The Optical Magic Lantern Journal (July 1899)

84 The Optical Magic Lantern Journal and Photographic Enlarger. the decomposition takes place at too high a temperature, the residue will be of a snuff colour, this means that other gases and tarry matter have been formed, with a consequent loss of illuminating power. With a very bad generator in which great heating occurs, the residue will consist of black carbon and white lime, while the yield of gas will be far below that which one has a right to expect, in some cases only half the proper quantity being obtained from the carbide. To test the carbide, a small quantity should be dropped into a glass of water; it will quickly decompose, and the colour of the residue should then be noted. If it is white and the residue from the same sample used in the generator brown or black and white, a faulty generator is indicated. Many people fancy they can judge the quality of carbide by its appearance, believing that a distinct crystalline structure denotes great purity, while a surface similar to pig iron isa sign of inferior quality. This is a fallacy. The purest carbide, that made by the “running” process, is of the fine grained variety, the rapidity with which it: is cooled causing the formation of smaller crystals. Burners and How To Avorp CaRBONISING.— There are many types of burner beside the small Bray, which until recently held the ‘field in England. It has been found better to make two entirely independent jets coming from burners inclined at a convenient angle impinge upon each other and form a flat flame. At a point below the outlet there is a hole or holes or slit, which allows air to be sucked in by the | rush of gas. This air surrounds the acetylene, and prevents it coming in contact with the steatite tip of the burner; consequently no carbon can be deposited. usually started by the presence of a trace of benzine or the like in the gas, which is absorbed by the steatite of the burner, and which becomes carbonised by the heat of the flame. This small beginning starts a catalytic action, and carbon is then rapidly deposited from the acetylene. THe Expiosipmity or ACETYLENE.—Pure acetylene may be exploded by means of a detonator, but in actual working there is no possibility of such an occurrence. With air, however, explosive mixtures are formed when the proportion of acetylene is anywhere between 38 and 90 per cent.—a much wider range than that of coal gas. Hence great caution is necessary in dealing with generators, gasometers, and the like, in which such mixtures may be formed. With regard to the fulminating compounds Carbonising is | said to be formed by the action of acetylene upon fittings composed of copper, brass, or other alloys, it has been proved by repeated experiments that it is impossible to form even the smallest quantity of such explosive in this way. It appears that the copper must be in ammoniacal solution for the explosive salt to be formed, and this condition does not obtain in or about any ordinary generator. In conclusion, pure acetylene is practically odourless. It is only the impurities which make one wish for ventilation when the gas is escaping. The two principal impurities, viz., phosphoretted and sulphuretted hydrogen, can be eliminated, but except in very bad cases, i.e., where very impure carbide has been used, it is hardly desirable to remove all traces, as it would then be difficult to detect a leakage of which a violent explosion might be the first intimation. Optical and Mechanical Effects for the Lantern.—No. IX. By EDMUND H. WILKIE. (Late Royan Ponyrrcunic Insritution). Ta en i HE natural course of ee has t) 2 BN radually led us from the conFay Cdevetied of simple mechanical ‘7 %>)} movements to those of a more ‘se complex nature in which several KS) plates of glass are used, each having =» a portion of the picture painted upon it. oS In designing effects of this class, the slide maker must take into consideration the character of the lantern in which it is proposed to work the motions, some being made in such a cramped fashion that the stages allow of little or no movement beyond the limits of the slide frame. ; Most lanterns of really good construction allow a little latitude in this respect, and the large lanterns used at the late Royal Polytechnic were arranged so that the back and front stage plates were not connected by pillars, but were simply built up opposite to each other on the foundation framework. This method of construction allowed slides | to be introduced which had projecting glasses | at the bottom and sides, without which it would have been quite impossible to realise some of the effects.