INDELIBLE WRITING 1N4» 241 



in the metallic salt decomposed, or the less the iron was dis- 

 oxided by hidrogen. The writing on which nitric acid has I»»^ifations of 

 acted strongly cannot be reproduced: but on passing snl-J^^^J^f^y^-^j^ 

 phuretted hidrogen over the paper where it was, waTingacid. 

 iincs of a green black will be formed on the remotest parts 

 to which the sulphuretted hidrogen penetrates. These lines 

 may be produced in great number, and in different direc- 

 tions. They are owing to the sulphuretted hidrogen com- 

 bining with the oxide of the ferruginous nitrate. If the un- 

 dulating lines, or the letters that haye been restored, shonld 

 disappear, they may be reproduced by dipping the paper into 

 cold water. Beside the traces of writing, and the nndnlat- 

 ing lines just mentioned, the paper takes a yellow colour 

 wh€^ it is not impregnated with an acid, and a green more 

 or less deep when it is. The green colour will be deeper, 

 in proportion as the acid was stronger, or in larger quantity. 

 In all cases the paper retains the colour of fresh butter after 

 it is dry. The hidroguretted sulphurets should be diluted 

 'with half or two thirds their quantity of water before they 

 are used, as in their ordinary state they are too strong. 



From what has been said, we may hope to restore writ- Method of re- 

 ing, that has been obliterated by any agent except the nitric ^^^'^JJ^j^J^J^^^ 

 acid ; and if this have been employed only in small quantity, 

 without the assistance of any other acid, audits action has 

 not been t«o long continued, on holding the paper to the 

 fire the writing will reappear of a rust colour. 



With regard to the improvement of ink, little progress Improvement 

 has been made since the time of Lewis. Inks made by in- ® ^^ • 

 fusion, and with green sulphate of iron, are of a Prussian 

 blue colour, light, pale when written with, but growing 

 black as t\\ey dry on the paper. Those made by decoction 

 are blacker, thicker, and form a more copious sediment, 

 which is of a dirty Prussian blue colour. Decoction ex- 

 tracts from galls all the soluble parts ; infusion takes up 

 chiefly the gallic acid, aad mucilage, with a little extract 

 and tannin. In the decoction the iron of the green sulphate 

 becomes more oxided, and the extract and tannin acquire 

 oxigen, by absorption from the atmosphere; and the iron ' 

 in a higher state of oxidation, and the oxigenized extract, 

 produce a deeper black with the gallic acid and tannin. 



Thft 



