THE VARIOUS KINDS OF PEOTEIDS 445 



Carbon 



Oxygen 



Nitrogen ...... 



Hydrogen ...... 



Suli^hur ..... 



There are several classes of these bodies known, the differ- 

 ences which they show depending chiefly on their solubilities in 

 different liquids. The best known groups are the following : — 



1. Albumins. These are soluble in distilled water, and if the 

 solution be heated the proteid is converted into a peculiarly 

 insoluble form, known as coag ulated ])roteid, and deposited. 

 Albumins can be precipitated unchanged by saturating their 

 solutions with sodio-magnesic sulphate. Their coagulation 

 temperature ranges from 70° to 80° C. They are not of 

 common occm-rence in plants, but ma}' be extracted from 

 certain roots. 



2. Globulms. These differ from albumins in not being soluble 

 in distilled water. They can be dissolved by adding a little 

 neutral salt, such as sodium chloride. Their solutions are 

 coagulated on heating, and they can be precipitated by satu- 

 rating them with magnesic sulphate. There are many 

 members of this class, which differ from each other by 

 varymg solubility in different strengths of the salt solution. 

 Their coagulation temperatures vary a good deal, some 

 bemg changed at about 55^ C. and others not till about 

 75°— 80° C. 



The proteids found in plants chiefly belong to this 

 group. They can readily be extracted from most seeds. 

 Probably this form is the one which occurs in the meso- 

 phyll cells. 

 8. Albuminates, or derived albumins. These are soluble in 

 weak acids and alkalies, but their solutions do not coagu- 

 late on boiling. They can be precipitated by carefully 

 neutralising the solution. 



4. Albumoses. These are soluble in water, and do not 

 coagulate on heating. Their characteristic reaction is that 

 they give with nitric acid a precipitate which dissolves on 

 warming the liquid, and reappears as it cools. 



5. Peptones. These are much like albumoses, but do not 

 give a precipitate with nitric acid. 



The members of the last two groups are capable of dialysing 

 through membranes, which those of the first three cannot do. 



