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24 DENTAL MEDICINE,

Although the symbol (W) is adopted in the United States
Pharmacopoeia to designate a drop, it should be remembered that
the size of a drop varies according to the greater or less fluidity
and gravity of the liquid, and the shape of the mouth of the bot-
tle from which it is dropped. It is best to use a glass medicine-
dropper, or a small vial with a thin edge of mouth, when great
precision is necessary, and to dilute the active medicine and ad-
minister it in the form of a mixture ; for in some preparations,
one hundred and fifty drops would measure but a fluidrachm,
while in others the same number of drops would be somewhat
more than three fluidrachms.
But there is a wide difference between a drop and a minim,
and although a drop is usually considered as one-sixtieth part
of a fluidrachm, this is only true of water and some other liquids,
and drops of the same liquid vary under different circumstances.
Specific gravity, viscidity or adhesiveness, and mobility modify
the size of drops. The heavier the liquid the smaller will be the
drops, and the greater number in a given measure.
The drops from a thick-lipped bottle are larger than those
from one having thin lips ; and drops from a full bottle are gen-
erally smaller than those from a bottle partially filled, on account
of rapidity of movement. The adhesiveness of a liquid to the
sides of a bottle increases the size of the drops, as a greater mass
is necessary to overcome the viscidity. Drops are also increased
in size by the greater mobility of a liquid.
A tablespoon/hi of any liquid is regarded as equal to half an
ounce by measure ; and a teaspoonful equal to 2i fluidrachm ; and
such measures are sufiiciently accurate where no great precision
'
is requisite.
A gallon contains eight pints.
A pint contains sixteen fluid ounces.
A fluid ounce contains eight fluidrachms.
A fluidrachm contains sixty minims (1U).
A wine glass (approximate measurement) contains two fluid
ounces.
A teacup (approximate measurement) contains four fluid
ounces.