THE PROTEINS 81 



may be obtained which is absolutely constant in both its physical and 

 chemical characters, we cannot ascribe to crystallisation the same 

 importance in securing purity and homogeneity of the substance 

 that we can when we are dealing with inorganic salts. This is due 

 to the fact that these crystals take up other colloids with great ease. 

 When haemoglobin, for instance, is crystallised from blood, the first 

 crop of crystals, although thoroughly washed from their mother liquor, 

 always contain a considerable proportion of serum albumin. Indeed, 

 the presence of colloidal material seems to render the production of the 

 so-called mixed crystals much more easy. Thus Schultz has shown 

 that in urine mixed inorganic crystals can be obtained. Human 

 urine is allowed to stand twenty-four to forty- eight hours -with di- 

 calcium phosphate and then filtered. On allowing the filtrate to 

 evaporate slowly, a crystalline precipitate is produced consisting of 

 whetstone-shaped crystals which are doubly refracting. On treating 

 these crystals with dilute acetic acid this acid extracts calcium phosphate 

 from the crystals. The original shape of the crystals is, however, retained. 

 The only difference under the microscope consists in the fact that they 

 have now lost their doubly refracting power on polarised light. They 

 consisted of a mixture of calcium sulphate and calcium phosphate, from 

 which, on treatment with acid, only the calcium phosphate was dis- 

 solved out. 



The Molecular Weight of Proteins. We may arrive at an approxi- 

 mate idea of the minimum size of the protein molecule in various ways, 

 though in all cases our calculations are apt to be vitiated by the diffi- 

 culty of obtaining a preparation which is homogeneous, i.e. chemically 

 pure, and by the ease with which molecules of the size which we must 

 assume for proteins form adsorption combinations in varying propor- 

 tions with other substances. If we assume that each molecule of 

 the respective protein contains only one atom of sulphur, we can 

 calculate its molecular weight. It is evident that the protein which 

 contains 1 per cent, of sulphur will have a molecular weight of 3200. 

 In this way the following molecular weights have been arrived at 

 (Abderhalden) : 



Sulphur per cent. Molecular weight 



Edestin . . . 0-87 .. 3680 



Oxyhaimoglobiii . 043 . . 7440 



(horse) 



Serum albumin . . 1-89 . . 1700 



(horse) 



Egg albumin . . 1-30 .. 2460 



Globulin . . . 1-38 .. 2320 



The greater part at any rate of the sulphur in the protein molecule 

 occurs as a constituent of a substance, cystine, each molecule of which 

 contains two atoms of sulphur. Each molecule of protein must also 



6 



