Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/38

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Dr. Mac Culloch on certain products, &c.

or evaporating the oil from the pitch, according to the process described above, a colour may be produced varying in tone from the warmest bistre brown down to black. At the same time the substance 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 distillation 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 requiring 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.

I 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 gums, some more useful and cheap compounds of this sort may not be produced.