104 HOW CROPS GROW. 
a curdy coagulum is formed, which agrees in its properties 
with casein. (Bopp, Ann. Ch. Ph., 60, p. 30; Gunning, 
Jour. fiir Prakt. Chem., 69, p. 52.) 
Lehmann has shown that when albumin is dissolved in 
potash, and mixed with a little milk-sugar and oily fat, the 
mixture coagulates with rennet exactly as milk gurdles, 
(Gorup-Besanez, Phys. Chem., p. 139.) 
Sullivan has observed that the casein of milk which was 
kept in closed air-tight vessels for a long time, at first co- 
agulated, but afterward dissolved again to a nearly clear 
liquid, which was found to contain no casein, but by heat- 
ing, coagulated, showing the conversion of casein into 
albumin, or a similar body. (PA. Mag., 4, XVIII, 203.) 
Some maintain that casein is not a distinct albuminoid, 
but a compound of albumin with potash, containing, ac: 
cording to Lieberkthn, 5.5°|, of this alkali. Its peculiarities 
are in part due to its natural association with phosphate 
of potash. Kiihne, Phys. Chem., 1868, p. 565. See, how- 
ever, Schwarzenbach, Ann. Ch. u. Ph., 144, p. 63. 
The Albuminoids in Animal Nutrition—We step 
aside for a moment from our proper plan to direct atten- 
tion to the beautiful adaptation of this group of organic 
substances to the nutrition of animals. Those bodies which 
we have just noticed as the animal albuminoids, together 
with others of similar composition, constitute a large share 
of the healthy animal organism, and especially characterize 
its actual working machinery, being essential ingredients 
of the muscles and cartilages, as well as of the nerves and 
brain. They likewise exist largely in the nutritive fluids 
of the animal—in blood and milk. So far as we know, the 
animal body has not the power to produce a particle of 
albumin, or fibrin, or casein; it can only transform these 
bodies as presented to it from external sources. They are 
hence indispensable ingredients of food, and have been 
aptly designated by Liebig as the plastic elements of nu- 
trition. It is, in all cases, the plant which originally con- 
