INTRODUCTION. 



THE proteins belong for the most part to that class of bodies which 

 Graham has designated the " colloids," to which the ordinary criteria 

 available for the identification and differentiation of simpler organic 

 compounds, such as boiling points, melting points, etc., are generally 

 inapplicable. The want of knowledge of the physical nature of 

 colloids has been one of the chief obstacles to advances in the experi- 

 mental investigations of the proteins. The principal problem 

 affected thereby is that of the separation of the proteins from one 

 another and from other substances. The proteins, furthermore, are 

 substances of large molecular weight, yielding on degradation a 

 great number of products, the isolation and quantitative estimation of 

 which have taxed the ingenuity of the chemist to the utmost. 



The investigations on the proteins may be consequently divided 

 into two main classes : (i) those connected with the elucidation of their 

 physical properties as colloids, with which are associated the names 

 of Hardy in this country, and of Hofmeister, Spiro, Pauli and many 

 others on the continent; (ii) those connected with their chemical 

 constitution, with which are associated the names of Emil Fischer 

 and of Kossel and their pupils. A short review of certain aspects 

 of protein chemistry in the light of the more recent advances will be 

 advisable before proceeding to a more detailed discussion of those 

 properties which may be utilised in the processes for the isolation 

 and identification of individual substances belonging to the class. 



ISOLATION AND SEPARATION OF THE PROTEINS. 



Proteins derived from different sources have markedly different 

 properties. Some are soluble in water, others are insoluble in pure 

 water, but soluble in saline solutions ; others, again, are soluble in 

 alcohol. Advantage was taken of these differences for the separation 

 of proteins from one another, especially in the earlier work on the 

 vegetable proteins. 



Another property, viz., that of precipitability from aqueous 

 solutions by the addition of neutral salts, also received early attention, 

 and the difference in behaviour of solutions of different proteins as 

 regards precipitability was soon turned to account in devising a 

 method of separation. It is of interest in this place to quote the 

 words of Denis, the first investigator who systematically employed 

 the method of " salting out ". On the title-page of his monograph, 

 Memoir e sur le sang, published in 1856, but containing results of work 

 commenced many years before this date, he describes his researches 

 as " e*tudes faites suivant la methode d'experi mentation par les sels, la 



