42 CHEMICAL CONSTITUENTS OF BODY AND FOOD. 



aqueous solutions, by the addition of certain neutral salts in large 

 quantities ; in some cases complete saturation is necessary. In some in- 

 stances, as in the precipitation of urates by ammonium chloride, 1 or 

 ammonium sulphate, 2 the formation of an insoluble compound with the 

 base of the salt used will explain the phenomenon. In other cases, 

 especially in the case of colloidal substances, the water-attracting power 

 of the salt is more probably the explanation. 3 The solutions used 

 should not be too concentrated, or the thick precipitate obtained is 

 difficult of filtration. 



The phenomenon is not confined to substances of a colloidal nature ; 

 thus, picric acid is precipitable by this means ; so are soaps, especially 

 potassium soaps by sodium chloride. But it is in connection with non- 

 diffusible substances, 4 and especially with proteids, that the method is 

 most used. 



Proteids differ from one another a good deal in the readiness by 

 which they are precipitated in this way. Ammonium sulphate added to 

 saturation, precipitates all proteids except peptones 5 and certain forms 

 of deuteroalbumose. 6 Half saturation with the same salt is sufficient to 

 precipitate globulins, 7 acid and alkali albumin and caseinogen. Speaking 

 generally, the globulins and nucleo-proteids are more readily precipitable 

 by neutral salts than the albumins. Thus, globulins are precipitated by 

 magnesium sulphate and sodium chloride, whereas albumins are not, and 

 some globulins, like fibrinogen, are precipitated by half-saturation with 

 sodium chloride. If the operations are carried out at the temperature 

 of the air, the precipitated proteids are not coagulated, but are 

 soluble in suitable liquids ; and they then again show their characteristic 

 properties. 8 



Heat coagulation. The albumins, globulins, and some nucleo-proteids 

 are coagulated at different temperatures, by heating their solutions. 

 The temperature varies with the reaction of the solution, 9 the quantity 

 and nature of the salts present 10 (minute quantities of calcium salts 

 favour heat coagulation as they do ferment coagulation), 11 and 

 under certain circumstances, especially in an alkaline solution, with its 

 concentration. 12 



1 F. G. Hopkins, Journ. Path, and Bacterial., Edinburgh and London, 1893, vol. i. p. 

 451. 



2 A. Edmunds, Journ. Physiol., Cambridge and London, 1894-5, vol. xvii. p. 451. 



3 0. Nasse, Arch. f. d. ges. Physiol., Bonn, Bd. xli. S. 504; F. Hofmeister and S. 

 Lewith, Arch. f. exper. Path. u. PharmaJcoL, Leipzig, 1888, Bd. xx. S. 247; xxv. 

 S. 1. 



4 On the precipitation of colloid carbohydrates by salts, see Pohl, Ztschr. f. physiol. 

 C/iem., Strassburg, Bd. xiv. S. 151 ; R. A. Young, "Proc. Physiol. Soc.," 1896-97, p. xvi. in 

 Journ. Physiol., Cambridge and London, 1897, vol. xxi. 



5 Wenz, Ztschr.f. BioL, Miinchen, Bd. xxii. S. 1. 



6 Kiihne, ibid., Bd. xxiv. S. 1 and 308 ; Chittenden, Journ. Physiol., Cambridge and 

 London, vol. xvii. p. 48. 



7 Kauder, Arch. f. cxper. Path. u. PharmaJcoL, Leipzig, Bd. xx. S. 411. 



8 On the precipitation of proteids by numerous salts, see Denis, " Memoire sur le 

 sang," p. 39 ; Schafer, Journ. Physiol., Cambridge and London, vol. iii. p. 181 ; 

 Halliburton, ibid., vol. v. p. 177; vii. p. 321; Hammarsten, Arch. f. d. ges. Physiol., 

 Bonn, 1878, Bd. xvii. S. 424. 



9 Traces of acid lower, of alkali raise, the temperature of coagulation ; more than 

 traces convert the proteid into acid or alkali-albumin respectively, and these substances do 

 not coagulate by heat. Halliburton, Journ. Physiol., Cambridge and London, vol. 

 v. p. 165. 



10 Limbourg, Ztscbr. f. physiol. Chem., Strassburg, Bd. xiii. S. 450. 



11 Ringer and Sainsbury, Journ. Physiol., Cambridge and London, 1891, vol. xii. p. 170. 



12 Haycraft, Brit. Med. Journ., London, 1890, vol. i. p. 167. 



