ANIMAL CHEMISTRY. 399 



about one third of the substance of the bones, phosphate 

 of lime being almost all of the remaining two thirds. It 

 is the gelatin in the skin that tannin so firmly unites with, 

 converting it into leather. What is commonly called glue 

 is gelatin. This substance dissolves readily in water. The 

 various jellies that we use prepared from animal substances 

 are gelatin. The gelatin is first separated from them and 

 dissolved by hot water, and then the solution on cooling 

 leaves a jelly. Isinglass, so called, is chiefly gelatin pre- 

 pared from the sounds or air-bladders of certain fresh-wa- 

 ter fishes, particularly one of the sturgeon class found in 

 the rivers of Russia, 



565. Two Kinds of Food. All the varieties of food are 

 divided into two classes according to the purpose which 

 they serve, the one class serving to build up the tissues, 

 and the other to maintain the animal heat. To the former 

 class belong those substances that contain nitrogen, as the 

 gluten of bread, the fibrin of meat, the casein of milk, etc. 

 The other class comprises those substances that have no ni- 

 trogen in them, as starch, sugar, and oily substances. These 

 all serve to maintain the warmth of the body, and it is sup- 

 posed have little or nothing to do with building its struct- 

 ures. They are burned up, as we may express it, in creat- 

 ing heat, which is just as essential to the maintenance of 

 life as nutrition is. This food is called respiratory food, 

 because it is supposed that the oxygen introduced by the 

 respiration is employed in consuming or burning it. The 

 products of this flameless combustion are the same with 

 those of ordinary combustion with flame, water, and car- 

 bonic anhydride. The heat-making food is often called 

 carbonaceous from the predominance of carbon in it, while 

 the building food is called nitrogenous because it is dis- 

 tinguished from the other by containing nitrogen. 



While the view thus presented is generally true, there 



