228 Intelligence and Miscellaneous Articles. 



rendered probable by observing that coins placed on a thick copper 

 plate seldom were in perfectly close contact, yet gave good spectra. 

 In order to come to a more definite conclusion on this point, I got a 

 lamina of bright copper, even thinner, and as elastic as the platinum 

 lamina above-mentioned. Gold or silver coins left twenty-four hours 

 on this, gave a spectrum scarcely visible, but on leaving a half-sove- 

 reign for two or three hours on it exposed to heat of 1 60°, as above, 

 and pressed down by exactly the same weight, the half-sovereign left 

 a permanent spectrum very well marked indeed. 



" The result of this experiment obviously shows, that, although 

 thinness and elasticity may have some little effect, the principal 

 cause for the formation of the spectrum is the peculiar chemical na- 

 ture of the metal, and that a spectrum cannot be produced on a non- 

 oxidable metal such as platinum. Bright silver and copper plates are 

 well known to tarnish by exposure to the atmosphere (the former 

 perhaps rather by forming a sulphuret than an oxide), but no matter 

 how. I have also found that spectra could be formed on tin and 

 zinc plates, both of which of course are oxidable. So on copper 

 coated with mercury, the mercury in such case no doubt readily tar- 

 nished (see section 7, Polished surfaces not receiving spectra, ante, 

 p. 226). Having decided that the effect in question is due neither to 

 light nor heat, to what cause it may be asked is it to be ascribed ? 



" Conclusions. — 1st. As brightness of the plate is indispensable, 

 and with brightness must exist an increased tendency to tarnish, 

 or enter into chemical combination ; 2ndly, as the plate must 

 be of an oxidable metal, and judging from the experiments with sil- 

 ver and copper the more oxidable the better ; 3rdly, as the more 

 perfectly the coins are cleaned and dried* the less the effect; and as a 

 dry perspiration (so to call it) must exist in a greater or less degree 

 on all coins, since they pass through so many hands, and as perspi- 

 ration is slightly acid ; 4thly, as even with clean coins the effectf by 

 actual contact must be admitted, but still is greater when there is a 

 difference in the nature J of the metal ; and 5thly, as when the metals 

 are not in contact (being removed only the one-twentieth of an inch 

 apart), no action or spectrum is evident, if the free circulation of 

 air, and the connexion with dust be prevented ; — taking all these and 

 minor considerations into account, we come to the conclusion that 

 the effect in question is dependent on a chemico-mechanical action, or 

 what Berzelius has called catalytic action. No doubt it may be urged 



* " Moisture much increases the effect. Thus when one surface of the 

 shilling was rubbed over with ink, and such surface put on the copper plate 

 and heated to 150", a mark much more difficult to be effaced was left than 

 when this degree of heat was applied without moisture." [Surely ordinary 

 chemical action was exerted in this case. — Edit. Phil. Mag.] 



-f- " This is equally true, as will be remembered, with regard to glass 

 plates." 



X " The general result of all the above experiments shows this, and of 

 course an alteration of affinity from contact is far more probable when me- 

 tals are different than when the same, though if one be dirty this makes it 

 approach the nature of a different metal." 



