The light powder should be used for dispensing in preference to the heavier crystals, because the former dissolves more easily.

Tannic Acid does not readily yield a bright solution with water; a stock-solution (1 in 2 of proof spirit) may be used, and yields, with water free from ammonia, a brilliant solution of a light-yellow shade. Tap-water makes an opalescent solution.

Zinc Chloride of commerce contains some basic salt, so that when treated with water the chloride does not completely dissolve. It should be strained from the insoluble oxy-chloride, and no attempt should be made to clear it by the addition of acid.

Dispensers should remember that the water of crystallisation in many chemicals suffices, when certain of them are mixed, to produce apparent decomposition. This fact is taken advantage of sometimes, as in the granulation of effervescing preparations, where the water of crystallisation in the citric acid leaves it to join the sugar or alkali, thus helping to mass the mixture. There are many similar instances. The liquefaction of camphor and chloral hydrate, phenol and quinine sulphate, and many other organic substances, when mixed together, is not of the same nature, but appears to be due to molecular rearrangement in some cases, and to actual chemical change in others.

Young dispensers should write for themselves paragraphs similar to those in this chapter regarding peculiarities in chemicals and drugs which they may observe or learn. There is much to be gained by a methodical system of annotation, as the following example from our own note-book proves:

Some dispensers get a water-white mixture from this, others cannot get it without free iodine; and even the water-white mixtures, when exposed to air and light, slowly acquire a brown colour and iodine odour. This is what might be expected when the fact is considered that potassium iodide and sulphuric acid react thus:

2KI + H2SO4 = K2SO4 + 2HI.

This takes place even in dilute solutions, especially when exposed to sunlight, and in presence of oxygen the hydriodic acid changes thus:

2HI + O = H2O + I2. The iodine thus liberated reacts with quinine to form an iodo-quinine, which crystallises in brown needles, and is a combination of four molecules of quinine with five atoms of iodine.

Thirty years ago a foreign chemist made a long series of experiments on the behaviour of acidified potassium-iodide solutions when exposed to light, and he conclusively demonstrated that all the mineral acids, even when very dilute, ultimately liberate iodine. Since then numerous studies of iodine and iodides have put it beyond dispute that the molecules KI and H2SO4 cannot exist together in presence of water, at least indefinitely, as the affinity between K and SO, is greater than that between K and I.

These facts can scarcely be missed in the course of study for the pharmaceutical examinations, and one has only to apply the knowledge to avoid disputes as to which kind of mixture is right. The iodide that gives the water-white mixture is alkaline in reaction.

This chapter is a direct proof of the remark made earlier -that the art of dispensing is the focussing upon the compounder's duties of the knowledge of chemistry and kindred subjects required by the Boards of Examiners. The connection between the laboratory bench and the dispensing-counter is exceedingly intimate, and the cleverest pharmacists find in compounding a ceaseless source of suggestive inquiry and professional pleasure.