18 PRODUCTS OF DECOMPOSITION OF PROTEIDS. [BOOK I. 



the solutions are not precipitated by neutralization. When heated with 

 strong solution of caustic potash potassium sulphide is not formed. 



/?. Casein, the chief proteid constituent of milk. Same properties as a, 

 but when heated with strong solution of caustic potash, potassium sulphide 

 is formed. In milk is coagulated by rennet. 



CLASS V. Fibrin : Insoluble in water and in weak solutions of sodium 

 chloride. White elastic solid, usually exhibiting fibrillation when examined 

 under a high magnifying power; swells up in cold hydrochloric acid of *1 per 

 cent., but does not dissolve; when thus swollen dissolves with ease when a 

 solution of pepsin is poured over it. When heated for a great many hours 

 at 40 in dilute hydrochloric acid, it dissolves and the solution contains 

 acid-albumin. 



CLASS VI. Coagulated Proteids : Insoluble in water, dilute acids 

 and alkalies. Give Millon's reaction. Are dissolved when digested at 

 350 40, in artificial gastric or pancreatic juice, giving rise to peptones. 



CLASS VII. Lardacein, so-called amyloid substance : Insoluble in 

 water, in dilute acids, in alkaline carbonates ; not dissolved by gastric juice 

 at the temperature of the body. Coloured brownish-red or violet by iodine. 



SEC. 4. PRODUCTS OF DECOMPOSITION OF PROTEIDS. 



The methods which the chemist follows in arriving at a know- 

 ledge of the constitution of a body are various ; his chief information 

 is derived from a careful study of the way in which the body is 

 decomposed under various circumstances, and of the structure and 

 amounts of the various products thus obtained ; subsidiary informa- 

 tion is derived from a consideration of physical properties, which 

 sometimes suggest analogies which otherwise would pass undetected. 

 The correctness of any view as to the structure of a body will be 

 tested by its being able, or not, to account for all known reactions, 

 and it will receive singular confirmation if it enable the experimenter 

 to effect the synthesis of the subject of speculation. 



Great though the progress of organic chemistry has been, and 

 remarkable the development of our knowledge of the constitution 

 of bodies, we are yet far from being able to unravel the constitution 

 of such complex bodies as the proteids. We can therefore merely 

 record the results of laborious experiments which shew the pro- 

 ducts, or rather the classes of products, yielded by the proteids, and 

 scarcely venture to surmise what the exact constitution of the 

 proteids may be. 



In the animal body, the proteids are ultimately subjected to 

 processes of oxidation of which the chief ultimate results are water, 

 carbon dioxide and urea ; what all the intermediate substances may 

 be we do not exactly know, though it is certain that glycine, leucine, 

 tyrosine and some other bodies are formed ; moreover it is certain that 

 substances destitute of nitrogen, such as carbohydrates, and also fats, 

 may take their origin in the decomposition of the proteids. Can 



