The British Pharmaceutical Codex Formulary: Special Notice
The quantities of ingredients in the following formulae are represented in both centesimal proportions and Imperial (avoirdupois) weights and measures, and it is important to note that, in certain instances, there may not be an exact correspondence between the two sets of figures, owing to the necessity of avoiding inconvenient fractional parts of grains and minims; any differences, however, in the products obtained as the result of employing the different systems will be so slight as to be practically negligible. Similarly, in the directions, where two sets of quantities appear in juxtaposition, it must not be assumed that the quantity in Imperial weights or measures is other than relative to the centesimal proportion which precedes it. Thus, in the case of Acetum Cevadillae, the directions are to macerate with "70 (14 fluid ounces) of distilled water"; the number "70" indicates 70 parts by volume in a total of 100 parts, whereas "14 fluid ounces" is the quantity to be used in order to obtain 20 fluid ounces of the product. The point to observe is that the two systems are not interchangeable; one or the other must be followed in its entirety throughout. Liquids should, of course, be measured in all cases unless the contrary is indicated. All the substances and preparations named in the formulae are of B.P., U.S.P., or B.P.C. standard, unless the contrary is stated.