90 On the Photogen Oils. 



still. The distillate from Ransroon tar is freer from odour than 



o 



that obtained from any artificial naphtha. As the natural and 

 artificial photogens are purified in a similar way, we may treat 

 of the purification of both at the same time. 



The Artificial Products. — Fluids very similar to, if oiot iden- 

 tical with, rock oil, are obtained by the destructive distillation 

 of cannel coal, shale, peat, etc., but most abundantly from 

 boghead coal, and the shales which bear the nearest resem- 

 blance to it. We may remark that naphtha and the photogenic 

 oils are extremely different in their properties. Photogen, the 

 paraffine oil of commerce, may be easily distinguished from coal 

 naphtha by its specific gravity, since no amount of redistillation 

 will bring naphtha below 0*850, while the specific gravity of 

 the ordinary kinds of photogen does not exceed about - 750. 

 Photogen may indeed have a specific gravity of 0'900, but then 

 it will most probably contain a large amount of paraffine. 



The manufacture of photogenic oils from coals and shales is 

 a recent application of science- to the further utilization of these 

 valuable substances. Not that their capability of affording such 

 products is a late discovery, for this important fact seems to 

 have been ascertained by Clayton, during his researches re- 

 garding the manufacture of gas from coal ; but the matter was 

 neglected by him, as beside the immediate object of his experi- 

 ments. So that, as occurred with coal gas, a long period inter- 

 vened between the discovery of photogen and its application 

 to any useful purpose. Though its production during the 

 destructive distillation of bituminous coal was frequently noticed 

 by succeeding investigators, and many experiments were made 

 with results more or less satisfactory, no thoroughly successful 

 process was invented for obtaining it until the middle of the 

 present century. The discoveries which led to the establish- 

 ment of this important manufacture are not due to any one 

 person, but it owes much to the labours of Reiehenbach, and 

 it was brought to its full development mainly through the 

 ingenuity of Young. The difficulty of obtaining a remunerative 

 product consisted chiefly in the necessity for a proper regula- 

 tion of the heat employed. The best temperature for the 

 production of photogen is between 650 and 700 degrees, ;md 

 the latter must never be execeded. Should it rise to 8<>(> 

 degrees, that is, to dull redness, the products are principally 

 seous. A less elevated temperature is required if super- 

 heated Bteam is thrown on the materials, either as an aid to an 



external fire, or without tl se of one. It is indispensable 



also that certain impurities shall be got rid of. Destructive 

 distillation of every kind may be looked upon as a combustion, 

 which is limited, because effected only by the oxygen which is 



contained in the substance itself: in all eases the results are 



