On the Photogen Oils. 91 



extremely complicated, and in most they include undesirable 

 compounds. If the body submitted to it contains nitrogen, 

 ammonia and other nitrogen combinations are among the pro- 

 ducts ; and in a similar way, if sulphur is present, there are 

 compounds of sulphur. The amount of photogen does not 

 depend on the temperature only, it has an important relation 

 also to the constitution of the crude tar. The tar furnished by 

 coal is more dense than that from bituminous schist ; that from 

 the Breckenridge cannel coal, which is supposed to be the most, 

 highly bituminous of all, yields about thirty-two per cent, of 

 tarry oil ; and the latter may be expected to give seventy per 

 cent, of the pure product. 



If pure, the photogenic oils contain almost exclusively 

 Tolmene, which boils at about 230 degrees, and Oumene, which 

 boils at 314 degrees, the less volatile oil being most probably 

 carried over by the vapour of the more volatile. The propor- 

 tion in which these are present depends very much on the 

 temperature at which the distillation is effected. In Germany 

 the distillate is usually divided into two portions, the more 

 volatile being set apart as photogen, and the less as solar oil; 

 but in this country the two are used together. The oils which 

 distil over between 340 and 400 degrees contain creosote ; 

 above 400 degrees there is cumene with other compounds, and 

 but little tolmene. It is evident, therefore, that in the manu- 

 facture of photogen too much attention cannot be paid to the 

 temperature -, and hence the numerous projects for securing a 

 proper one. If a naked fire is used, a due regulation of the 

 heat becomes extremely difficult, and the material is very likely 

 to become overheated in certain places, which causes the evolu- 

 tion of gaseous instead of vaporous products ; but this may, to 

 some extent, be prevented by keeping the coal, shale, etc., 

 contained in the retort, in motion. To obviate the necessity of 

 using a naked fire, or to render a more moderate one sufficient, 

 steam is employed, but it should be superheated, and must not 

 be derived from water, with which the materials have been pre- 

 viously wetted, as this would tend to the production of cold 

 rather than heat. Bituminous shale is, however, an exception 

 in this respect, as the presence of water protects it from too 

 high a temperature, and the vapour generated from it, helps 

 mechanically to carry off the photogenic products as fast as 

 they are volatilized, so as to prevent their decomposition. 

 Superheated steam both heats the material and aids in its 

 decomposition, and it may be raised to the proper temperature 

 by being made to traverse coils of tubing, placed within a 

 furnace — that used in the distillation, if any such is employed. 

 But since the steam is decomposed at a high temperature, its 

 gaseous constituents entering into combination, and forming 



