72 PHYSIOLOGY 



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, altera- 

 tion of reaction, or addition of neutral salts. Coagulation by heat forms a 

 distinguishing feature of a number of members of this class, which are there- 

 fore 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 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 coagula- 

 tion on change of temperature is a reversible process. Thus an alkaline 

 solution of caseinogen, the chief protein of milk, if treated with a little cal- 

 cium 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 com- 

 bustion. 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 adsorption 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 solutions differen- 

 tiates 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 protoplasm of the cells. These crystals consist of proteins belonging to 

 the class of globulins. By chemical means they can be separated from the surrounding 

 tissues and, after washing, dissolved in a solution of magnesia. Drechsel showed that 

 on dialysing such a solution against alcohol, the fluid undergoes gradual concentration, 

 and crystalline granules of the magnesia compound of the protein separate out. These 



