2 THE GENERAL CHARACTERS OF THE PROTEINS 



seule qui, dans 1'etat actuel de la science, semble pouvoir etre applique 

 avec fruits a des reserches sur ces substances". 1 Although these 

 words were written more than forty years ago, when the protein 

 investigations were still in their infancy, they contain a statement 

 which is substantially true to-day, for the process of " salting out " 

 is the only one which is capable of general application for the 

 separation of the proteins. Like the analogous processes of fractional 

 distillation and fractional precipitation, the process of fractional 

 " salting out " seldom leads to a complete separation of mixtures. 

 The efficiency and the gradual development of the technique is 

 discussed in that section of this work, which, owing to the actual and 

 historical importance of the subject, precedes all others dealing with 

 the general character of the proteins. It has purposely been treated 

 in a somewhat empirical fashion ; the physical nature of the process 

 will, it is hoped, be dealt with in greater detail in a later volume of 

 this series. 



THE CHEMICAL NATURE OF THE PROTEINS. 



Although it was clearly recognised that the nitrogenous matters 

 obtainable from plants and animals varied considerably in their 

 physical properties, such as in solubility, appearance, etc., it was 

 nevertheless held by Liebig that only one protein existed. The 

 differences in the nitrogenous substances obtained from various sources 

 were ascribed entirely to variations in physical conditions. The slight 

 differences in empirical composition in the preparations obtained 

 accounted, no doubt, for this view of Liebig. Mulder, to whom many 

 valuable observations are due, expressed later a similar view in a 

 somewhat modified form. Referring to " protein " he remarks : "It 

 exists in various forms, being either soluble or insoluble in water. It 

 forms different compounds with sulphur or with phosphorus or both 

 and hence the differences it presents in appearance and physical 

 properties. The substance has received the name of ' protein ' 

 because it is the origin of so many dissimilar bodies, and is itself a 

 primary substance " (Mulder, Vegetable and Physiological Chemistry \ 

 p. 291. English translation, 1845-49). 



Subsequent investigations have shown that these early views of 

 Liebig and Mulder are incorrect, more especially the careful and 

 reliable experiments of Schiitzenberger, Hlasiwetz and Habermann, 

 Horbaczewski, Schulze and Barbieri and others, most of which were 

 carried out thirty years or more after the publication of the above- 

 quoted words of Mulder. It is unnecessary to enter into detail in this 

 place as to the results of the experiments on the hydrolysis of the 

 proteins, due to these and other workers, culminating subsequently in 

 the elaboration by E. Fischer and his pupils of an approximately 

 quantitative method for the isolation of the hydrolysis products ; these 

 are discussed in the volume devoted to this subject. Suffice it here to 

 say that the individual proteins have been found to differ from one 

 another both qualitatively and quantitatively as regards the amino- 

 acids which they yield on hydrolysis. 



Modern work tends to confirm the theory due to Hofmeister that 



l i.e., proteins, 



