VOL. LXXVII.] PHILOSOPHICAL TRANSACTIONS. 353 



formed by these two substances very much exceeds that of the iron alone, the 

 bulk of colouring matter would thus be greatly augmented. M. Bergman was 

 of opinion, that the blue precipitate contains only between a 5th and a 6th part 

 of its weight of iron ; and though subsequent experiments tend to show that, in 

 some cases at least, the proportion of iron is much greater, yet on the whole it 

 is certainly true, that if the iron left by the stroke of a pen were joined to the 

 colouring matter of phlogisticated alkali, the quantity of Prussian blue thence 

 resulting would be much greater than the quantity of black matter originally 

 contained in the ink deposited by the pen ; though perhaps the body of colour 

 might not be equally augmented. To bring this idea to the test, I made a few 

 experiments as follows. 



The phlogisticated alkali was rubbed on the bare writing, in different quanti- 

 ties ; but in general with little effect. In a few instances however it gave a 

 bluish tinge to the letters, and increased their intensity, probably where some- 

 thing of an acid nature had contributed to the diminution of their colour. 



Reflecting that when the phlogisticated alkali forms its blue precipitate with 

 iron, the metal is usually first dissolved in an acid, I was next induced to try the 

 effect of adding a dilute mineral acid to writing, besides the alkali. This 

 answered fully to my expectations ; the letters changing very speedily to a deep 

 blue colour, of great beauty and intensity. It seems of little consequence as to 

 the strength of colour obtained, whether the writing be first wetted with the 

 acid, and then the phlogisticated alkali be touched on it, or whether the process 

 be inverted, beginning with the alkali ; but on another account I think the 

 latter way preferable. For the principal inconvenience which occurs in the pro- 

 posed method of restoring mss. is, that the colour frequently spreads, and so 

 much blots the parchment as to detract greatly from the legibility ; now this 

 appears to happen in a less degree when the alkali is put on first, and the dilute 

 acid is added on it. The method I have hitherto found to answer best has been, 

 to spread the alkali thin with a feather over the traces of the letters, and then 

 to touch it gently, as nearly on or over the letters as can be done, with the di- 

 luted acid, by means of a feather, or a bit of stick cut to a blunt point. Though 

 the alkali has occasioned no sensible change of colour, yet the moment that the 

 acid comes on it, every trace of a letter turns at once to a fine blue,* which 



* The phlogisticated alkali (which is to be considered simply as a name) appears to consist of a 

 peculiar acid, in the present extensive acceptation of that term, joined to the alkali. Now the theory 

 of the above-mentioned process I take to be, that the mineral acid, by its stronger attraction for the 

 alkali, dislodges the colouring (Prussian) acid, which then immediately seizes on the calx of iron, 

 and converts it into Prussian blue, without moving it from its place. But if the mineral acid be put 

 on the writing first, the calx of iron is partly dissolved and diffused by that liquor before the Prussian 

 acid combines with it ; whence the edges of the letters are rendered more indistinct, and the parch- 

 ment is more tinged. The sudden evolution of so fine a colour, on the mere traces of letters, aftbrds 

 an amusing spectacle. — Orig. 



VOL. XVI. Z Z . 



