PART I. 

 THE PHYSICAL PROPERTIES OF THE PROTEINS. 



SECTION I. THE SOLUBILITY OF PROTEINS IN SALT SOLUTIONS ; 

 THE " SALTING OUT " FROM SOLUTIONS. 



THE method which has been most commonly employed hitherto, for 

 the separation of proteins from one another and from other bodies 

 is that commonly known as "salting out". It was noticed as long 

 ago as 1853 by Panum that dry sodium chloride does not precipitate 

 egg-white in the cold, whereas blood-serum gives a distinct precipi- 

 tate which redissolves in distilled water. Claude Bernard had also 

 observed that pancreatic juice yielded a precipitate with magnesium 

 sulphate; Robin and Verdeil (1853) made a similar observation 

 with ascitic fluid. The latter investigators noticed also that mag- 

 nesium sulphate gave a precipitate with egg-white and with serum, 

 the filtrate from which coagulated on heating, whereas the filtrate 

 from a similar precipitate with pancreatic juice did not. 



Virchow, in the following year, carried out further investigations 

 on this subject, and found that, besides magnesium sulphate, other 

 salts, such as potassium sulphate, sodium sulphate, alum, calcium 

 chloride, sodium chloride, possess the property of precipitating 

 proteins ; he also made the fundamental observation that the 

 precipitates formed by salts redissolved in water, and that the 

 solutions thus obtained behaved as true protein solutions. He 

 confirmed also the French observers, in noticing that the salt does 

 not always precipitate the whole of the protein, and he assumed that 

 the latter must exist in solution in different conditions. At this 

 time the existence of several kinds of protein was unknown, and 

 Virchow did not recognise the fact that many such bodies could 

 exist, and that their different behaviour towards salt solutions was a 

 factor which could be employed for a partial separation, at any rate, 

 of one protein from another. 



D6nis l first clearly discriminated between proteins in this respect. 

 He noticed that some are readily soluble in salt solutions, which are 

 insoluble in water. He found, for example, that certain proteins are 

 soluble in 10 per cent, sodium chloride, potassium nitrate, and 

 sodium sulphate solutions at 30 C, from which they could be 

 reprecipitated on dilution with water ; he studied also the precipita- 

 tion of proteins from blood-serum by saturation with neutral salts, 

 employing for this purpose sulphate of sodium, chloride of sodium, 

 and sulphate of magnesium. Denis must be regarded, therefore, as 



1 De"nis had commenced his investigations on the proteins in 1835. His mono- 

 graphs on the subject were published in 1856 and 1859. 



