PHOTOGEN 



561 



are interested in the commercial value of the different varieties of coals and bitumens 

 as sources of illuminating oils : 



None of the cannel or bituminous coal, shales, or other substances used for yielding 

 burning fluids by distillation, give distillates of such purity and freedom from odour, 

 as Rangoon tar. The more volatile portion of the distillate from the latter has 

 obtained in commerce the absurd name of Shenvoodole; it is used instead of coal- 

 benzole for removing grease, &c. The paraffin obtained from Rangoon tar has a 

 greater value for commercial purposes than that from Boghead coal, inasmuch as it 

 has a higher melting-point, which renders it better adapted for candles. The follow- 

 ing are the melting-points of various samples of paraffin : 



Melting-point. 



Fahr. 



Boghead-coal paraffin . . . . . , .114 

 another specimen . . . .108 



The last, after being distilled 108 



Turf paraffin . 116 



Bituminous-coal paraffin, prepared by Atwood's process . 110 

 Rangoon-tar paraffin 140 



It is curious to observe the effect of light upon photogen. Some samples of ex- 

 tremely dark colour, when exposed to its influence for a few days, become as completely 

 bleached as animal oils would under these circumstances. At the same time, as we 

 have before hinted, the odour becomes much improved. A photogen of good quality 

 has by no means a repulsive odour, but if much of the more volatile constituents be 

 present, it is impossible to avoid its being disagreeable if spilled about. The less 

 volatile hydrocarbons have comparatively little odour. It should not be too inflam- 

 mable, that is to say, it must not take fire on the approach of a light. If it does, it is 

 owing to the more volatile portion not having been sufficiently removed. See 

 PARAFFIN for a description of its manufacture. 



VOL. III. 



