in reply to his Animadversions. 425 



in an exceedingly minute quantity," that it will condense va- 

 pours irregularly. But in the communication which I made 

 to the British Association at Cork, I stated several experi- 

 ments, which appear to me to prove that the images are pro- 

 duced quite independently of any layer of organic matter. 

 Copper plates were polished with water only and boiled, and 

 all the things placed on these plates were boiled also, yet very 

 perfect images were produced. I have since tried the effect 

 of exposing the plates, &c. separately to a very strong heat, 

 and when cold placing them in contact; there has been no 

 apparent difference between the images formed under these 

 circumstances, and those formed upon plates, and with medals, 

 &c. which have been purposely covered with very slight films 

 of organic matter. I have also repeated these experiments 

 many times, in the best vacuum which could be maintained 

 with a good air-pump, to avoid the action of any vapours on 

 the metal or glass plates, and in every case the images have 

 been well defined. I cannot imagine any volatile film of the 

 kind described, exerting an influence to so great a depth into 

 the solid metals as I find to be the case. Often I have, by 

 polishing, removed layers of metal, and yet the images have 

 been reproduced by exposing the plate to vapour; and in pa- 

 ragraph 8 of my paper on Thermography, above alluded to, 

 I have stated the result of an experiment in which electricity 

 reproduced a succession of images which had been obliterated 

 from the copper plate. 



It has been suggested that electricity may be engaged in 

 the production of these spectral figures. I have just tried an 

 experiment which appears to show the probability of this ele- 

 ment's being involved in some way in these very complicated 

 phaenomena. I arranged four electro-positive metals, nickel, 

 bismuth, cadmium and silver, and two electro-negative ones, 

 arsenic and antimony, on a copper plate, and they were allowed 

 to rest upon it for three hours. Being removed, the plate was 

 submitted to the vapour of mercury. The space covered by 

 the nickel was marked, by being left free of vapour ; that on 

 which the cadmium lay was still more decidedly marked in 

 this way; where the bismuth was placed the image was ex- 

 ceedingly faint, but still it was observable by a deficiency of 

 vapour ; and the silver was more decidedly outlined with va- 

 pour, but none on the spot it covered. On the contrary, the 

 space occupied by the antimony was covered in a most remark- 

 able manner with vapour, presenting a perfectly white spot, 

 which in all positions distinguished it from the other parts of 

 the plate, whilst the arsenic left no trace behind. 



I think I have now shown, that many different causes may 



