This section is from the book "Dental Medicine. A Manual Of Dental Materia Medica And Therapeutics", by Ferdinand J. S. Gorgas. Also available from Amazon: Dental Medicine.
In medical prescriptions, letters, parts of words, or certain symbols, are employed as abbreviations, to designate the substance, quantity, etc., as follows:
Anodal closure contraction,
C. or Cong.,
A small paper.
Let them be strained.
Bruised or broken.
F. or Ft.,
Fiat vel fiant,
Let them be made.
Folium vel folia,
A leaf or leaves.
Granum vel grana,
A grain or grains.
Gutta vel guttae,
A drop or drops.
Pilula vel pilulae,
A pill or pills.
Pulvis vel pulveres,
A powder or powders.
A sufficient quantity.
Write or give directions.
A fluid ounce.
A fluid drachm.
Although the symbol 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 bottle 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 administer 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 generally 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 tablespoonful of any liquid is regarded as equal to half an ounce by measure; and a teaspoonful equal to a fluidrachm; and such measures are sufficiently 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
A wine glass (approximate measurement) contains two fluid ounces.
A teacup (approximate measurement) contains four fluid ounces.
A tablespoon of powder (approximate measurement) contains two drachms.
A teaspoon of powder (approximate measurement) contains one-half drachm.
One drop of water (small drop, approximate measurement) contains one minim.
One drop of essential oils (approximate measurement) contains one-half minim.
A graduated measure-glass is the most accurate measure, as spoons, glasses, etc., vary greatly in size.