92 On the Photogen Oils. 



gaseous compounds with the carbon, burnt air is often used 

 instead of it ; and this is found to give nearly twice as large a 

 product as a naked fire. As, however, the cost of fittings for 

 either steam or burnt air is, in some instances, more important 

 than the saving they effect, a naked fire is still being" generally 

 used, both in the production and the purification of photogen. 

 Baths of fusible metal also have been employed with great 

 success, as a means of regulating the temperature ; and they 

 have been found to improve not only the quantity but the 

 quality of the result. 



Bitumens and bituminous schists give the same products, 

 but the former leave a less amount of earthy residue : the schists 

 may be expected to yield about fourteen per cent, of oil. The 

 heavy oil, obtained among the products from peat, answers well 

 for burning ; it gives a dazzling white light, but on account of 

 the large quantity of carbon which it contains, the wick requires 

 to be trimmed after about eight hours. If bituminous slate is 

 used for the production of photogen, it must not be too minutely 

 divided, or the high temperature, which will arise from the free 

 escape of the vapour being prevented, will cause gases to be 

 formed in abundance. 



To obtain the photogen in a state of purity, it is necessary 

 to subject the crude product to certain processes, which are of 

 great importance. The demand at present existing for coal 

 oil is so great, as to render the manufacturer careless about an 

 adequate purification; but a neglect of this, however strong 

 the temptation, can arise from only a short-sighted policy; and 

 the difficulty of obtaining a good article deters many persons 

 from using an oil which, with a proper and honest system of 

 manufacture, is capable of giving a convenient and brilliant 

 light. In France, where the vegetable oils are abundant, pure 

 schist oils are easily had; even in Germany, where the induce- 

 ment to supply an inferior article is strong, since oils and fats 

 are comparatively less plentiful there than in Britain, which is 

 supplied with an abundance of animal oil by its whale fishery, 

 a good photogen is easily obtained ; and in America, where 

 great care is bestowed on the preparation of the oil, and also 

 on the construction of the lamp, to which far less attention is 

 paid by us than is requisite, photogen is preferred to almost 

 every other source of artificial light. There is enough to encou- 

 rage the manufacturer to a due purification of his product, since 

 it is possible to obtain so much as one hundred and forty gallons 

 of good oil from one ton of cannel coal ; or from one hundred 

 parts cannel e<»al, forty parts oil, and ten parts paraffine, with 

 other substances applicable to lubrication, to the solution of 

 caoutchouc, and other useful purposes. 



The purification of photogen must bo effected by chemical 



