ried far enorgh ; and, from the foregoing sketch of its na- 

 ture, it will also be seen that the yellowish varieties will be 

 the most disagreeable in working. The obvious remedy for 

 these oils is to evaporate the oil and the acid ; and that eva- 

 poration may be regulated to the greatest nicety, by adopting 

 the liquid tar of the charcoal and vinegar distilleries. Thus 

 it may be procured of the colour of sepia ; which indeed it 

 may be made to rival in use, as it is the same substance, con- 

 sidered as a chemical compound. 



There is one other objection to the use of bistre, in water- 

 colour painting, and that is its powdery nature; as it is, 

 like other cpiours, merely suspended in gum. This evil is 

 remedied by the same process which converts it into an ink. 



It is soluble in the pure alkalies, both potash and soda ; 

 with which it forms a compound analogous to soap, and which 

 is in fact the &ame, very nearly, as that which common rosin 

 forms with these in that mixture of this substance with tallow 

 which produces what is called brown soap. 



But it musfcbe remarked, that the combination with potash 

 remains liquid, if not too far evaporated, while that with soda 

 gelatinizes, even, when much water is present. The process 

 itself is extremely simple, as it consists merely in boiling the 

 bistre in the alkaline solutions, taking care that they are fully 

 saturated. The degree of dilution may be regulated at any 

 time, according to the uses for which the colour is intended. 



It is not easy to give precise directions for the state in which 

 the bistre ought to be used, for want of a scale of reference, 

 But it cannot well be too brittle and too dark in the colour, 

 provided it has not been so far evaporated as to destroy its 

 solubility. To bring it to the condition of asphaltum, is per- 

 haps a good general rule. 



In a fluid state, this is the indelible ink in question. There 

 is no powder here, as the bistre is in a state of solution, and 

 it requires neither gum nor any other addition. It remains 

 unchanged in a bottle, never depositing its colour, like com- 

 mon ink. It flows freely through the pen, and can be used 

 to write as rapidly as ordinary ink. It is so incapable of 

 change as to resist even oxymuriatic gas ; nor is it affected 

 by any exposure to damp. I may add, that I have kept a 

 manuscript for ten years, and more, exposed to the vapours 

 of a large laboratory, where acid was always present, and 



