COOLING OR PRECIPITATING.
When the boiling has proceeded suffi ciently far. the vessel containing the emul sion is set aside in a dark cupboard to solidify and ripen (by ripening is meant the increase of speed which results from keeping); or it may be precipitated imme diately by the use of alcohol. The best plan perhaps is. to pour out the emulsion into a, flat dish. The dish should be one scrupulously clean, and preferably one which has not been used for any other purpose. A flat porcelain or glass dish, similar to those used in developing, will answer well. A form often employed is shown in Fig. 134 ; even a pie dish may be used, but it is open to the objection that the enamel is usually not good, and frequently cracked. In any case, enamelled iron dishes should not be used. The advantage in using a. dish, over a beaker or jar, is that it pre sents a much larger surface to the air, and consequently sets more rapidly. At the same time, it must be remembered that the emulsion should be placed in a. cool place where there is a free passage of air, otherwise it will not set for a considerable time. Do not shut it up in a box, or in a close cupboard. After about two hours the emulsion will have set, the actual time varying with the tem perature and the dryness of the air, as will no doubt have been gathered from the foregoing remarks. In commercial practice it is usual to cool the emulsion with ice water, as the effect is better. The principle is explained in a later section. The precipitation of the emulsion by means of methylated spirits or alcohol is a method introduced by a well-known firm of plate makers, but is very little practised at the present day. For this purpose, ordinary methylated spirit may be used (preferably of low specific gravity), not containing gum. Some workers recommend an equal quantity of alcohol or spirit, whilst others prefer to use double the quantity. Of the two methods, the latter is perhaps the safer, as the great desideratum is to completely remove the soluble nitrates. Take, then, twice as
much alcohol as of water used in the making of the emulsion. In the present case, 5 oz, should be used. The bottle, or other closed vessel, containing the emul sion, is then violently shaken, so as to completely impregnate the solution with the spirit. As this proceeds. the spirit having a strong affinity for water will extract it from the gelatinous matter, with the result that it subsides to the of the vessel (on allowing the solution to stand), in the form of a thick pasty mud In removing the water the soluble nitrates are removed also which remain dissolved in it. When working on a large scale, the spirit may be saved, so as to be re-dis tilled for use again. It has been said, however, that this method, although offer ing as it does many advantages in the way of simplicity, is not so applicable to the preparation of rapid emulsions on a. small scale. The soluble nitrates do not appear to be so effectively removed as may be done by washing. Supposing, therefore, that washing the emulsion is the method adopted ; after allowing it to set in the dish as already described, it is redissolved and cooled down to a temperature of about 70° to 80° F. This may be clone by allow ing a stream of cold water to run clown one side of the vessel containing it. The 210 grs. of gelatine are now rapidly rinsed in a few changes of distilled water to free it from dust or any adherent matter, and then covered with 2 oz. of distilled water and placed in a water bath, the tempera ture of which should only be sufficient to dissolve it properly. Something about 100° F., probably. Do not be in too great a hurry for the gelatine to dissolve. The less heat used in dissolving this the better, as it is liable to lose its power of setting properly. When thoroughly dissolved it is added to the emulsion and shaken vio lently. It is now ready for filtering.