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678 UKINE.
acidity and the ion acidity it follows that no direct relation exists between
these and that the extent of these two acidities may be independent of
each other.
The osmotic pressure of the urine varies considerably even under
physiological conditions. The limit for the freezing-point depression
has been found by a number of investigators to be A 1.3° to 2.3° C. After
partaking of considerable water it may be markedly lower, and on
diminished supply of water it may be considerably higher.
In regard to the further physical-chemical imvestigations of the urine
and as to the conclusions drawn from a combination of the chemical
and the physico-chemical investigations of the urine, we must refer to
the extensive work of Carl Neuberg.1
The specific gravity of urine, which is dependent upon the relation
existing between the quantity of water secreted and the solid urinary
constituents, especially the urea and sodium chloride, may vary con-
siderably, but is generally 1.017-1.020. After drinking large quantities
of water it may fall to 1.002, while after profuse perspiration or after
drinking very little water it may rise to 1.035-1.040. In new-born
infants the specific gravity is low, 1.007-1.005. The determination
of the specific gravity is an important means of learning the average
amount of solids eliminated from the organism in the urine, and on this
account the determination becomes of true value only when at the same
time the quantity of urine voided in a given time is determined. The
different portions of urine voided in the course of the twenty-four hours
are collected, mixed together, the total quantity measured, and then the
specific gravity taken.
The determination of the specific gravity is most accurately obtained
with the pycnometer. For ordinary cases the specific gravity may be
determined with sufficient accuracy by means of areometers. The
areometers found in the trade, or urinometers, are graduated from 1.000
to 1.040; for exact observations it is better to use two urinometers, one
graduated from 1.000 to 1.020, and the other from 1.020 to 1.040.
To determine the specific gravity of urine, if necessary filter the
urine, or if it contains a urate sediment, first dissolve it by gentle heat,
then pour the clear urine into a dry cylinder, avoiding the formation of
froth. Air bubbles or froth, when present, must be removed with a glass
rod or filter-paper. The cylinder, which should be about four-fifths full,
must be wide enough to allow the urinometer to swim freely in the liquid
without touching the sides. The cylinder and urinometer should both
be dry or previously washed with the urine. On reading, the eye is
brought on a level with the lower meniscus—which occurs when the sur-
face of the liquid and the lower limb of the meniscus coincide; the read-
1
Der Ham sowie die iibrigen Ausscheidungen und Korperfliissigkeiten von Mensch
und Tier. Teil. 2, Berlin, 1911.
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