de in the preceding chapter, namely, phosphotungstic,
phosphomolybdic, tannic, picric, ferrocyanic, and trichloracetic acids, the
double iodide of potassium, mercuric iodide, etc. The precipitates produced
by strong mineral acids are often soluble in excess of the acid, with the
formation of certain so-called "derived proteins," which are probably
products of the partial hydrolysis of the protein.

The proteins are also precipitated out of solution by the addition of small
amounts of salts of various heavy metals, such as the chlorides, sulfates,
and acetates of iron, copper, mercury, lead, etc. This precipitation is
different than that caused by the saturation of the solution with the salts
of the alkali metals, as in this case the metal unites with the protein to
form definite, insoluble salts, which cannot be redissolved except by
treatment with some reagent which removes the metal from its combination
with the protein (hydrogen sulfide is commonly used for this purpose).

(2) Color reactions.--Certain specific groups which are present in most
proteins give definite color reactions with various reagents. It is
apparent that any individual protein will respond to a particular color
reaction, or will not do so, depending upon whether the particular group
which is responsible for the color in question is present in that
particular protein. Color reactions to which most of the common plant
proteins respond are the following ones:

(_a_) _Biuret Reaction._--Solutions of copper sulfate, added to an alkaline
solution of a protein, give a bluish-violet color if the substance contains
two, or more, --CONH-- groups united together through carbon, nitrogen, or
sulfur atoms. Inasmuch as most natural proteins contain several such
groups, the biuret reaction is a very general test for proteins.

(_b_) _Millon's Reaction._--A solution of mercuric nitrate containing some
free nitrous acid (Millon's reagent) produces a precipitate which turns
pink or red, whenever it is added to a solution which contains tyrosine, or
a tyrosine-containing protein.

(_c_) _Xanthoproteic Acid Reaction._--This is the familiar yellow
coloration which is produced whenever nitric acid comes in contact with
animal flesh. It is caused by the action of nitric acid on tyrosine. The
color is intensified by heating, and is changed to orange-red by the
addition of ammonia.

(_d_) _Adamkiewicz's Reaction._--If concentrated sulfuric acid be added to
a solution