remarltable Properties of Iodine,, 8^-c. 211 



image upon a plate of copper or polished silver, upon which 

 it is pressed without any other preparation. This operation 

 is very readily effected, and will therefore be very useful to 

 the tool-engraver. 



On Nitric Acid. 



With nitric acid I obtained the following results. On ex- 

 posing an engraving (whatever may be the composition of tlie 

 black parts) to the vapour which is evolved by pure nitric 

 acid, afterwards la3'ing it upon a plate of silver or copper, and 

 leaving it there for some minutes, we obtain a distinctly visible 

 negative impression. The white parts are coated with a white 

 mist, the black are the pure copper. 



An oiled engraving, and figures drawn with charcoal or 

 black crayon upon white paper, yielded the same results. I 

 subsequently exposed a block made of white wood and ebony, 

 and found that the white band alone was copied. 



If an engraving is left exposed to the vapour of this acid 

 for a long time, the black parts are ultimately impregnated 

 like the white; and the plate of metal upon vvliich the engra- 

 ving has been applied is then covered with a uniform la^'er, 

 which presents no further trace of the drawing. 



An engraving will only serve to make one or at the most 

 two impressions ; after this it must be left exposed to the air 

 for twenty-four hours before it can be again used, and fre- 

 quently it ceases to reproduce its image. We thus see that 

 the action is not characterized in the same manner as in the 

 case of iodine and phosphorus. This vapour is deposited 

 equally upon the parts in relief and upon the depressions : 

 thus an oil-painting and embossed impressions or stamps with- 

 out ink are easily copied by this means. The same effects 

 occur with dry chloride of lime, but it requires to be gently 

 heated before exposing the engraving to the vapour which is 

 evolved from this substance, and which gives a negative im- 

 pression, like nitric acid. 



Appendix to the preceding Memoirs^: 



On expoiing the black and white feathers of birds (as those 

 from the wings of a magpie or the tail of the lapwing) to the 

 vapour of iodine, the black differed sensibly from the white; 

 and with the same feather I have made eight or ten impressions 

 upon copj)er, all of which presented a well-marked line of 

 demarcation between the black and the white parts. I after- 

 wards immersetl an engraving in tincture of iodine, and after 

 having produced several successive impressions upon starched 



* Conimimiciitcd to tlie Aciulcmy, Octol)cr 2."), 1817- 

 1'2 



