72 PHYSIOLOGY 



04 to 0-8 per cent. It is doubtful, however, how far this phosphorus 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 defined, and the 

 relative proportions of the ingredients vary according to the conditions under 

 which the compound was formed. As is the case with most colloids when 

 in solution 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 dis- 

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

 fluid sets to a solid white mass (hydrogel). This change is irreversible, 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 cool- 

 ing 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 preparation 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 adsoiption of substances possessed by such solu- 

 tions, there is no need to regard these salts as essential constituents of the 

 protein. 



Crystallisation of Proteins. Although the indiffusibility of protein solu- 

 tions differentiates them from the crystalloid substances such as sugar or 

 sodium chloride, under certain conditions it is possible to obtain crystals 

 consisting, largely at any rate, of proteins. Thus in the seeds of certain 

 plants, e.g. hemp seeds, Brazil nut, pumpkin and castor-oil seeds, the so-called 

 aleurone crystals may be seen under the microscope enclosed in the proto- 

 plasm of the cells. These crystals consist of proteins belonging to the class of 



