorous animals 

 chiefly by plants, in the food they derive from 

 the vegetable kingdom. Plants, as we have 

 seen, obtain their nitrogen chiefly from the 

 minute quantity of ammonia contained in the 

 air; and hence it is manifest that the health and 

 vigour, and even the very existence of the 

 whole animal world, is most intimately con- 



* Bread is well called the staff of life. The gluten it con- 

 tains is a nitrogenised compound. It is well known that upon 

 bread alone life can be supported for a very long period ; a 

 fact due, in great part, to the circumstance of its containing 

 this nitrogenous substance, together with earths and salts. 



