23 Dr. Mac Culloch on certain products^ ^c, 



or evaporating the oil from the pitch, according to the process des- 

 cribed above, a colour may be produced varying in tone from the 

 warmest bistre brown down to black. At the same time the sub- 

 stance loses a great portion or the whole of its disagreeable tenacity, 

 according to the degree of boiling it has undergone. By treatment 

 in alcohol, results in some measure similar are produced, and the 

 residuum of this solution is equal in colour to seppia, and totally void 

 of tenacity. In either or both of these ways may the quality of this 

 colour be improved. 



It might perhaps be a matter worthy of trial, whether useful 

 varieties in colour and quality might not be produced by the distilla- 

 tion of different woods. That which I used was" procured either 

 from willow or alder — the two woods chiefly used in the royal 

 powder mills, but I cannot ascertain from which of them. The 

 solution in lixivium of potash or of soda, a substance analogous to 

 the resinous soaps, answers the purpose of ink, possessing a colour 

 sufficiently intense and flowing freely from the pen without re- 

 quiring gum. As it is indestructible by time, by the common acids 

 or by the alkalies, perhaps it may be found a valuable substitute 

 for this useful but fugacious substance. The compound of bistre 

 and soda appears peculiarly well fitted for drawing in monochrome, 

 since as it does not consist of a powder suspended in a vehicle, it is 

 free from the peculiar defects, so well known to artists, which occur 

 in colours thus compounded. 



1 may also add that it forms a substitute for asphaltum in drying 

 oil where such a coloured varnish is wanted, and that it makes a very 

 good japan varnish for metal if dissolved in spirit of wine, and 

 heated strongly after its application. It is for practical men to see 

 whether by combining it with asphaltum, lac, or the gu ns, some 

 more useful and cheap compounds of .his sort may not be produced. 



