THE PROTEINS 79 



Nearly all the proteins contain a small trace of phosphorus vary in 

 from 04 to 0-8 per cent. It is doubtful, however, how far this phos- 

 phorus forms an integral part of the protein molecule. 



Physical Characters. The proteins are amorphous indiffusible 

 substances belonging to the class of bodies known as colloids. Most 

 of them are soluble either in water, weak: salt solutions, or in dilute 

 acids or alkalies. They are inert bodies and tasteless. Although they 

 form compounds with various metallic salts, acids, or alkalies, these 

 compounds are but ill denned, and the relative proportions of the 

 ingredients vary according to the conditions under which the com- 

 pound was formed. As is the case with most colloids when in solu- 

 tion or pseudo-solution, they can be brought into an insoluble form 

 by various simple agencies, such as shaking, change of temperature, 

 alteration of reaction, or addition of neutral salts. Coagulation 

 by heat forms a distinguishing feature of a number of members of 

 this class, which are therefore spoken of as ' coagulable proteins.' 

 For instance, white of egg is a solution of different proteins. On 

 diluting it with weak salt solution no precipitation takes place. If, 

 however, the solution be heated to about 80 C. a precipitate of coagu- 

 lated protein is formed. If a strong solution be boiled the whole 

 fluid sets to a solid white mass (hydrogel). This change is irrever- 

 sible, i.e. it is not possible by lowering the temperature to bring the 

 white of egg again into solution, and many properties of the protein 

 have been changed in the act of coagulation. With certain proteins 

 and their allies the coagulation on change of temperature is a reversible 

 process. Thus an alkaline solution of caseinogen, the chief protein 

 of milk, if treated with a little calcium chloride and heated, undergoes 

 coagulation and sets into a jelly, but on cooling the mixture the 

 coagulum once more enters into solution. Ordinary gelatin, which 

 is closely allied to the proteins, with water forms a solid jelly below 

 20 C., and a fluid solution above this temperature. 



If a protein be heated in a current of air or oxygen it undergoes 

 combustion. In all cases a certain amount of incombustible material 

 is left, consisting of inorganic salts which were closely attached to 

 the protein molecule. If a solution of protein be subjected to long- 

 continued dialysis, the proportion of ash may be diminished very largely, 

 but in no case has any experimenter succeeded in obtaining a prepara- 

 tion of protein absolutely ash-free. On this account it has been 

 thought that the salts of the ash must be in chemical combination 

 with the protein ; but having regard to the physical character of 

 colloidal solutions, which we shall study in the next chapter, and the 

 power of adsorption of substances possessed by such solutions, there 

 is no need to regard these salts as essential constituents of the protein. 



Crystallisation of Proteins. Although the indiff visibility of protein 



