32 THE PBOTEINS. 



70. Mix: a little lecithin with dry, powdered potassium nitrate 

 in a small porcelain crucible, and warm, at first gently, then, after 

 deflagration, until the dark color has disappeared. After cooling 

 dissolve in water and test for phosphoric acid by nitric acid and 

 ammonium molybdate. At once or after warming a yellow pre- 

 cipitate will appear. 



THE PKOTEINS. 



The protein compounds constitute the greater part of 

 the solid matter of the blood, muscles, nerves, and other 

 organs of the animal hody. The urine, tears, and perspira- 

 tion, in a normal condition, never contain more than a 

 trace. The proteins contain carbon, hydrogen, nitrogen, 

 oxygen, and usually sulphur. A few contain phosphorus 

 and a few others iron. When heated they are charred, 

 giving off water, inflammable gases, and ammonia, at the 

 same time emitting a strong odor similar to that of burnt 

 horn or wool. Upon further ignition they leave an ash, 

 though whether this was originally a part of the protein 

 molecule has not been decided. They are often spoken of 

 simply as the nitrogenous constituents of the body or the 

 food, although not all of the nitrogenous compounds found 

 there belong to this class. 



The proteins are very complex substances with a high 

 molecular weight, and it is probably owing to this fact 

 that they are so easily decomposed, as is seen by the putre- 

 faction which sets in soon after life has ceased. To the 

 large molecule, too, is probably due the inability of most 

 of them to pass through a parchment or animal membrane. 



GENERAL PROPERTIES OF THE PROTEINS. 



71. Burn a small piece of dry albumin or other 

 protein compound on a piece of porcelain or platinum foil, 



