ON PETROLEUM AND PHOTOGEN. 25 



" a method of extracting or making tar, pitch, and other substances 

 from pit-coal," and in this manner is said to have turned the mines of 

 coal on his own and other estates, to considerable profit. Later, the 

 attention of some French chemists, especially M. Selliguier, was turned 

 to the purification of these products, and their exertions were attended 

 with some success, and the purified oils acquired an extensive sale in 

 Europe for illumination and lubrication. But the first real practical 

 success was made in this country by Mr. Young. Previous to his in- 

 vestigations, the thickness, coarseness, and unpleasant odour of the oils 

 then manufactured, were such that they had fallen into disuse in Europe, 

 when the attention of Mr. Young, a manufacturing chemist of Glasgow, 

 was called to some petroleum which had been obtained from Biddings, 

 in Derbyshire. The spring was an old coal-mine, from the sandstone 

 roof of which a dirty rock-oil exuded. This source soon became ex- 

 hausted, and Mr. Young then investigated the somewhat similar oils 

 which Beichenbach and Selliguier had previously shown might be ob- 

 tained by the distillation of coal, lignite, peat, &c. Since that period, 

 enormous impulse has been given to the manufacture, especially by the 

 recent discoveries in America. 



We now, without any lengthened or scientific details, proceed to 

 consider generally the substances capable of affording oils by distillation. 

 The materials which are employed at the present time are coals, bitu- 

 minous shales, asphalt, bitumen, bituminous sands and clays, petroleum, 

 lignite, and peat. 



I. Coal. — When coal, as in the ordinary process of gas manufacture, 

 is submitted to distillation in closed iron retorts, three substances may 

 be said to be the result of that process. 1. Illuminating gas of complex 

 composition, consisting chiefly of gaseous vapour of hydro-carbons. 2. 

 Tar. 3. Coke which is left in the retorts. The quantity and composition 

 of these several products are dependent on several causes, but principally 

 on temperature and the nature of the coal. These circumstances will 

 guide the manufacturer in the selection of the conditions under which 

 he distils the coal, according to the products which he wishes to obtain. 

 The higher the temperature employed, the greater is the quantity of 

 volatile matter or gas which is produced. Therefore, in gas-works, when 

 it is the object to procure as much volatile matter as possible, the coals 

 are distilled at a high temperature, sufficient to decompose the oils. 

 These the manufacturer of photogen desires to preserve, and accordingly 

 distils his materials at a heat which does not destroy the liquid hydro- 

 carbons. The following is a general outline of the process usually 

 adopted for the preparation of illuminating oils and other substances of 

 economic value from coal : — 



The coal is distilled at a temperature of 700°, and the tar collected. 

 The latter substance is put into a large iron retort (which is of different 

 fashion in several manufactories), connected with a coil of iron pipes 

 surrounded with cold water, called the condenser. The retort is heated, 

 and a light oil or naphtha (sp. gr. 0.830) comes over, and is succeeded, by 

 VOL. IV. D 



