COPIES OF WRITTEN PAPER BY PRESSURE. 131 



No. 3. A pale ink formed with green* sulphate of iron 

 and solution of tan, having excess of sulphate. 



No. 4. An ink as above with excess of tan. 



Pour on the writings hidrosulphuret of ammonia; Nos. 

 1 and 4- will remain unchanged, Nos. 2 and 3 will instantly- 

 become intensely black, but No. 2 changes by exposure to a 

 rusty brown, and No. 3 becomes faint. 



If hidrosulphuret of ammonia is poured into solution of sul- 

 phate of iron, a black powder precipitates; when this is ex- 

 posed to the air it turns to a red rust. 



Although the hidrosulphuret appears to have no effect on 

 dry tannate of iron, yet when poured into the ink No. 4, it 

 changes it to a red colour ; writing written with this mixture 

 becomes nearly black in an hour. When filtered, a red sub- 

 stance remains, and the filtered liquor is of the same colour. 



I have somewhere seen it asserted, that ink consists of a. , -. . 



Ink not simply 

 black powder suspended in water, so extremely fine as to pass ablackpowdei 



with the liquor through a paper filter; this is not exactly sus P ended m 



the case. 



If ink, prepared as No. 3, be exposed to the air a short time 

 -and filtered, a black mass remains on the filter, and the li- 

 quor that passes through is of a fine deep blue colour: if a 

 drop be let fall from the filter on a piece of ivory, and exa- 

 mined immediately, it will appear a homogeneous liquor, but 

 in the course of a minute numerous black particles will 

 be seen floating in it. 



These effects arc best perceived with a glass. Some of the 

 filtered ink placed in a wine glass is speedily covered with a 

 film; on shaking the glass, black pieces will be seen in the 

 apparently colourless liquid, that trickles down the sides. 



From this it is evident, that new ink consists of at least two Asolubleblue 

 substances, one soluble in water, and communicating to it a and insoluble 

 dark blue colour, the other an insoluble black powder. stance ;" * 



* This was the green -vitriol of commerce boiled on iron filings to de- 

 prive it of any excess of acid, and to bring it to a minimum of oxigen j 

 but I do not know whether it was exactly in that state. The ink is of a 

 blue colour, and passes through the filter without leaving scarce any resi- 

 duum : the writing written with it is at first excessively pale, but gradu- 

 ally becomes black. 



It 



