26 ON PETROLEUM AND PHOTOGEN, 



a series of heavier oils, which constitute the lamp-oil. Steam is now 

 forced into the retort, and a heavy, lubricating oil passes over, and, as a 

 residue, there is left a black, tarry matter used to grease heavy machinery, 

 and a black coke employed as fuel. When the oils first come over, they 

 are in a very impure condition ; the amount of foreign matter which 

 they contain renders their purification a matter of difficulty, as well as 

 of expense, and although much has been effected in this direction, much 

 remains to be done before they are rendered free from all offensive odour, 

 and free from colour. The process of purification consists in the treat- 

 ment with concentrated oil of vitriol to remove the coloured and 

 odorous constituents of the crude distillate, and washing with an alkali 

 to remove carbolic acid and its congeners, as well as that portion of sul- 

 phuric acid which remains suspended in the naphtha, and the sulphurous 

 acid produced by the decomposition of a portion of the sulphuric acid 

 by the carbon of some of the organic compounds. The alkalies also 

 serve to remove some sulphuretted hydrogen and other fetid sulphur 

 compounds and their resulting products. By a careful purification, coal- 

 oil is now obtained perfectly free from colour. It often exhibits the 

 phenomena of dichroism. This is diminished by perfect purification 

 with acids and the removal of the less volatile portion of the distillate. 



But few common bituminous coals can be successfully employed for 

 the production of oils, their distillates abounding in creosote, carbolic 

 acid, &c, and their purification is both troublesome and expensive. Few 

 coals produce over 100 gallons per ton ; some cannels will not yield over 

 50, others 30 gallons per ton. The quality of the crude oils also differ ; 

 some afford a large quantity ol paraffin, or heavy oil, and but a small 

 percentage of light oil, and others yield the opposite. The lighter 

 qualities yield the largest proportion of burning oil. 



II. Boghead Coal occurs at Torbane hill, in the county of Linlith- 

 gow, in the carboniferous limestone of the Frith of Forth. It is used 

 largely by Mr. Young for obtaining his paraffin and paraffin oil, his 

 manufactory being situated in the immediate vicinity of the mines. At 

 one time this mineral was largely exported to America, and in 1859 the 

 Kerosene Light Company imported upwards of 20,000 tons at 1 8 dollars 

 per ton ; but the discovery of strata of cannel coal, and of petroleum, 

 has done away with the necessity of that importation. 



The nature of Boghead coal is at the present time an undecided 

 question, even after an action at law. At that trial a great array of 

 scientific witnesses were examined on this matter ; they consisted of 

 chemists, geologists, botanists, mineralogists, microscopists, as well as 

 practical coal engineers and owners. The evidence was conflicting, one 

 party maintaining that it was coal, whilst the other declared it to be an 

 nndescribed mineral or bituminous schist. But the evidence is in favour 

 of its right to be ranked as a true coal ; it rests on a bed of fireclay full 

 of Stigmaria, and is surmounted by shale and ironstone, with plants and 

 shells (Anthracosia). It has the microscopical structure of coal, burns 

 with a flame, and yields 70 per cent, of volatile matter. 



