luences.

The physiological variations which the specific gravity undergoes under
the influence of the taking in and excretion of fluid do not exceed
0.003 (Schmaltz). From what has been said, it follows that all
variations must correspond with similarly occurring variations in the
factors that underlie the amount of haemoglobin and the number of
corpuscles.

More recent authors, in particular Hammerschlag, v. Jaksch, v. Limbeck,
Biernacki, Dunin, E. Grawitz, A. Loewy, have avoided an omission of many
earlier investigators; for besides the estimation of the specific
gravity of the total blood, they have carried out that of one at least
of its constituents, either of the corpuscles or of the serum. The red
blood corpuscles have consistently shewn themselves as almost
exclusively concerned with variations in the specific gravity of the
total blood; partly by variations in number, or changes in their
distribution; partly by their chemical instability; loss of water and
absorption of water, and variations in the amount of iron.

The plasma of the blood on the contrary--and there is no essential
difference between plasma and serum (Hammerschlag)--is much more
constant. Even in severe pathological conditions, in which the total
blood has become much lighter, the serum preserves its physiological
constitution, or undergoes but relatively slight variations in
consistence. Considerable diminutions in the specific gravity of the
serum are much less frequently observed in primary blood diseases, than
in chronic kidney diseases, and disturbances of the circulation. E.
Grawitz has lately recorded that in certain anaemias, especially
posthaemorrhagic and those following inanition, the specific gravity of
the serum undergoes perceptible diminutions[3].

There are still therefore many contradictions in these results, and it
is evidently necessary in a scientific investigation always to give the
specific gravity of the serum and of the corpuscles, in addition to
that of the total blood.

A method closely related to the estimation of the specific gravity is
the direct estimation of the dried substance of the total blood,
"HYGRAEMOMETRY"; the clinical introduction of which we owe to Stintzing
and Gumprecht. This method is really supplementary to those so far
mentioned, and like them can be carried out with the small amounts of
blood obtainable at the bedside without difficulty. Small quantities of
blood are received in weighed glass