230 Intelligence and Miscellaneous Articles. 



attraction of glass and oxidable metallic plates for dust, &c. is very 

 great, and is perhaps dependent on the same causes as their attrac- 

 tion for oxygen. Whether or not, I feel pretty well convinced, after 

 a laborious investigation of the discovery in question, that it is not 

 of that wonderful character that Moser and others have supposed ; 

 nor calculated to alter our ideas of vision or of the nature of light. 

 On the contrary, I think with Fizeau (a short notice only of whose 

 memoir I have seen), that no effect of any consequence is produced 

 where organic matters are carefully removed by boiling water and po- 

 lishing ; for such is perhaps the philosopher's opinion just named, 

 and in as far as our opinions agree he has the priority. Begun by a 

 purely catalytic action, it is only continued and developed in any 

 marvellous degree when those circumstances are present that permit 

 it to assume a more strictly chemical character." 



In the Athenaeum, No. 815 (1843, p. 557), our correspondent, 

 Mr. Hunt, has replied to Mr. Prater's observations : — " I am not," 

 he says, " satisfied that Mr. Prater has succeeded in proving that the 

 effect is due neither to light nor heat, and I must most decidedly ob- 

 ject to his conclusion, that the effect is due ' to a mechanico-chemi- 

 cal action, or a catalytic action ; meaning thereby an action so slightly 

 chemical as, in the present state of the science to be scarcely appre- 

 ciable/ I have shown, in a paper published in the Philosophical 

 Magazine for April [preceding volume, p. 270], that chemical decom- 

 position could be brought about, in some metallic salts, by the jux- 

 taposition of metallic plates ; that the iodides of copper and of gold 

 were reduced to the metallic state by being allowed to remain for a 

 few weeks under a copper plate with a well-amalgamated surface, 

 the salts and the plate being nearly a quarter of an inch apart. I 

 have since that time succeeded in decomposing many other salts in 

 a similar manner. These facts may appear to confirm Mr. Prater's 

 idea. We must of course consider the decomposition as a chemical 

 phenomenon : but this change is effected by the influence of an 

 agent which bears a strong analogy to light, but which is separated 

 from it by many broad distinctions. I am by no means wedded to 

 the opinion that heat is the active principle. That the phsenomena 

 described by Moser and myself are accelerated by the application of 

 heat, all my experiments render certain, and this is indeed admitted 

 by Mr. Prater. This gentleman has erred, as it appears to me, in 

 exposing his plates so long to the influence of a high temperature, 

 during which both the coins and the plates were heated in an equal 

 degree. I have shown, in the paper above alluded to, that all that 

 is necessary to prepare a metal plate for the reception of vapours on 

 defined spaces, is to disturb the equilibrium of the caloric latent in the 

 plate. Another point, of great importance in these investigations, is 

 entirely overlooked by Mr. Prater. In my paper on Thermography, 

 I have stated that the vapours of mercury and iodine attack the plate 

 differently. I find that many impressions which we cannot render 

 visible by breathing on the plates, or by exposing them to the vapour 

 of water, are developed with beautiful distinctness by the vapour of 

 mercury : others again, which remain invisible under the influences 



