Pru/tan Blue. [Book VI.* 



precipitate the metal of a blueifh grey colour, if the 

 folution is frefh, but if long kept, in the form of a 

 yellow powder. Mild vegetable alkali feparates a 

 yellow calx from the folution of iron in the nitrous 

 acid, which foon becomes of a beautiful red orange 

 colour. If the mixture is agitated during the eflfer- 

 vefcence, the precipitate is re-diflblved in much 

 greater quantity than by the pure vegetable alkali. 

 This folution is known by the name of Stahl's martial 

 alkaline tincture. 



But the moft remarkable precipitation is that pro- 

 duced by a fixed alkali prepared with animal inflam- 

 mable matter. The alkali is treated by mixing ir, 

 when dry, with twice its weight of blood, which has 

 been indurated and reduced to powder; the mixture is 

 put into a crucible, and a gentle heat applied ; parti- 

 cular care muft be taken to ufe the due degree of heat, 

 which is known by the difappearance of a blue flame 

 and fmoke, which is at firft obfervable on the furface. 

 The matter thus prepared, being infufed in water, 

 affords an alkaline folution, which precipitates iron of 

 a deep blue colour, efpecially if a little muriatic acid 

 is added. The efiential ingredients of the matter ob- 

 tained by the above procefs feem to be a peculiar matter 

 called prufllc acid, and an alkali. The bafis of the 

 pruffic acid, according to M. Berthollet, confifts of 

 carbon, hydrogen, and azote. When the alkali, com- 

 bined with this peculiar acid, is added to a folution of 

 iron, the alkali takes the acid from the iron, while the 

 pruflic acid unites with the metal and falls to the bot- 

 tom in the form of a blue powder. This effect of the 

 alkali, thus prepared, is the foundation of the procels 

 for obtaining that valuable pigment, known by the 

 name of Pruffian blue. 



Another fingular, and not lefs ufeful circumftance, 



in 



