Having decided upon the various ingredients which are to enter into the prescription, the next consideration is the desirable amount of each.

The bottles found in pharmacies have capacities of 1,2, and 4 fluidrams, and 1, 2, 3, 4, 6, 8, 12, 16, and 32 fluidounces, or 4, 8, 15, 30, 60, 90, 120, 240, 500, and 1000 Cc, and it is wise to prescribe mixtures of these sizes; otherwise, the patient may fear some error from receiving a bottle only partly full, as when a 10-ounce mixture is placed in a 12-ounce bottle.

As a rule, it is better to prescribe small bottles rather than large. Always avoid ordering more of a medicine than the patient will probably need.

Having decided, then, how many doses to order, and the dose of each ingredient, it is a simple matter of multiplication to figure how much of each ingredient shall go in the prescription.

The following is a very simple rule for estimating amounts in Apothecaries' Measure:

In an eight-ounce mixture, the dose being a dram, take as many drams of the medicine as there are wanted minims or grains to the dose. It will be observed that in this case the basis is an eightounce mixture, yet it typifies the rule which, when thoroughly understood, may easily be applied to a four-ounce or a two-ounce mixture, one-half or one-fourth as many drams; while if the dose is to be a dessertspoonful, or two drams, it is only necessary to take one-half as many drams to an eight-ounce mixture, reducing for smaller mixtures in accordance with the rule. If the dose be a tablespoonful, or four drams, one-fourth as many drams must be taken to an eight-ounce mixture as there are minims or grains to the dose. This rule, while not fractionally exact, is sufficiently accurate for all practical purposes.

Examples: We desire to give an eight-ounce mixture, with a dram for a dose, each dose to contain 12 grains of potassium bromide and 10 grains of chloral, the vehicle to be syrup of orange and water. We have here, then, 64 doses of a dram each: to be exact, therefore, we should have 768 grains of potassium bromide, or 12 drams and 48 grains; but, following the rule, we put in the mixture 12 drams, since we desire 12 grains to the dose. Of chloral we would require exactly 640 grains, or 10 drams and 40 grains, but we use the round number, 10 drams, in the mixture. We see that in each case there is but the fraction of a grain short in the dose.

The prescription would consequently be written as follows:

M. et. Sig. - Teaspoonful for a dose.

Or, if we wish the medicaments in greater dilution, we may halve the amounts and double the dose, as follows:

M. et. ft. sol. Sig. - A dessertspoonful for a dose.

The amount of each ingredient thus varies with the size of the mixture, and inversely as the dose - i. e., the larger the mixture the greater the amount of the ingredients, the dose being the same; and, the larger the dose the smaller the amount of the ingredients, the size of the mixture remaining the same.

When writing a prescription put down first all the ingredients which are to enter into the combination, and, after the last one, which is usually the vehicle, write the whole amount - i. e., if it is to be a four-ounce mixture, write after the name of the vehicle "q. s. ad 3iv." Then figure the total amount for each ingredient, by multiplying the amount of each dose by the number of doses, and write it down. In other words, decide upon the doses to be given after the medicines have been selected.

The next thing to be determined is the manner in which thedicine should be measured out to the patient for internal use. A graduated medicine-glass is always preferable to a domestic measure, and should be ordered in all cases. Teaspoons, as well as dessertspoons and tablespoons, vary considerably in size. A teaspoonful, considered to be equivalent to one fluidram, may contain from one-half to two fluidrams; a dessertspoonful, which should be equivalent to two fluidrams, and a tablespoonful, equal to one-half fluidounce, vary almost as much in capacity.

Ordinarily, it is unwise to prescribe medicines to be dropped out, since a drop varies greatly in dimension according to the viscosity and specific gravity of the fluid, the shape, size, and character of the neck and lip of the bottle, the degree of fulness of the bottle, and the steadiness of the hand in dropping.

Drops, therefore, are not accurate measures. Sometimes, however, it is desirable to order medicines in drops, and then it is well to remember that aqueous liquids and fixed oils drop about one drop to the minim, and volatile oils and alcoholic liquids, such as tinctures or fluidextracts, drop about two drops to the minim.

There are exactly 60 minims of any fluid in 1 fluidram, while 60 drops may be greater or less than 1 fluidram, as the followinist shows .