170 PEACTICAL PHYSIOLOGY 



also be detected "by its smell, or by holding the stopper of the con- 

 centrated hydrochloric acid bottle over the mouth of the test tube 

 when fumes of ammonium chloride are formed. 



EXPERIMENT II. The loosely combined Sulphur. To about five 

 cubic centimetres of 20 per cent, caustic potash add two drops of 

 lead acetate solution; a precipitate at first forms, which, however, 

 redissolves on shaking. When clear, add some solution of egg-white 

 and boil, when a brown to black colour will be developed, due to the 

 lead sulphide which is formed. 



By studying the decomposition products resulting from their 

 hydrolysis, many attempts have been made to construct the con- 

 stitutional formula of proteids, and, although this has not been attained, 

 still much knowledge has been gained of their chemical nature. 



Eecently Emil Fischer and his pupils have succeeded in linking 

 various amido acids together, the resulting bodies being named peptides. 

 This discovery justifies the hope that in the near future proteid syn- 

 thesis will be possible. 



The basis of construction of all proteids is, according to Kossel, a 

 body called protamin (C 30 H 57 N ir 6 ), which yields on hydrolysis three 

 basic substances, lysin, histidin, and arginin, each containing six 

 carbon atoms, and hence called hexone bases. Protamin has been 

 found loosely combined with nucleic acid (see p. 428) in the sper- 

 matozoa of certain fishes. In the proteid molecule it is firmly 

 combined with amido-acids (e.g. leucin, glycin, etc.), and usually with 

 aromatic bodies (e.g. tyrosin, etc.), and inorganic elements, (e.g. sulphur 

 and phosphorus). The nature and the amount of the decomposition 

 products yielded by different forms of proteid varies, and even in the 

 case of protamin there is no doubt that the molecule is of enormous 

 size, though the accurate determination of this is impossible on 

 account of the peculiar physical properties of proteids. 1 



In the animal body the ultimate decomposition products of proteids 

 are urea, carbon dioxide gas, water, ammonia, and sulphuric acid; 

 the intermediate bodies, such as hexone bases, amido-acids, aromatic 

 bodies, etc., being produced during digestion in the intestine, in con- 

 nection with which they will be more closely studied. 



PHYSICAL PROPERTIES OF PROTEIDS. 



I. Diffusibility. Proteids belong to the class of bodies called 

 Colloids, which do not diffuse through animal membranes or parch- 



J The decomposition products also vary with the nature of the decomposing 

 agency employed, but the subject is too complicated for study here. Attempts 

 at the synthesis of proteids have as yet proved futile. 



