It is always convenient, if there are no chemical reasons to prevent it, to take the ingredients of a formula, especially where these are numerous, in the order in which they are written, and use, weigh, or measure them out in this order. It is less tax on the memory to know, when you have got so far among them, that you have used all the items previous to the one being weighed or measured; and the prescriber who has any knowledge of dispensing generally writes them in this order. The following is a good example of the value of this rule:

To compound in the order written is the best way possible in such a case as this. Dissolve the first two oils in the tincture of cubebs, then the ol. menth. pip. in 2 drachms of rectified spirit; add this to the first solution, then the tincture of buchu, and gradually and with smart shaking the decoction and infusion. If the last two are recently prepared, the mixture is much more presentable than when it is compounded with concentrated preparations.

We take another example, a prescription for a bronchitis-mixture which seems to present no difficulty:

Here the carbonate of ammonium must first be dissolved in the infusion, and for that purpose a mortar will be required, as the salt does not dissolve readily enough in aqueous menstrua to enable us.to make the solution in a measure-glass with the aid of a glass rod, as might be done with bicarbonate of potassium, and in this case also shaking causes undesirable frothing. Having got the solution of ammonium carbonate in clear condition- say, about a couple of ounces of it- in the bottle, syrup of tolu comes second on the formula. But this should not be added now, because it would adhere to the glass, and we should not be able to measure the other ingredients accurately after it in the same graduate. The ipecacuanha wine should be added at this stage, but, before doing so, the solution should be diluted in the bottle with about 7 ounces more of the infusion. Now mix the spirit of chloroform and tincture of tolu, and pour them into the bottle, being careful to let the liquid fall into the middle of the solution, and not touch the neck or side; agitate by a little sudden jerking. In this way the tolu and chloroform will be equally diffused through the mixture.

On adding the spirit of chloroform by itself to such a mixture, without shaking, the chloroform might separate and descend, and on pouring in the tincture of tolu by itself, without shaking, the resin would separate and float on the top. By having the resin of tolu held in solution by the additional quantity of alcohol of the spirit of chloroform, less separation takes place on addition to the mixture. In other words, a better emulsion will be made of the tolu than if the tincture had been poured in by itself. The syrup of tolu should now be added quickly, and the measure rinsed with more infusion - sufficient to make the required quantity of mixture. The bottle should then be gently shaken- not too much, or it will have a tendency to make the resin separate on the sides of the bottle.

A word of caution about infusions. When freshly prepared they should be allowed to become quite cold before being used for dispensing. Sometimes dispensers do not cool them, and if a hot infusion is used in this instance the heat partly volatilises the ammonia, chloroform, and some of the spirit, and causes the resin of tolu to deposit on the sides of the bottle.

The rule in preparing such mixtures should therefore be: Make a solution of the salts first, using the vehicle, which is generally aqueous, as a menstruum; strain into the bottle, dilute with more vehicle passed through the strainer, add the tinctures, or spirits, measuring small quantities first, shaking after each addition ; then add the syrup or any mixed preparation ordered ; lastly fill up with the vehicle, and shake again. Sometimes syrup of squill is ordered in such a mixture as the foregoing. In this case, the dispenser should mix the carbonate solution with the syrup of squill before adding any other ingredient, to prevent excessive frothing. If spirit of nitrous ether is also ordered, it should be mixed with the carbonate solution before tinctures and the like are added.

In the following another course has to be adopted:

There is so little aqueous menstruum here that, if the former course is adopted, effervescence is most persistent, and it may be an hour before the froth subsides. Powder the carbonate and, in place of adding the syrup of squill, take a proportionate quantity of acetum scillae- viz., 78 minims- and put it over the carbonate in the mortar. When the effervescence has ceased, which is almost immediately, transfer the solution to the bottle, add the vin. ipec, vin. antim., and syr. mori, and make up to 1 1/2 ounce with simple syrup.

Another example is a mixture containing insoluble salts with tragacanth to suspend them. The great object in this is to give the patient in each dose of the mixture an equal quantity of each ingredient, having the solids suspended in it with the aid of the mucilage formed.

Misce.

Rub the three powders together in a mortar, then pour on 3 ounces of peppermint-water, gradually stir to form a uniform mixture, which should be transferred to the bottle, and the liquids added (the acid last) in the bottle, not the mortar, else loss of hydrocyanic acid will take place. The mortar is finally rinsed with a little more of the water, this poured into the bottle, and the quantity required to fill the bottle added. N.B.- It is requisite in all cases that the dispenser should make up the measure exactly, not according to the bottle but by actual measure.