fjS On rendering Writing indejlru&ibh by Fire. 

 while the oxydated bifmuth was converted into metal *; 

 The invifible letters made with a folution of this metal can 

 be read alfo, if the paper be dipped into pure water. By 

 this fimple procefs the characters appear of a white colour, 

 as the paper becomes fomewhat darker. The writing, how- 

 ever, may be read better if the paper be then held to the 

 light, becaufe the paper, which becomes fomewhat tranf- 

 parent, fuffers the characters, which are altogether dark, to be 

 feen. 



I thought iturinecefiary tomakeahy experiments with dif- 

 folved arfenic and quickfilver, becaufe their oxydes eafily 

 fuffer the oxygene to efcape ; fublime in the fire, and thert 

 entirely fly off from the burning body. 



Of all the metallic falts, I found only nitrite of copper and 

 zinc fit for my propofed object ; for the calces of thefe metals 

 Continue fixed in the fire, and fhew themfelves afterwards in 

 durable and bright colours. 



The characters which I wrote with diffolved nitrite of 

 copper were exceedingly vifiblc, of a copper colour on the 

 charred paper, and this colour Can be well diftinguifhed on a 

 dark ground. When I wrote with diflblvecl ammoniacal 

 copper, inftcad of nitrite of copper, the characters afmmed 

 in the fire a dark red colour, which approached that of cin- 

 nabar. Diffolved nitrite of zinc, however, is the beff. of all 

 the fubflances with which I made experiments, becaufe 

 characters written with it on paper prepared by liquor oi 

 flint were legible ; and it i;. therefore preferable to folutions 

 of copper, or of any other fubftance. 



When I wrote with diffolved nitrite of zinc, the characters 

 Were not vifible ; but when the paper was burnt they were 



* To obferve this phenomenon better, it will be fufheient to write on a 

 leaf of paper which has been impregnated with fal-ammoniac, and then 

 to burn it over the flame of a candle. The charred paper remains frftooth : 

 but at the places where the characters were written a beautiful fmall 

 border of regulus of bifmuth is fcen, fo that when the paper is held to the 

 light many of the words written upon it may be read. 



fo 



