On the Photogen Oils. 93 



means, because the impurities cannot be separated by any 

 method of filtration. The viscous, semi- solid, and solid hydro- 

 carbons suspended in the lighter oils must be thoroughly 

 removed, since a very small amount of them, in the photogen 

 would cause the production of smoke during combustion, on 

 account of the extra quantity of carbon which would be present. 

 Most of them may be got rid of by careful distillation, at a 

 regulated temperature ; and the remainder by treatment with 

 sulphuric acid, which unites with them, while it has very little 

 effect on the lighter constituents with which, they are associated. 

 The more volatile impurities, which render the oil odorous, and 

 increase its inflammability, may be separated by bichromate 

 and manganate of potash, since they are easily oxidizable, or 

 even by animal charcoal. The more offensive the smell of the 

 oil the more easily it takes fire, and, therefore, the more dan- 

 gerous it is. Any sulphuric acid which may remain in suspen- 

 sion, on account of not having been removed by washing, is 

 neutralized by soda, which takes away also any carbolic acid or 

 creosote which may have been generated by decomposition of 

 sulphuric acid, determined by the carbon of the easily decom- 

 posible compounds in the distillate. The alkali takes away any 

 sulphide of hydrogen present, and perhaps other fetid sulphur 

 compounds. Alternate use of soda and sulphuric acid renders 

 the oil very pure. Sulphuric acid, besides being cheaper, is 

 more effective than nitric or nitro-muriatic. A very common 

 process of purification consists in adding sulphuric acid to the 

 crude oil after the tar has been separated from it, agitating 

 the mixture, and allowing it to settle ; then drawing off the 

 clear liquid into another purifier, in which caustic soda or lime 

 water is added, agitating and allowing to settle. When the 

 resulting clear liquid is distilled at a temperature between 400 

 and 600 degrees, tar remains in the still. If the distillation is 

 not very carefully conducted, volatile, and therefore highly 

 explosive compounds will result, and the oil will be dangerously 

 inflammable. In Hamburgh, which is remarkable for the good- 

 ness of its photogen, the distillation is repeated several times, 

 after which the oil is treated with sulphuric acid ; it has then 

 but little smell, since the material from which it is made is 

 very free from sulphur. The purified oil should be without 

 colour, and inodorous, or at least with only a slight aromatic 

 odour. The smell of bad photogen is diminished by exposure 

 to the air, and even by keeping it for a few days in casks, and 

 its colour is destroyed very rapidly by mere exposure to the 

 light. Should creosote be present, it will cause a smell, and 

 will char the wick. The freedom of the wick from charring is, 

 therefore, a test of the goodness of the oil. So much as six or 

 seven per cent, creosote is often found in what is sold as doubly 



