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450 THE PHOTOGRAPHIC NEWS. undissolved portion, and so prevent the fresh spirit from reaching the resin. It is for this reason that spirit var nishes require frequent shaking while being prepared. With the resin in the bag at top of the spirit, as it becomes dissolved the heavy mixture falls, and so the bag is con tinually in contact with fresh spirit. When varnishes are used on the large scale, they often contain small particles which make them turbid ; they therefore have to be clarified. This can only be done with volatile varnishes by allowing them to stand for weeks in large bottles in places where they are safe from all disturbance. The solid bodies will then gradually settle in the bottom, and the clear varnish can be poured off by careful handling. By this method a great number of ex pensive and easily broken bottles, as well as considerable space for storing them, render the clarification costly, and besides, a certain percentage of the varnish is lost by unavoidable evaporation of alcohol, by spilling, &c.; and, to obtain bright and light colour varnishes; they should be filtered. Filtration of Varnishes.—Owing to the viscosity of varnishes, their filtration occupies considerable time, and, if filtered in uncovered vessels, quite a quantity of alcohol, benzole, &c., is lost by evaporation. Fig. 1 shows a simple yet efficacious apparatus, that is easily made at little cost, for the perfect filtration of small quantities of volatile varnishes, and without loss of spirit by evaporation. A is a large bottle, either of glass or tin. This is hermeti cally closed by a cork with two holes; a caoutchouc stop per is very handy in this case, or a wide bung. B is a glass funnel, the neck of which passes through one of the holes in the cork of A, while a piece of bent glass tubing, connected with a piece of india-rubber tubing, C, passes through the other hole ; a plug of cotton-wool—medicated wool I find the best—is put in the neck of the funnel, or else a filter paper is used. I prefer the former for both spirit and volatile varnishes. The top of the funnel is ground smooth, and a piece of mahogany made to fit accurately thereon as a cap, D. A ring of rubber placed on the underside of this cap renders it air-tight. In the centre of this cap is a hole, through which passes a piece of bent glass tubing, E, which is connected with the other end of the rubber tubing, C. The volatile varnish is placed in the funnel B, the cap put on, and, as the varnish filters through into A, the air in the latter vessel is dis placed, and, passing through C, enters E, and so keeps up the equilibrium. The air in B becomes saturated with the vapour of the spirit; but, as such vapour cannot escape when the air is saturated therewith, no more vapour is given off from the varnish; thus, the consistency of the filtered varnish is uniform, which is not the case when it is filtered in an open funnel, because, as the spirit evapo rates, the proportion mixed with the resin is lessened, and so the latter portion of varnish that is filtered becomes thicker than the first, due to this loss of spirit. When quantities of the varnish, the de ¬ small The the finished product is secured in the vessel A. For preparing colorizing can be done at the same time as the filtering by plunging the animal charcoal in the funnel B, and pouring the varnish to be filtered upon this. This method, the plug of cotton-wool or filter-paper ceases to act well —i. e., when the varnish filters very slowly—whatever is in B should be poured off, and the filtering body be replaced with fresh material. The Decoloration of Varnishes. —One great drawback to using varnishes is that they possess too much colour for the purpose to which they are required to be put. There are many means of bleaching varnishes and of bleaching the natural resins before use, but the safest and simplest material for general use is animal charcoal. To use this material, it is best employed in small pieces like coarse sand, because when powdered animal charcoal is used the pores soon choke up, and filtering becomes a very tedious operation. Before using the charcoal at all, it must be freed from the salts contained in it to render it suitable for our purpose. This is done by treating it with hydro chloric acid in the following manner. A quantity of raw animal charcoal is placed in a suitably sized stone ware jar with a lid to it, and a quantity of raw hydrochloric acid equal in weight to half or three-quarters of the weight of charcoal used is poured on it, and the whole allowed to stand for one day in the covered jar with frequent stirring. The contents of the jar are then poured into a tub containing about five times the amount of water that the jar would hold, the charcoal allowed to settle, the fluid poured off, and clean water poured over it. This operation is repeated until the water is entirely free from acidity (ascertained by means of a blue litmus paper not becoming red when dipped in it). The washed animal charcoal is then dried by heat. however, has its disadvantages in respect to chang ing the filtering substance or the animal charcoal, if either one of these substances should lose its effect. Therefore it is best to carry on the filtering and decolor izing in separate vessels. In fig. 2, the varnish that is to be decolorized and filtered is put into the bottle F, which has a stopcock, G, fitted to it near the bottom; this is connected with the bent pipe, H, by a bit of rubber tubing. This pipe dips just below the lid of the vessel P, and the neck of this vessel enters the cap D of vessel B, while the pipe E fits into a hole in the cork of F. This arrangement allows the varnish to flow into G to be de colorized, and from thence into B Fig. 2. to be filtered, while