i26 THE CHEMISTRY OF THE TISSUES AND ORGANS. 
enable the ether to attack the fat more easily. Moreover, Hoppe-Seyler 
states thai it is not so difficult to remove the fat simply with ether; 
the fluid still remains cloudy, it is true, but solutions of caseinogen 
are always opalescent, and this is increased by the presence in the 
milk of particles of proteid or proteid-like substances, as described by 
Kehrer. 
The reaction of milk. — Milk readilyturns sour from the fermentation 
of lactose and formation of lactic acid. In carnivora fresh milk has an 
acid reaction, but in most animals it gives either an alkaline or, more 
frequently, an amphoteric reaction; the acid phosphates in the milk 
turn neutral litmus red, and the alkaline phosphates turn it blue. The 
proportion between these salts varies very considerably in different 
animals, in the same animal at different stages of lactation, and even 
between the first and last portions of the same milking (Thorner, 1 
Sebelien, 2 Courant :; ). 
Courant estimated the alkaline constituent by titration with 
decinormal sulphuric acid, with blue lackmoid as indicator, and the acid 
constituent with decinormal soda with phenolphthalein as indicator. 
He found as a mean for the first and last port inns of the milking of 
twenty cows, that the alkalinity of 100 c.c. of the milk was equal to 
41 c.c, and the acidity equal to 19 - 5 c.c. of the respective solutions 
used. In human milk the proportional alkalinity is higher: the average 
of the numbers was ID'S and '■'<■(> c.c. respectively. 
Constituents of mill,-. — These are water, three proteids (caseinogen, 
lactalhumin, lacto-globulin), two carbohydrates (lactose, animal gum?), 
fats, extractives (traces of urea, creatine, creatinine, hypoxanthine, 
lecithin, cholesterin, citric acid 4 ), salts and gases. Most of these de- 
mand separate discussion. 
Effect of boiling milk. — When milk is heated to, or near to, the 
boiling point, a scum forms on the surface; on the removal of this skin 
it is rapidly renewed, and this can lie repeated over and over again. 
This is probably in part produced by the coagulation of the lact- 
alhumin : this carries to the surface some caseinogen and fat. 5 Contact 
with air appears to he the chief influence in causing the solidification 
which results in the formation of the scum : evaporation is rapid from 
the surface exposed to the atmosphere, and thus partial drying occurs 
there. 
The boiling of milk before it is used as a food is advantageous in 
two ways — ( 1 ) all micro-organisms are destroyed : (2) the gastric juice, in 
virtue of its rennet, causes a flocculent and not a bulky precipitate. 
These quite outweigh any slight diminution of digestibility alleged to 
occur. 6 The reason that boiled milk curdles with rennet with greater 
difficulty than fresh milk appears to be that, by boiling, a part of the 
dissolved calcium salt is precipitated as tricalcium phosphate. 
As milk turns sour, it is possible to get a bulky heat coagulum by 
boiling. 7 
1 Chen. Ztg., Calien, Bd. xvi. S. 1469. - Ibid., S. 597. 
s Inaug. Diss., Bonn, 1S91 ; ami Arch. f. d. ges. Physiol., Bonn, Bd. 1. 
4 Soldner, Landio. Versuchs. Stat., Berlin, Bd. xxxv. 
5 See D. F. Harris, Journ. Ann/, and Physiol., London, 1S94, vol. xxix. p. 188. 
6 Raudnitz, Ztschr.f. physiol. Chan., Strassburg, Bd. xiv. S. 1. 
7 Recent work on this question will be found in a paper by Cazeneuve and Haddon, 
Comjyt. rend. Acad. d. sc, Paris, 1895, tome cxx. p. 1272. See also influence of boiling mi 
the proteids of cows' milk, Ccntralbl. f. d. med. Wissensch., Berlin, Bd. xxxiv. .S. 145. 
