BIOLOGICAL METHODS 77 



CONCLUDING REMARKS. 



In the foregoing pages the chief properties of the proteins have 

 been passed in review with the object of determining those which 

 might serve for the purposes of isolation and identification. As a 

 result it must be admitted that the methods available at the present 

 moment are extremely defective. 



The separation of the proteins from one another depends almost 

 entirely on their differences of solubility in alcohol, water, salt solu- 

 tions, or dilute acids and alkalis. To the incompleteness of the 

 separation by differential extraction or by salt precipitation attention 

 has been already drawn. Furthermore, there are large classes of pro- 

 teins, to which even these methods are inapplicable, viz., those which 

 are quite insoluble in the solvents mentioned. For the separation of 

 mixtures of proteins of these classes no methods are available. 



The methods for the identification of proteins are again extremely 

 defective. The unreliability of the physical constants has been 

 repeatedly emphasised. There remain the biological methods, which 

 in recent years have received considerable attention, and a few isolated 

 chemical factors, such as the sulphur content and the distribution of 

 nitrogen in the hydrolysis products. The biological methods are, 

 however, in many cases uncertain, and whilst they are generally 

 available for the physiologist or pathologist, they are entirely beyond 

 the scope of the worker whose only resource is a laboratory devoted 

 to pure chemistry ; the biological reactions, furthermore, require a 

 considerable interval of time for their accomplishment. For these 

 reasons their general application must be limited, and they are, for 

 the most part, quite unavailable for the purpose of the technical 

 examination of products, such as falls, for example, within the range 

 of work of the food analyst. 



For these reasons reliance will have to be placed chiefly on the 

 purely chemical methods for the identification of proteins. Much 

 work remains to be done in the elaboration of such methods, and it 

 is not too much to hope that, with the rapidly increasing knowledge 

 of proteins, a reliable technique will be developed in the near future, 

 such as exists already for the identification and differentiation of fats. 

 It is a necessity for the physiologist, the pathologist and the technical 

 chemist. 



