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 casemogen. 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 ; ami 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. x - 
I F. G. Hopkins, Journ. Path, and Bactcriol., Edinburgh and London. 1893, vol. i. p. 
451. 
- A. Edmunds, Journ. Physiol., Cambridge and London, 1894-5, vol. xvii. p. 451. 
3 0. Nasse, Arch. f. d. ges. Physiol., Bonn, Bd. xli. 8. 504; F. Hofmeister and S. 
Lewith, Arch. f. exper. Petit, u. Pharmakol., Leipzig, 188S, Bd. xx. S. 247 ; xxv. 
S. 1. 
4 On the precipitation of colloid carbohydrates by salts, see Pohl, Ztschr. f. physiol. 
Chew,., 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. 
,; Kuhne, ibid., Bd. xxiv. S. 1 and 30S ; Chittenden, Journ. Physiol., Cambridge and 
London, vol. xvii. p. 48. 
7 Kauder, Arch./, exper. Path. u. Pharmakol., Leipzig, Bd. xx. S. 411. 
8 On the precipitation of proteids by numerous salts, see Denis, "M&noire sur le 
sang." p. 89 ; Schafer, Journ. Physiol., Cambridge and London, vol. iii. p. 181 ; 
Halliburton, ibid., vol. v. p. 177; vii. p. 321 ; Hammarsten, Arch. f. d. ges. Physio/., 
Bonn, 1878, Bd. xvii. S. 421. 
9 Tracts of acid lower, of alkali raise, the temperature of coagulation : more than 
traces convert the proteid into acid or alkali-albumin respective^, and these substances do 
not coagulate by heat. — Halliburton, Journ. Physiol., Cambridge and London, vol. 
v. p. 165. 
lu Limbourg, Ztschr. f. physiol. Chem., Strassburg, Bd. xiii. S. 450. 
II Ringer and Sainsbury, Jov/rn. Physiol., Cambridge and London, 1891, vol. xii. p. 170. 
J - Haycraft, Brit. Med. Journ., Loudon, 1S90, vol. i. p. 167. 
