44 PROTEIDS. 



Preparation of peptones. For this the works of Maly 1 , Herth 2 , 

 Hemiinger 3 , Kossel 4 , Hofmeister 5 and Low 6 should be consulted. The 

 general properties and reactions of the peptones obtained by the above 

 authors may be stated as follows. As precipitated by alcohol they 

 consist of a white or yellowish powder, which is hygroscopic and 

 extraordinarily soluble in water, and in some cases may even be 

 deliquescent. Unless thoroughly dehydrated the powder may melt 

 on gentle warming. From their neutral aqueous solutions they are 

 precipitated with difficulty by a large excess of alcohol, being unchanged 

 in the process and not becoming coagulated or insoluble by prolonged 

 exposure to the action of the precipitant. The precipitation occurs 

 with difficulty if at all in presence of hydrochloric acid. Peptones 

 are not precipitated by many of the reagents which precipitate 

 other proteids, but are precipitated by tannic acid, mercuric chloride, 

 nitrates of mercury, and by phosphotungstic and phosphomolybdic 

 acids in presence of hydrochloric or other mineral acids ; also by 

 the double iodides of potassium and mercury or potassium and bismuth, 

 in presence of strong mineral acids. A very characteristic reaction is 

 the 'biuret' or pink coloration which is obtained on the addition of 

 an excess of caustic soda and a mere trace of sulphate of copper. The 

 slightest excess of the copper salt gives a violet colour, as is the case 

 with all other proteids, which deepens in tint on boiling. This biuret 

 reaction is however now known to be yielded also by the albumoses 

 (see above). Peptones are all laevorotatory and diffusible. 



The diffusibility of peptones is relatively great in comparison with that of other 

 forms of proteids : it is however absolutely small when compared with that of 

 crystalline substances such as sodium chloride, and hence they may be separated 

 from admixed salts by dialysis. All statements as to their absolute diffusibility, as 

 based on earlier statements, must however be received with caution, in view of the 

 transitional state of our information as to the properties of the true peptones. 



Of late years it has been observed that the complete separation of 

 peptones from albumoses is possible by taking advantage of the fact 

 that the latter are all completely precipitable by saturation with 

 neutral ammonium sulphate, whereas the former are not 7 . By means 

 of this difference in the behaviour of the two classes of substances to 

 the ammonium salt, Kiihne and Chittenden have prepared what they 



1 Pfliiger's Arch. Bd. ix. (1874), S. 585. 



2 Zt.f.physiol. Chem. Bel. i. (1877), S. 273. 



3 "De la nature et du role physiologique des Peptones." Paris, 1878. 



4 Zt.f. physiol. Chem. Bd. in. (1879), S. 58. 



5 Ibid. Bd. v. (1881), S. 129. 



6 Pfliiger's Arch. Bd. xxxi. (1883), S. 408. 



7 Wenz, Zt.f. JBiol. Bd. xxn. (1886), S. 10. Kiihne, Verhandl. d. naturhist.-med. 

 Ver. Heidelberg, Bd. in. (1885), S. 286. 



