The question occasionally crops up whether natural or artificial acid should be or has been used, owing to the practice in a few dispensing houses of using the former. Between the 'physiologically pure' artificial acid (which is 'B.P.') and the natural there cannot be detected any difference in therapeutic action; and as the British Pharmacopoeia permits either to be used, the dispenser must allow common-sense or sentiment to decide the matter for him. Artificial salicylate of sodium is white; the natural salt is creamy in colour. The red to brown colour produced in salicylate mixtures containing spirit of nitrous ether never becomes quite so dark with the natural acid (see page 299). Salicylic acid dissolves to a slight extent in water (1 in 500), but very readily in solution of ammonium acetate. In the latter case ammonium salicylate is really formed, acetic acid being set free. So also when other alkali salts of organic acids are combined with salicylic acid. Some salicylates are but slightly soluble in water- e.g., antipyrin salicylate or salipyrin- and when the readily soluble sodium salicylate is added to an equally soluble body (by itself) the mixture may not be clear.

The powerful antiseptic property of salicylic acid suggests its addition to solutions which are prone to decomposition; when so used it is a good plan to prepare salicylic water by boiling a pint of distilled water in a flask and adding to it 16 grains of the acid, plugging the flask with a piece of cotton wool which has been scorched in the Bunsen flame, and setting aside to cool. This water is sterile, antiseptic, and saturated with salicylic acid. Microbes cannot live in it, but it is possible to introduce along with the microbes something upon which they may live, as is the case with ergotin.

Silver Nitrate in solution should be sent out in amber-tinted glass bottles, which glass minimises the action of light on the solution, and, being more transparent than blue-glass bottles, the contents may be more easily seen.

Silver Oxide parts with its oxygen so readily when brought into contact with certain organic matters as sometimes to induce an explosion. The subject is fully treated in the section on pills; but here the student is especially warned regarding the reducing-power of the compound.

Spirit of Nitrous Ether is one of the most troublesome products which the dispenser has to handle; but during the past thirty years much light has been thrown upon it, so that it is no longer the 'dark continent' that it was to our forefathers. The spirit is essentially a solution of ethyl nitrite and aldehyde in rectified spirit. When fresh there is little aldehyde in it (less than 1/2 per cent.), but the water in the rectified spirit, under the influence of light and air, decomposes the ethyl nitrite, C2H5N02, which splits up, forming alcohol and nitrous and nitric acids. The acids at once react with alcohol to form fresh ethyl nitrite and a little aldehyde; some of the ethyl nitrite evaporates. As some of the nitrous acid remains in the spirit for a long time, when it is compounded in a mixture with an alkaline iodide, the acid and iodide react, nitrite of the alkali being formed, and iodine and nitric oxide liberated. This reaction, which has been the bugbear of dispensers for generations, is utilised by the Pharmacopoeia to estimate the percentage of ethyl nitrite in the spirit by measuring the volume of nitric oxide evolved (see page 241). Spirit of nitrous ether should be kept in well-filled and inverted bottles, and the dispenser should bear in mind that although it is a weak diuretic it is a powerful chemical agent, especially prone to decomposition through mixture with water, which simply hydrolyses the ethyl nitrite.

The old-fashioned prescriptions for diaphoretic mixtures such as the following were really scientific:

Hydrolysis (or decomposition) of the ethyl nitrite quickly begins in this mixture, but the nitrous radicle as soon as it is liberated combines with the ammonia of the ammonium acetate, hence the mixture retains a large measure of its potency because the ammonium nitrite has the same, though milder, action upon the arterial system as the ethyl nitrite.

In mixtures containing antipyrin and spirit of nitrous ether, the latter, if acid, reacts with the former, iso-nitroso-antipyrin and a cyanogen compound being amongst the products, and the mixture becomes green. Some dispensers keep a crystal or two of potassium bicarbonate in the spirit, and thus have it always neutral; that does not of course prevent decomposition of the ethyl nitrite, but as hydrolysis proceeds nitrite of the alkali is formed and retained in solution, so that the spirit may on analysis show less loss than when alkali is not employed.

Sugar for pharmaceutical purposes should be the kind made from cane. It is purer and less liable to alteration than beetroot sugar. It rarely happens that solid sugar is prescribed, except in the form of pulv. sacchari albi in powders. Should it occur in any mixture, the best plan to follow is to take the equivalent of syrupus B.P., 3ix. of which is equal to 1 avoirdupois ounce of sugar, or mlxxiv. equals 60 grains of sugar.