THE PHOTEIN SUBSTANCES. 31 



II. Proteid. 



With this name, as suggested by HOPPE-SEYLER, we designate 

 a class of bodies which are more complex than the albuminous 

 bodies and which yield as nearest splitting products albuminous 

 bodies on one side and non-protein bodies, such as coloring matters 

 and carbohydrates, on the other. 



The most important substances belonging to this group are the 

 blood - coloring matter, haemoglobin, which will be thoroughly 

 treated in a following chapter (Chapter IV), and mucin substances 

 or allied bodies. 



Mucin Substances. We designate as mucin colloid substances 

 whose solutions are mucilaginous and thready, and which when 

 treated with acetic acid give a precipitate insoluble in an excess 

 of acid, and on boiling with dilute mineral acids yield a substance 

 capable of reducing copper oxyhydrate. This last-mentioned fact, 

 which was first observed by EICHWALD, differentiates mucin from 

 other bodies which have long been mistaken for it and .which have 

 similar physical properties. On the other hand, bodies whose phy- 

 sical properties differ from it but which give a reducible substance 

 on boiling with dilute mineral acids have also been designated as 

 mucin. 



The different bodies characterized as mucin substances cor- 

 respond, first, either to true mucin or, second, to mucoid or mucinoid. 



All mucin substances contain carbon, hydrogen, nitrogen, 

 sulphur, and oxygen. Compared with albuminous bodies they 

 contain less nitrogen and, as a rale, less carbon. As close decom- 

 position products they yield albuminous bodies on one side and 

 carbohydrates or acids related thereto on the other. On boiling 

 with dilute mineral acids they all give a reducible substance. 



The true mucins are characterized by their natural solution, or 

 one prepared by a trace of alkali, being mucilaginous, thread-like, 

 and giving a precipitate with acetic acid which is insoluble in excess 

 of acid. The mucoids do not show these physical properties and 

 have other solubilities and precipitative properties. As we have 

 intermediate steps between different albuminous bodies, so also we 

 have such between true mucins and mucoids, and a sharp line 

 between these two groups cannot be drawn. 



