^2 CHEMISTRY. 



Besides the foregoing vegetable alkaline principles, there exist a 

 number of neutral principles in plants, possessed of very active 

 properties. Some contain nitrogen, and some do not. The most 

 important of them are Fhloridzin, Salicin, Asparagin, Gentianin, 

 ElateriUy Cathartin, quassin, Lupulin, Ergotin, &c., &c. 



SECTION VIII. 



ANIMAL COMPOUNDS. 



Protein and its compounds. — The most important protein com- 

 pounds are Albumen, Fibrin, and Casein. 



Albumen exists in eggs and the serum of the blood,— being com- 

 bined in the latter with soda : the white of an egg affords a good 

 example of it. It is not soluble in water, unless a little alkali be 

 present ; coagulates by heat, acids, creasote, alcohol, and electricity ; 

 it gives precipitates with most of the metallic salts, particularly cor- 

 rosive sublimate, for which it is the best antidote. 



It is composed of carbon, oxygen, hydrogen, nitrogen, sulphur, 

 and phosphorus ; — or supposing protein to be represented by Pr., the 

 formula for albumen would be, Pr + P-f S^. 



Fibri?i constitutes the chief portion of muscular flesh ; it is also 

 an important constituent of the blood, in which it exists in the 

 soluble siaXe. It may be procured either from muscle, or preferably, 

 by whipping freshly drawn blood with a twig ; the fibrin adheres to 

 it in long white filaments. Its characteristic is its spontaneous 

 coagulation ; it is in consequence of this tendency that blood coagu- 

 lates when drawn from the body. Its composition is very nearly 

 identical with that of albumen, — containing one equivalent less of 

 sulphur. Albumen is converted into fibrin, in the Hving body, in the 

 process of organization. Its proportion in the blood is liable to 

 variation by disease. 



Casein is found in milk, and is the basis of cheese. It closely re- 

 sembles albumen, but diflfers from it in not being coagulable by heat. 

 In composition it is nearly identical with the two foregoing sub- 

 stances, but it contains no phosphorus. 



From either of the above three compounds, protein may be pro- 

 cured, by dissolving them in an alkaline solution, and then precipi- 

 tating by an acid. 



Gelatin and Chondrin. — These principles constitute the bases of 

 skins, tendons, cartilage, and fibro-cartilage, &c. Any of these, 

 when boiled for a long time in water, yield a jelly, which, on cooling, 

 solidifies into gelatin or glue. Isinglass is the dried swimming- 

 bladder of the sturgeon. Chondrin is very analogous to gelatin : 



