How to Write Metric Prescriptions.1

In writing prescriptions it is sufficiently accurate and safe to consider 1 gramme as exactly equal to 15 troy grains, and to consider 1 cubic-centimetre as equal to 15 minims.

1. TO CONVERT TROY GRAINS INTO GRAMMES, OR MINIMS INTO CUBIC CENTIMETRES: a. Divide by 10, and from the quotient subtract one-third; or, b. Divide by 15.

2. TO CONVERT APOTHECARIES' DRACHMS INTO GRAMMES, OR FLUIDRACHMS INTO CUBIC-CENTIMETRES, multiply by 4. In writing prescriptions the "gramme" (abbreviated "Gm.") and "cubic centimetre," (abbreviated "C. C.," which may be called "fluigramme," and written "f Gm") only, should be used.

The centigramme, a convenient unit in medicine and pharmacy, is used in books, but not in prescriptions.

All other terms, and units, and prefixes, used in the metric system, may be wholly ignored by the physician and pharmacist.

The use of a decimal line prevents possible errors.

To write a prescription for fifteen doses of any medicine, write it first for one dose in grains and minims, and then substitute the same number of "grammes" and "cubiccentimetres," thus:

From a paper on Metric or French Decimal System of Weights and Measures. By O. Oldberg, Phar. D., Med. Purveyor, U S. Marine Service.

1 Gm.

2 Gm.

and to get fifteen such doses in metric terms, write:-

The gramme and the cubic-centimetre (fluigramme), when referring to liquids, may be considered as equal quantities, unless the liquids be very heavy (as in the case of chloroform) or very light (as in the case of ether).

Measures may be discarded and weights exclusively employed, if preferred. All quantities in a prescription would then be expressed in grammes.1

The average "DROP" (water) may be considered equal to 0.05 C. C., or 0.05 Gm. An average TEASPOON holds 5 C. C., and an average TABLESPOON 20 C. C. Decimal numbers should be used as far as practicable without sacrifice of accuracy as to strength and dose of the preparation. It is safe to prescribe 30 Gm. for one troy ounce, and 250 C. C. for eight fluidounces.

The above contains ALL THAT IT IS NECESSARY TO KNOW OR LEARN of the metric system IN ORDER TO WRITE METRIC PRESCRIPTIONS with or without a metric posological table.

To become familiar with the system, the rules previously given for the conversion of Apothecaries' into metric weights and measures, may be profitably used.

1 As any liquid medicine must necessarily be administered to the patient in measured, and not in weighed doses, it will of course be more convenient to the physician to continue to make use of fluid measures in writing prescriptions, especially as he is already accustomed to this, and would not then have to bear in mind the specific gravity of any liquid ingredient in the prescription. To the pharmacist it makes but little difference, as he will have both weights and measures, and can use one or the other, as may be directed. If the physician discards measures, he must, of necessity, adjust the proportion in his formula to produce a mixture of which after all the dose must be a 66 teaspoonful," or other convenient measure, and this is as unnecessary as it is difficult.0.0.

Number of Drops in a Fluidrachm.

The importance of becoming familiar with the relation of the drop to the fluidrachm is at once evident by an inspection of the following table. The wide range between the smallest number of drops (44) in one of the fluids cited, and the largest number (276), suggests to the prac titioner the exercise of very great caution in the administration of medicines calculated guttatim. In writing prescriptions based on an estimate of a certain number of drops to each dose, serious error might be committed in the employment of a large number of the potent articles here enumerated, by a blind adherence to the view that any average number of drops is equivalent to a fluidrachm. The only true course for the practitioner is to make a special study of these agents, as offered in some such table as that here furnished, and to endeavor to fix in his mind the relative tenuity of the liquid articles he may desire to prescribe.

1 According to measurements by Durand, Procter, Parrish, Farquharson, and others. Some of these measurements are from ordinary bottles, others from Oj tincture bottles. Such a table is merely an approximate one, not absolutely exact under all circumstances.

As this table does not include a number of liquid preparations, which are also in common use, it is desirable that the practitioner should possess general rules for his guidance:

1. Liquids which contain a small proportion of water afford a small drop; while, on the contrary, liquids containing a large quantity of water furnish a large drop. For instance, concentrated acids, ethers, rectified alcohol, fixed and essential oils, etc., which contain but a very small proportion of water, yield a smaller drop than diluted acids, weak alcohol, wine, vinegar, etc.

2. Amongst the liquids containing a large proportion of water, those which are not charged with remedial substances give a larger and heavier drop than these same liquids containing extraneous bodies in solution. Thus, weak alcohol, wine, vinegar and water furnish a larger and heavier drop than the tinctures prepared from them.1

Relative Value of the Drop and Minim.

The view, so long entertained and even loosely taught, that the minim was usually to be accepted as synonymous with the drop, has been wholly abandoned in the light of modern experimental research. Sixty minims always, of course, constitute a fluidrachm, but sixty drops are seldom equivalent to the same measurement, as may be deduced from the table just given. The fact becomes of vital importance in the calculation of quantities in prescription

E. Durand, Journal Phila. Col. of Pharmacy, i. 168.