Mr. Hunt and Mr. Prater on Moseys Discovery. 231 



of the vapours of water or of mercury, are readily evoked by the 

 vapours of iodine, bromine, or by sulphuretted hydrogen gas. It 

 appears to me that some very curious relations are yet to be traced 

 out between the vapours of bodies and solids on which they may be 

 condensed." 



In No. 817 of the Athenaeum (1843, p. 598), Mr. Prater replies 

 to Mr. Hunt in the following terms : — 



" Moser's Discovery. — It is easier to demolish old theories than 

 establish new ones ; and though I must continue to think the expe- 

 riments detailed in my essay on Moser's discovery prove the effect 

 is neither due to heat nor light, I did not presume to say they proved 

 the correctness of the chemico-mechanical theory proposed by my- 

 self. That theory seems to me the most satisfactory one hitherto 

 advanced ; but perhaps future experiments may require it to be mo- 

 dified. At present however I have met with no experiments whose 

 results are incompatible with such theory. I have lately looked over 

 the papers of Prof. Draper and Mr. Hunt in the Philosophical Ma- 

 gazine, published during the last three or four months ; but as in 

 none of these experiments interposed substances or screens were 

 used, nor the necessary precaution of boiling as well as polishing 

 used, I cannot see that either Fizeau's or my own experiments [are] 

 proved, in any way, inconclusive by the results obtained by the gen- 

 tlemen just mentioned. Mr. Hunt did a great service to the subject 

 by his discovery of the power of heat to increase the effect ; but as 

 he has not shown he can decompose the iodides of copper or gold by 

 mercurial plates placed nearly a quarter of an inch above them, when 

 a screen is interposed, the probability is that the iodides merely vo- 

 latilized (as Mr. Faraday some time ago proved, even mercury itself 

 as well as many other substances, to do) at the common temperatures 

 of the air. The effect then in question seems to be a mere chemical 

 action produced by direct contact. If future experiments prove it 

 can be done without such contact, it will then be time to think about 

 some new mysterious agency. But even admitting Prof. Draper's 

 very valuable experiments to countenance such an agency (at pre- 

 sent doubtful, as Mr. Hunt himself has well attempted to show), I 

 do not see we are at all nearer proof that such agency is concerned 

 in Moser's images ; fob these cannot be taken at any distance 



FROM THE PLATE WHEN POLISHING, BOILING, OR SCREENS are Used ; 



a fact that was not known when Prof. Moser or Mr. Hunt wrote 

 their essays. 



" I did not use mercury or iodine in my experiments, because I 

 believe results equally satisfactory may be got without them if we 

 lengthen the time of experiment." 



On the subject of the foregoing discussion see also our preceding 

 volume, p. 324 ; and Scientific Memoirs, part xi. 



OBSERVATIONS ON M. MILLON's MEMOIR ON NITRIC ACID. BY 

 M. GAY-LUSSAC. 



The researches of M. Millon were printed in a pamphlet of 47 



