of a protein to which some acetic acid (or better, glyoxylic
acid) has previously been added, a violet color is produced. This color
will appear as a ring at the juncture of the two liquids, if the sulfuric
acid is poured carefully down the sides of the tube, or throughout the
mixture if it is shaken up. It depends upon the interaction of the
glyoxylic acid (which is generally present as an impurity in acetic acid)
upon the tryptophane group, and is therefore given by all proteins which
contain tryptophane.

(_e_) _Molisch's reaction_ for furfural will be shown by those proteins
which contain a carbohydrate group. In applying this test, the solution to
be tested is first treated with a few drops of an alcoholic solution of
[alpha]-naphthol, and then concentrated sulfuric acid is poured carefully
down the sides of the test-tube. If carbohydrates are present, either free
or as a part of a protein molecule, a red-violet ring forms at the juncture
of the two liquids.

(_f_) _Sulfur Test._--If a drop of a solution of lead acetate be added to a
solution containing a protein, followed by sufficient sodium hydroxide
solution to dissolve the precipitate which forms, and the mixture is heated
to boiling, a black or brown coloration will be produced if the protein
contains cystine, the sulfur-containing amino-acid.

FOOTNOTES:

[5] Since the proteins are essentially _colloidal_ in nature, many of the
terms used in the discussions of their properties, and these properties
themselves, will be better understood after the chapter dealing with the
colloidal condition of matter has been studied. A more logical arrangement
so far as the systematic study of these properties is concerned would be to
take up chapter XV before undertaking the study of the proteins (this order
is actually followed in some texts on Physiological Chemistry). But from
the standpoint of the consideration of the various groups of organic
components of plants, it seems a better arrangement to consider these
groups in sequence, and then to discuss the various physical-chemical
phenomena which govern their activity. However, it is recommended that the
student refer at once to Chapter XV for an explanation of any terms used
here, which may not be familiar to him; and that after the study of Chapter
XV, he return to this chapter dealing with the proteins for an illustrative
study of the applications of the principles presented there.

THE CLA