226 Dr. Mac Culloch on the Lignites. 



relating to the chemical composition of vegetahle matter and of 

 peat ; but it is necessary, in practice, to adopt a more compendi- 

 ous method of estimating the nature of the compounds subjected 

 to investigation, particularly where the lignites are to be examined 

 for the purpose of discovering the condition in which they stand 

 relatively to peat, placed at one of the extremities, and coal, stand- 

 ing at the other. It was mentioned that a resinous matter was 

 obtained from wood or peat, on distillation, which is distinguished 

 by the name of bistre ; but that substance is obtained, in the first 

 instance, only in combination with such proportions of hydrogen and 

 oxygen as materially alter its character ; or, rather, it is procured 

 as the residue of other distillations, by which the compounds of a 

 fluid nature, first procured, are decomposed. A certain proportion 

 of acetic acid and carburetted hydrogen being freed, according to 

 the quantity of heat and the mode in which it is allowed to act on 

 the first volatile products, a liquid matter resembling tar is pro- 

 cured ; which, on subsequent distillation, gives over a volatile oil 

 and acetic acid, the resinous matter remaining in the retort. By 

 still urging the heat, additional quantities of tar are produced, 

 and thus the process may be continued, until charcoal alone at last 

 remains ; the oxygen as well as the hydrogen being at length en- 

 tirely dissipated, first in the compounded states already indicated, 

 and ultimately in the form of gases. 



The whole of this process presents an exact analogy to that 

 which happens in the case of turpentine and of the bitumens ; and 

 it illustrates the nature of both these substances, of which the 

 composition and general habits so much resemble each other. It 

 is unnecessary to dwell on these differences with chemical minute- 

 ness, but it may be stated generally, that they appear to consist 

 in the different proportions of the several ingredients in these 

 analogous compounds, as far, at least, as this species of analysis 

 enables us to discover at present. Now, as the bistre, the tar, and 

 the volatile oil of wood, and of peat, resemble the resin, turpen- 

 tine, and oil, of all the resinous compounds, so they are precisely 

 analogous, in their relations to each other, to the asphaltum, 

 petroleum, and naphtha of the bitumens. As it is with this latter 



