228 Dr. Mac Culloch on the Lignites. 



varieties ; and that, in all, it is different from those, either of coal, 

 or of peat, or of the ligneous fibre. If the sense of smell is really 

 capable of thus analyzing compound odours, it appears to be a 

 mixture of those derived from both these sources ; and it is at least 

 abundantly sensible that the odour of burning jet is not far dif- 

 ferent from that of coal ; while that of the pale lignites is of a very 

 different nature, as if from a predominant quantity of the smell of 

 wood-tar, or the volatile oil of the distilled ligneoifs fibre. 

 • The differences in the relative proportion of acetic acid, pro- 

 duced by the different lignites, is also remarkable, as it diminishes 

 exactly in the degree to which they approach to coal, be'nfg 

 greatest in the pale varieties, and least in jet; and indicating, 

 without further analysis, the different proportions in which oxygen 

 is contained in the several kinds. Lastly, an easy test of the 

 degree to which the process of bituminization has advanced in the 

 lignites, is to be found in the nature and properties of the pure 

 volatile oil which they yield on distillation. 



The distillation of coal, or petroleum, with a regulated heat, is 

 well known to produce naphtha, a colourless volatile oil, which, as 

 it does not act on potassium, is presumed to contain little or no 

 oxygen. By distilling the tar of wood, or wood itself, with simi- 

 lar care, an analogous oil is produced, colourless also, but dis- 

 tinguished from the former, not only by its very peculiar odour, but 

 by its chemical properties. Although these two oils are miscible in 

 any proportions, yet while naphtha is a ready solvent of the 

 asphaltum, which remains after it is separated from the petroleum, 

 it will not dissolve bistre, or the analogous solid product which 

 remains after separating the oil of wood from the vegetable tar. 

 In the same manner the oil of wood is not the proper solvent of 

 asphaltum. Both these substances are, however, soluble in the 

 mixed oil, of which the smell is also very peculiar, and after some 

 experience, easily recognised. 



Now, in distilling the lignites, it will be found that the volatile 

 oil differs in smell, both from naphtha and the oil of wood, and 

 that the odour is not unlike to that produced by mixing them, 

 while, at the same time, it is capable of dissolving a certain pro- 



