96 HOW CROPS GROW. 
potash and soda-lye; acids separate them from these solutions, strong 
acetic acid dissolves them with one exception. In very dilute mineral 
acids (sulphuric and chlorhydric) some of them dissolve in great part, 
others swell up like jelly. 
Coagulation.—A remarkable characteristic of the group 
of bodies now under notice is their ready conversion from 
the soluble to the insoluble state. In some cases this 
coagulation happens spontaneously, in others by elevation 
of temperature, or by contact with acids, metallic oxides, 
or various salts, 
The albuminoids, when subjected to heat, melt and burn 
with a smoky flame and a peculiar odor—that of burnt 
hair or horn,—while a shining charcoal remains which is 
difficult to consume. 
Tests for the Albuminoids.—The chemist employs the 
behavior of the albuminoids towards « number of reagents * as tests 
for their presence. Some of these are so delicate and characteristic as 
to allow the distinction of this class of substances from all others, even 
in microscopic observations. 
1. Jodine colors them intensely yellow or bronze. 
2, Warm and strong chlorhydric acid colors all these bodics blue or 
violet, or, if applied in large excess, dissolves them to a liquid of these 
colors. 
8. In contact with nitric acid they are stained a deep and vivid yellow. 
Silk and wool, which consist of bodies closely approaching the albumin- 
oids in composition, are commonly dyed or printed yellow by means of 
nitric acid. 
4, A solution of nitrate of mercury in excess of nitric acid, + tinges 
them of a deep red color, This test enables us to detect albumin, for 
,cxample, even where it is dissolved in 100,000 parts of water. 
Albumin.— Animal Albumin.—The white of a hen’s 
egg on drying yields about 12 per cent of albumin in a 
state of tolerable purity. The fresh white of egg serves 
* Reagents are substances commonly employed for the recognition of 
bodies, or, generally, to produce chemical changes. All chemical phenomena re- 
sult from the mutual action of at least two elements, which thus act and react on 
each other. Hence the substance that excites chemical changes is termed a re- 
agent, and the phenomena or results of its application are called reactions. 
+ This solution, known as Millon’s test, is prepared by dissolving mercury 
in its own weight of nitric acid of sp. gr. 1.4, heating towards the close of the 
process, and finally adding to the liquid twice its bulk of water. 
