Since the introduction of the metric system, its use has become almost universal among scientific men as the standard of weights and measures. It was not, however, until 1890 that the United States Pharmacopoeia adopted it for use in that work, and as it is likely to continue as the standard, it were well that all practitioners who have occasion to write prescriptions based on the official list of remedial agents should be familiar with it.

The metric system is based on the meter, which is the unit of linear measurements and is equivalent to 39.37 inches. The meter is subdivided into Decimeter, or one tenth of the meter ;

Centimeter, or one hundredth of the meter ; Millimeter, or one thousandth of the meter.

In writing prescriptions the decimeter is rarely employed.

The unit of weight is the gramme, which is the weight of one cubic centimeter of water at 4° C, the point of maximum density ;

1 gramme is equivalent to 15.432 Troy grains, and is written 1 ;

1 decigramme is equivalent to 1/10 of a gramme, and is written 1 ;

1 centigramme is equivalent to 1/100 of a gramme, and is written .01 ;

1 milligramme is equivalent to 1/1000 of a gramme, and is written •001.

The abbreviation for the gramme is gm. Quantities by weight are employed both for fluids and solids, but in a few instances it is necessary to allow for differences in specific gravity of some liquids.

Instead of Roman numerals, quantities are expressed in Arabic figures. Thus, for x is written 10.

The following rules for quantity should be remembered:

One Troy grain or minim is the equivalent of 0.06 gramme.

One drachm, or fluid drachm, is the equivalent of 4 grammes.

One ounce, or fluid ounce, is the equivalent of 30 grammes.

One gramme, or fluid gramme, is the equivalent of 15 grains or minims.

Four grammes, or fluid grammes, is the equivalent of 1 drachm.

Thirty grammes, or fluid grammes, is the equivalent of 1 ounce.

In writing a prescription by the metric method, it is usual to draw a vertical line to separate the grammes from the centigrammes and milligrammes—thus :

Rx Quininae sulphatis............................4

Ferri sulphatis............................... 1

Extracti nucis vomicae........................ 5

M. Ft. pil. no. 30.

S. : One pill three times a day.

Or the same might be written without the line, but by decimal points, thus :

Rx Quininae sulphatis............................. 4.

Ferri sulphatis................................ 1.

Extracti nucis vomicae......................... .5

M. Ft. pil. no. 30 ; or, M. ut fiant pilulae no. 30.

S. One pill three times a day.

One of the great merits of the metric system is its simplicity. Our French colleagues rarely use other weights than the gramme and centigramme, and although designating the article required in the terms of the French Codex in Latin, write the instructions and directions in their own tongue. It is best to conform to the French method in these respects.

As already remarked, in prescribing fluids by this system, parts by weight and volume are made to be mutually convertible. Some fluids, however, weigh considerably more than they measure ; others less. Thus, while one drachm or one teaspoonful of water weighs 3.75 grammes, the same quantity of chloroform weighs about 5.50 grammes, and the same quantity of ether only 2.80 grammes. Allowance must be made for these articles, but, generally speaking, parts by weight and volume are written in the same way.

In the following table the equivalents are sufficiently accurate to be employed in prescribing by the metric system :

Measures of Length.

Fluid Measures.

1 cubic centimeter is equivalent to 16·231 minims.

The following are the equivalents of metric weights in grains: