27-4 Tke Knowledge of Fossil Shells 



equal in freedom and force to seppia. Bv distilling 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 tlie 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 treat- 

 ment in alcohol, results in some measure similar are produced, 

 and the residiuun of tliis solution is equal in colour to seppia, 

 and totally void of tenacitv. 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 qnalitv might not be produced by the di- 

 stillation of different woods. That which I used was jirocured 

 Vither from willow or alder — the tu-o woods cliiefly used in the 

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

 Tlie solution in Ifxivium of potash or of soda, a substance ana- 

 logous to the resinous soaps, answers the pin pose of ink, pos- 

 ses>ing a colour sufliciently intense and fiowiug 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 

 l)ractical 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. 



XLIX, Anearnesl Remnniendathn to mtrlous Ladies and Gen- 

 tlemen res'id'nig or visiting in. the Country, to examine t/ie 

 Quarries, Ci>ffs, steep Banh, &fc. and collect and preserve 

 Fossil Shells, as kighly curions Objects in Cvnctiology, and, 

 OS 7nost important Aids in identifi/ing Strata in distant Places; 

 u/i which Knowledge the Progress of Geology in a principal 

 degree, if not entirely, depends. By a Cokrksi'onuent. 



To Mr. Tilloch. 



SiK, — Xjaving long been a constant Reader of your very use- 

 ful Magazine, 1 have been induced by what I have therein read 

 cg;u:ding Gcologv, to become a reader and very zealous promoter 



of 



