212 M. Niepce (!e Saint-Victor on some 



paper, I ultimately obtained a positive and perfectly distinct 

 impression, just as if I had used the vapour of iodine: the same 

 occurs if the engraving be immersed in an aqueous solution 

 of iodine. 



I must not omit to n)ention, that in copying an engraving 

 all the black or coloured points which almost always exist in 

 the substance of the paper are copied in the same manner as 

 the lines of the engraving ; in such cases they must be removed 

 by touching them with animonia, or some other means. 



Before leaving the positive impressions to pass to the 

 negative, I may state that I obtained the same results with 

 iron pyrites as with the sulphuret of arsenic; the latter is how- 

 ever preferable on account of the facility of execution of the 

 process, and because it leaves no mark upon the engraving. 

 'J'hese impressions resist the action of nitric acid. 



1 have also obtained a positive impression with the bichlo- 

 ride of mercury. If the design on the copper be exposed to 

 the vapour of ammonia, it appears much more distinct, and 

 is well-fixed. 



I shall now 'allude to the negative impressions which are 

 obtained by means of substances possessing the power of being 

 deposited upon the white parts of an engraving in preference 

 to the black, as nitric acid. The new results obtained were 

 as follows: — I immersed printed characters in pure nitric acid 

 (taking care to withdraw them immediately) ; 1 laid them upon 

 a plate of copper, and on removing them after a certain time, 

 found the characters in relief resembling a page of type. 



if an engraving is immersed in water acidulated with nitric 

 acid, and allowed to dry until but little moisture is left, and 

 is subsequently laid upon a metallic plate, we almost always 

 obtain a very evident negative impression ; and when this is 

 not the case, simply blowing upon the plate is sufficient to 

 make it visible. A black and while feather treated in the 

 same way, also yielded an impression in which the white part 

 only was copied; a result the reverse of that obtained by im- 

 pressing upon the metal a feather after exposure to the vapour 

 of iodine. Hydrochloric acid produces almost the same effect 

 as nitric acid ; but the latter is much preferable. 



I have already stated that chloride of lime (hypochlorite of 

 lime) produces a negative impression when an engraving is 

 exposed to the vapour evolved from it, an opposite result to 

 that ohtaineil with chlorine. The impression is also negative 

 if we plunge an engraving into solution of chloride of lime, 

 whilst it is positive when immersed in pure chlorine. When 

 an engraving is placed in contact with chloride of lime dis- 

 solved in water or the vapour which it exhales by its heat, on 



