18 THE GENERAL CHARACTERS OF THE PROTEINS 



substances not precipitated. Certain inorganic bodies which readily 

 adsorb proteins have also been suggested for the removal of the latter 

 from solution. These inorganic adsorbents have been investigated 

 by Landsteiner and Uhlirz, who found that the most effective sub- 

 stances were precipitated silicic acid, meerschaum and iron oxide. 

 Two grams of these powders, shaken with 20 c.c. of 0x25 per cent, 

 solutions of proteins, can remove the whole of the latter from solution. 

 The more readily a protein is precipitated by salts the more easily is 

 it adsorbed by these non-crystalline powders. 



Method. 



It has already been stated that the mastic emulsion should be in 

 large excess. In the case of liquids rich in protein, such as serum, a 

 preliminary precipitation with alcohol can be carried out, or the 

 mastic can be added in portions at a time. The following example 

 will illustrate the method : 



One volume of serum is mixed with three volumes of alcohol. 

 After separating the precipitate, one volume of a 50 per cent, solu- 

 tion of mastic in absolute alcohol is added and the mixture is then 

 diluted with water till the alcohol does not form more than 30 per 

 cent, of the total fluid. The liquid is then just acidified with acetic 

 acid, and 10 per cent, magnesium sulphate solution is added, 10-15 

 c.c. being employed for each litre of the solution. The precipitated 

 mastic will then carry down the last traces of the protein remaining 

 in solution. 



SECTION V. CRYSTALLISATION OF PROTEINS. 



Proteins, as colloidal bodies, were at one time thought to be 

 incapable of existing in crystalline form. Later investigations have 

 shown, however, that this is not the case, and the elaboration of 

 methods for the crystallisation of certain substances of this class must 

 be considered as a distinct advance in the chemical technique for the 

 preparation of pure substances. 



Crystalline bodies of a protein character were first observed in 

 plants where they occur naturally. They were known as " aleurone " 

 grains, and were first noticed in gluten by.Th. Hartig in 1850. Their 

 protein character was first demonstrated by Radlkofer in 1858. They 

 occur in the seeds of a large number of plants, such as the pumpkin, 

 in hemp seeds, in castor-oil seeds, and in Brazil nuts. 



In some respects these aleurone grains differ from ordinary crys- 

 tals. They can, for example, imbibe water and swell, during which 

 process they lose to some extent their capacity for refracting light ; 

 they retain, however, their original contours. The increase in size, as 

 determined by accurate measurement by Schimper, is not the same 

 in every axis. Another peculiarity is that aleurone grains will par- 

 tially dissolve on treatment with glycerine, and an undissolved portion 

 will be obtained which still retains the original crystalline contour, 

 but which has nearly the same refractive index as water. 



These peculiarities have caused the question to be raised as to 

 whether these bodies are true crystals. 



Similar naturally occurring protein crystals have also been de- 

 scribed in animal organisms, notably in the intestinal epithelium of 

 meal-worms and in the eggs of certain fish and amphibia. These 



