This is the method of coating pills which is done by making the pill-powder, French chalk, adhere to form a cemented layer on the surface of the pills. Full particulars were given in the 'Art of Pharmacy'(' The Chemist and Druggist Diary,1898') in regard to coating pills on the large scale. A similar process is used at the dispensing-counter. The apparatus employed is a tin globe copper-lined, about 5 1/2 inches diameter, intersected in the middle, the two sections being fixed with two brass pins. This arrangement permits of the apparatus being easily taken to pieces and thoroughly cleansed. In practice, two globes are generally sufficient (one is figured above)- namely, one to coat and one to burnish the pills ; but if a very high finish is required, a third is necessary to polish the pills. This is done by thinly coating the warmed globe with hard paraffin. The pills, after passing the second globe, are transferred to the waxed globe, and slowly rotated for a time in it. This gives them a brighter, more uniform, and probably, also, a more permanent coating.

Many dispensers succeed in coating pills by using covered pots or tins as coaters.

The coating solutions which are recommended vary in composition, but a good one is made by mixing 1 drachm mucilage of acacia, 1 drachm simple syrup, and sufficient water to make 1 ounce. Of this mixture pour sufficient upon the pills to damp the surface thoroughly- the exact quantity can only be determined by experience, but a dozen 5-grain pills require not more than 2 drops- and after rotating in the coater in order to distribute the gum-mixture uniformly over the surface, transfer to the French-chalk tin, and rotate uniformly until all the chalk has been taken up or the pills are thoroughly coated. If too little chalk has been used, or too much mucilage, more chalk has to be added from time to time until the coating is uniform.

The following are other formulae for moistening-solutions:

The second formula is Mr. Martindale's, who advocated the three covered-pots method. Mr. E. W. Lucas recommends a mixture of acacia mucilage and syrup, 2 ounces each, and sandarac varnish, 1/2 ounce.

Fine French chalk is used for coating, and it is much improved by having 3 per cent, of light carbonate of magnesia added to it. This mixture should be well sifted through a lawn sieve several times. The carbonate of magnesia is beneficial because the gastric acid acts upon it, and thus aids in disintegrating the coating. The coating-powder may be sweetened by the addition of 1 ounce of powdered sugar or 5 grains of saccharin to each pound. Obviously the secret of a thin coating is the use of as little mucilage as possible, just as in coating with silver. After a uniform coating has been imparted, the pills are transferred to a clean coater, and rapidly rotated in order to give them a polish.

The following process has been specially recommended for use at the dispensing-counter. It turns out a finished pearl-coated pill in a few minutes. Shake the pills in a covered pot with sandarac-and-ether varnish (page 133) and throw into very fine French chalk, rotate for a minute, and separate excess of powder by sifting. Shake the pills in another pot with a mixture of equal parts of whipped white of egg (strained), syrup, and water, sufficient to wet the pills thoroughly, and throw them into excess of very fine French chalk ; shake for a minute, remove the pills to a flat marble slab, and rotate very lightly under a pill-finisher, sprinkling on a very little chalk until a smooth surface is produced. If time permits they should be exposed to the air in a tray to dry thoroughly.

If the pills are silver-coated first, two coats of chalk suffice to make them pearly white, and the colour does not show through.

Plumbago gives the opposite in colour of pearl-coating by simply rubbing the pills in ordinary powdered blacklead. The method is old, but has never got into favour, although the coating is an excellent protective.

Gelatin-coating.

Gelatin coating is the most soluble of all, and is now easy of application on account of the simple apparatus which is obtainable for doing the job expeditiously. The gelatin coating has the advantage of being transparent, so that the colour of the pill is seen, and the coating is more quickly imparted than any other.

In the absence of better apparatus we have used the following simple coating device with good results. If, say, three dozen pills are to be coated, take three soda-water corks, and stick into one end of each the eye-ends of a dozen needles at an angle of 45°; then place a pill on each of the needles. The pills are now ready to receive the coating. Having melted the coating-solution on a water-bath, dip the 'corkful' of pills into it, withdraw, allow the drops of superfluous gelatin to form, and remove them by allowing the pills just to touch the surface of the liquid, then twirl the cork between the forefingers for a few seconds, and set aside to harden.

A convenient and simple apparatus for dispensing-counter work is that devised by Mr. A. J. Palethorpe. It consists of a tray in which recesses (a) for the pills are carved, and a needle-disc pill-dipper (b) which fits on a quadrangular centre-piece to which a check-bolt (c) is fitted. The pills are put in the tray, and the check-bolt adjusted according to their size, so that the needles just pierce the pills enough to hold them tight. The disc, with the needles, being pressed home, is lifted, dipped in the hot gelatin solution, reversed for a moment or two, and waved in the air until the solution sets. The pills are then removed with d. The points to be observed in gelatin-coating are:

(1) An expeditious arrangement for affixing and removing the pills from the needles, so as to avoid touching them with the fingers. A pill-tray with as many holes in it as there are needles, and each hole opposite a needle, is used for charging the needles. For removing the pills a plate which slides between the disc and the needle-points is the best idea, and is provided in the 'Unique'coater (see'The Chemist and Druggist Diary, 1898,'page 498).

(2) The coating-solution should be kept as limpid as possible without heating much above 1500 F. This is to ensure a thin coating. Coating-solutions are made either of (1) gelatin, (2) acacia and gelatin, or (3) acacia. The second kind is that generally adopted, and the following are formulae for it:

Soak the gelatin in the water until soft, heat on a water-bath until dissolved, add the mucilage, and mix.

The mucilage should be made by dissolving 4 ounces of gum ('flaked 'or Trieste picked grain) in 7 1/2 ounces of water.

(3) Allow the pills to dip completely in the solution, withdraw from the solution slowly (so that superfluous coating may be removed by capillary attraction), and when out, if drops form, touch the surface of the solution with the drops. Finally, turn upside down and allow to dry.

(4) Most gelatin pill-coating machines provide for rotation of the pills after they are dipped. Pill-manufacturers allow the pills to remain at rest at a temperature of 8o° F. The object of rotation is to dry quickly, and it is a substitute for rest at a higher than normal temperature. The pills should be removed from the needles as soon as the surface is hard, and before the inner layer of the coating hardens, so that the hole may close with the soft solution and thus exclude air.

On the manufacturing scale gelatin-coating is done with machines which have tubes instead of needles; these terminate in a shallow box connected with a suction-machine, and the pills are thus picked up by atmospheric pressure. One half of each pill is first coated and dried, then the other half.