above alluded to, where a little imperfection will be compen- 

 sated by the advantages it holds out. 



It is prepared from the substance called Bistre, the nature 

 of which I formerly investigated in a paper published in the 

 Transactions of the Geological Society. 



This substance is a compound of carbon, or charcoal, and 

 hydrogen, principally ; very analogous to the bitumens, but 

 differing from them in some particulars, which I need not 

 here detail. It is most conveniently procured from the de- 

 structive distillation of wood, and can now be obtained, at no 

 price, and in any quantities, from the distillers of charcoal^ 

 for the gunpowder manufactories, and from the manufacturers 

 of new vinegar. 



When thus obtained, it is a liquid resembling common tar 

 in consistence ; and it is this substance which forms the beau- 

 tiful brown varnish that covers the inside of a Highland cot- 

 tage ; being deposited from the smoke of the peat. On being 

 subjected to evaporation or distillation, it gives out an essen- 

 tial oil, either colourless or brown, according as the process is 

 managed, analogous to naptha and to petroleum, together with 

 acetic acid. Being thus treated, it becomes first tenacious, 

 like soft pitch, then hard or brittle, like asphaltum, with a 

 bright, clean, conchoidai fracture. The longer the heat is 

 continued, the more brittle it becomes, till at length it falls to 

 po"wder. 



The chemical change which here takes place is that of gra- 

 dually diminishing the hydrogen in the compound ; and, if 

 the process be pushed to an extremity, charcoal alone at 

 length remains. At the same time, the colour, which was at 

 first a yellow brown, becomes gradually darker, till it settles 

 into one that is nearly black. 



In a hard state, this is the bistre of artists, although that 

 substance is ignorantly obtained from the soot of wood, parti- 

 cularly beech, if possible ; being generally furnished to the 

 colourmen by the chimney-sweepers, who collect it from those 

 places where wood is burnt. For this reason it is a very un- 

 certain colour as to its tone ; while artists also know well that 

 it is often glutinous and disagreeable in use; adhering and 

 returning to the pencil, so as to render it difficult to put on 

 clear and repeated washes. 



This fault consists in the evaporation not having been car- 



