340 



prussiateof 

 lime or po- 



hidroguretted 

 alkaline sul» 

 phurets. 



INDELIBLE WRITING INK. 



alkaline carbonates, cannot combine with the gallic acid to 

 form ink, unless the carbonic acid have been expelled from 

 the oxide of iron by some more potent acid. It is the 

 same with respect to the oxalic acid, and acidulous oxalate 

 of potash: when this acid or this acidulous salt has seized 

 the oxide of iron, the gallic acid cannot destroy the com- 

 bination, because it has an inferior attraction for the oxide 

 of iron. 



If the writing have been destroyed by nitric or oximu- 

 riatic acid, the gallic acid in tincture, infusion, or decoctioa 

 of galls will revive it. 



Liquid prussiate of lime or potasb is a good re- 

 agent, to detect the presence of iron. If the ink have 

 disappeared in consequence of the decomposition of gallic 

 acid, as when oximuriatic acid has been employed, either 

 tof these will render it legible, causing it to appear of a 

 fight greenish blue Awhile wet. If oxalic acid have been 

 employed to obliterate the writing, the prussiates will re- 

 store it of a reddish brown colour. If nitric or sulphuric 

 acid have been employed, the prussiate of lime will shovir 

 this by staining the paper blue, but it cannot reproduce the 

 Writing. 



Hidroguretted sulphurets of the alkalis, or of the al- 

 kaline earths, are very prompt and powerful tests of fer- 

 ruginous salts. The alkali, or earth, combines with the 

 acid; and the sulphuretted hidrogen with the oxide of iron, 

 forming an hidroguretted sulphuret of iron. Iron in the 

 State of red oxide is partly disoxidated by the hidrogen, 

 water is formed, and the iron passes to the state of black 

 oxide. This is the case with writing turned rusty: these 

 reagents immediately change it to a green black, much 

 deeper than gallic acid would give. A solution of sulphatd 

 of iron mixed with an hidroguretted sulphuret produces a 

 very deep green black ink. 



The same attractions are exerted when the hidroguretted 

 tests are applied where writing has been obliterated by the 

 oxalic acidule or the oximuriatic or nitric acid. If the 

 oxalic acidule were employed, the characters will reappear 

 of a green black or brown red. If the oximuriatic acid, 

 of a green black or pale rust colour. The less the revived 

 writing approaches a blacky the more the iron w^s oxided 



