454 Dr. Blagden’s Obfervations on 
cipitate formed by thefe two fubftances very much exceeds that 
of the iron alone, the bulk of colouring matter would thereby 
be greatly augmented. M. Bergman was of opinion, that the 
blue precipitate contains only between a fifth and a fixth part of 
its weight of iron ; and though fubfequent experiments * tend 
to fihew that, in fome cafes at leaft, the proportion of iron is 
much greater, yet upon the whole it is certainly true, that if 
the iron left by the flroke of a pen were joined to the colouring 
matter of phlogifticated alkali, the quantity of.’Pruffian blue thence 
refulting would be much greater than the quantity of black mat- 
ter originally contained in the ink depofited by the pen ; though 
perhaps the body of colour might not be equally augmented. 
To bring this idea to the teft, I made a few experiments as 
follows. 
The phlogifticated alkali was rubbed upon the bare writing, 
in different quantities ; but in general with little effect. In a 
few in fiances, however, it gave a bluifh tinge to the letters, 
and increafed their infenfity, probably where fomething of an 
acid nature had contributed to the diminution of their colour. 
Reflecting that when the phlogifticated alkali forms its blue 
precipitate with iron, the metal is ufually firft diflolved in an acid, 
I was next induced to try the effeCt of adding a dilute mineral 
acid to writing, befides the alkali. This anfwered fully to my 
expectations ; the letters changing very fpeedily to a deep blue 
colour, of great beauty and intenfity. It feems of little confe- 
quence as to the ftrength of colour obtained, whether the wri- 
ting be firft wetted with the acid, and then the phlogifticated alkali 
be touched upon it, or whether the procefs be inverted, begin- 
ning with the alkali; but on another account, I think, the 
* Crell Beytrage, B. I. ft. i. p. 42, &c. 
latter 
