"xhich are produced upon it by Paper and Parchment. 113 



intensity of colour. This fact enables us to explain the cir- 

 cumstance, that ink which does not contain gum becomes of 

 as intense a colour as that in which it is present, after being 

 written with upon paper, — the paper serving the same pur- 

 pose of affording a substance with which the ink may combine 

 and develop the colouring properties of it. 



Ink is considered as a compound of tannin and gallic acid 

 with sulphate of iron, but I believe it does not contain any 

 tannin whatever. It has been already remarked, that a copious 

 precipitate falls down when ink is made ; this appears to con- 

 tain the tannin. When a solution of gelatine is added to a 

 decoction of galls so as to precipitate the tannin, the fluid still 

 affords an equal quantity of ink with sulphate of iron, and 

 without the Ibrmation of any additional precipitate. When 

 a decoction of galls is kept exposed to the action of the at- 

 mospheric air, oxygen is absorbed and carbonic acid gas is 

 extricated ; the decoction loses its astringent taste, becomes 

 acid, and no longer precipitates gelatine; the tannin has in fact 

 been converted into gallic acid. The addition of sulphate of 

 iron does not now cause a precipitate till after the interval of a 

 day or two, and then is very scanty. I infer therefore that it 

 is a trijile compound, a gallo-sulphate of protoxiile of iron. 



In consequence of the change which, galls thus undergo, 

 the quantity of ink which they afford is increased nearly three- 

 fold; 413 grains of galls require fcr combination 144 grains 

 of sulphate of iron, but when the tannin has been converted 

 into gallic acid the same quantity requires 336 grains. 



\A'hen the persulphate of iron is combined v.ith a decoc- 

 tion of galls, an intense blue compound results, but after a 

 short time the colour changes to a dirty green. It is unfit for 

 any practical purpose ; but it deserves notice, because it en- 

 ables us to ascertain the requisite quantity of protosulphate 

 v.'hich a given quantity of galls requires. The persulphate is 

 made by conibiiiing the protosulphate with nitric acid. For 

 this purpose expose Gi minims of the latter and an ounce of 

 the Ibrmer together, using an increase of temperature as long 

 as the fumes of nitric oxide gas are extricated. This com- 

 pound may for the sake of convenience be dissolved in a cer- 

 tain proportion of water, and after the subsidence of the super- 

 abundant oxide is fit for use. Having made a solution of galls 

 or gallic acid, adtl of this solution gradually as long as it causes - 

 ati increase of colour, which, owing to its great intensity, rc- 

 (|uires very considerable caution. The method I have ibund 

 to answer best, is to agitate the compound in a glass vessel so 

 us to wet the sides of it, and while wet to touch it with a wire 

 (lipped in the ))ersulphate. The tpiantity of persulphate re- 



New Series. Vol. 2. No. 8. y///i;. 1827. ' Q quired 



