NITROGENOUS COMPOUNDS 63 



is associated, in definite chemical union, with a prosthetic 

 group or non-protein radicle. These are to be regarded as 

 compound or conjugated bodies, in contradistinction to the 

 simple substances in which the typical compound is not so 

 associated. Both types occur in nature. 



In addition to the native proteins above referred to, the 

 group includes a number of transformation products, derived 

 from them by artificial means, but which retain the essential 

 characteristics of the group. 



Nomenclature. The study of this group of substances has 

 been greatly complicated by an unfortunate confusion in 

 nomenclature. White of egg was long regarded as a typical 

 member, and the terms Eiweisskorper (egg-white bodies) or 

 Proteinstoffes were used in German as equivalent to the French 

 substances albuminoid. In English, the terms " protein " and 

 "albuminoid" have been, and are still occasionally, used in a 

 differential sense, to distinguish between the simple and com- 

 pound types, and also to distinguish between the " true or 

 genuine " proteins, such as white of egg, and other substances, 

 such as horn, gelatine, etc., which were not regarded as true 

 proteins, though closely allied to them. 



These substances are now regarded as members of the 

 group, and the term " protein " has been definitely accepted as 

 the scientific name for the whole class. The term "albu- 

 minoid " is now rarely used, except for commercial purposes, 

 but may be regarded as an alternative name. 



Physical Properties. In the dry state, the native proteins 

 are obtained as white or nearly colourless amorphous, non- 

 hygroscopic powders. Some of them are soluble in pure 

 water, others are soluble in solutions of neutral salts, or in 

 dilute acids or alkalis. They are all insoluble in alcohol, 

 ether, chloroform, etc., and are therefore precipitated from 

 aqueous solution by these reagents. 



The solutions of those that are soluble in pure water offer 

 great resistance to the passage of the electric current. The 

 boiling points and freezing points of these solutions, when they 

 can be determined, differ very slightly from those of the 

 solvents under. similar conditions. The substances have very 



