Mr. Prater on Moser's Discovert/. 229 



against this view, that the action takes place when the coins and 

 plate are both heated, and hence quite dry. But this is no solid 

 objection, for the adage ' corpora non agunt nisi sint soluta ' is not 

 true, as hundreds of examples in chemistry show. The very fact of 

 heat itself increasing the effect is all in favour of a chemico-mecha- 

 nical view, for heat increases the tendency of copper to oxygenation, 

 and tends also to volatilize any feeble acid matter on the coins. But 

 again, if it be said the spectrum rubs off, even when permanent and 

 clearly defined (as we have shown), and leaves a polished surface under 

 it, — this we admit ; but still this surface has suffered an almost im- 

 perceptible degree of oxygenation ; for so slowly does this effect take 

 place, that it is only visible when much advanced, as will be evident 

 to any person who watches the gradual tarnishing of copper plates. 

 Moser's discovery shows that very slight chemical action is often 

 going on, which has been previously overlooked. 



"The chief difficulty that occurs to the above view is, that the 

 effect takes place to a slight extent on glass : but in all my nume- 

 rous experiments I have found that the effect is much less on glass 

 than on well-polished copper ; for in no case has a permanent spec- 

 trum been made on glass even by the longest contact*. It will also 

 be remembered that I found no effect whatever produced on talc. Now 

 the talc scratches easily, glass of course does not ; but talc is pro- 

 bably less soluble in acids than glass ; at least in my trials it did not 

 seem at all acted on either by nitric, muriatic, or sulphuric. To be 

 sure, you perceive no effect of these on glass, but it does not seem 

 impossible but that some very slight effect takes place, and that the 

 alkali of the glass is very feebly acted on, as glass is a compound 

 body. Contact, at all events, may be presumed to have an influence 

 on the affinities of one of its elements, whether there be even the 

 slightest degree of decomposition or not. Now this influence is the 

 catalytic influence ; for it has been shown above, that without actual 

 contact, and when all dust is kept off, neither silver nor copper, even 

 at the one-twentieth of an inch from the glass plate, produces any 

 effect, though kept there ninety- six hours. In consequence of this 

 slight alteration in affinity, the parts of glass which have been in 

 contact some time with coins or other substances, condense the breath 

 differently from those parts which have not : hence the spectrum. 



" The effect of glass, supposing it not susceptible of a gradual change 

 by the action of air similar to oxidation, is rather in favour of the spec- 

 trum depending on a mechanical than a chemical action. I have in 

 consequence ascribed the effect to a mechanico-chemical action, or a 

 catalytic action, meaning thereby an action so slightly chemical as, 

 in the present state of the science to be scarcely appreciable f. The 



* " A permanent spectrum has been proved (see experiments) to be but 

 a higher degree of an evanescent one." 



f " In coming to this conclusion I have not forgotten another difficulty 

 viz. why a vrcW-^polished and boiled copper produces a spectrum on copper 

 plate. The effect, even when continued an hour or two at a heat of 160° is 

 very slight, and I found it to disappear entirely by mere breathing on the 

 plate. Contact then of the same metal slightly modifies chemical properties ; 

 such on the present view is the inference to be drawn from this fact." 



