of the nitrogen in any
given protein into these various groups is characteristic for that
particular protein, and the process serves both as a means of
identification of individual proteins and a method for tracing their
changes through various vital, or biochemical, transformations.

GENERAL PROPERTIES OF THE PROTEINS

Individual proteins differ slightly in their characteristics, but in
general they are all alike in the following physical and chemical
properties.[5]

=Physical Properties.=--(1) The proteins are all
_colloidal_ in character, that is, they form solutions in water, out of
which they cannot be dialyzed through parchment, or other similar
membranes. (2) All natural proteins, when in colloidal solution, may be
_coagulated_, forming a semi-solid _gel_, which cannot again be rendered
soluble except by decomposition. The most familiar example of this type of
coagulation is that of egg-albumin, when eggs are cooked. This coagulation
may be produced by heat, by the action of certain enzymes, or by the
addition of alcohol to the solution. (3) All solutions of plant proteins
are optically active, rotating the plane of polarized light to the left, in
every case. (4) Proteins are precipitated out of their solutions, without
change in the composition of the protein, by saturating the solution with
various neutral salts of the alkali, or alkaline earth, metals, such as
sodium chloride, ammonium sulfate, magnesium sulfate, etc. This is only
another way of saying that the proteins are insoluble in strong salt
solutions. Separation from solution by the addition of salts is different
from coagulation by heat, etc., as in this case simple dilution of the salt
solution will cause the protein to redissolve, whereas a coagulated protein
cannot be redissolved without some change in its composition.

=Chemical Properties.= (1) Precipitation reactions.--The proteins have both
acid and basic properties (due to the presence in their molecules of both
free NH_{2} groups and free COOH groups). Bodies of this kind are known as
"amphoteric electrolytes," since they yield both positive and negative
ions, if dissociated. The proteins readily form salts, which are generally
insoluble in water, with strong acids. For this reason, they are generally
precipitated out of solution by the addition of the common mineral acids.
They are also precipitated by many of the "alkaloidal reagents," to which
reference has been ma