Mr. Prater on Moseys Discovery. 227 



cave*, with the same result. Each substance left its spectrum, 

 the part where the coin rested on such layer not being at all distin- 

 guishable. The spectral image of the square piece of talc was per- 

 fect to the minutest outline, and left its straight mark under the 

 sixpence equally well as at other points. These experiments render it 

 clear that the effect is not due to latent light, for otherwise how could 

 it happen that a coin does not leave a spectral image when left on 

 transparent substances, glass or talc, even a fortnight ? They also 

 show it does not depend on heat (at least alone), for a heat of 

 1 60° soon passed through thin glass and talc, and I found it impos- 

 sible to keep my finger on glass or talc so placed. Yet we have seen 

 above that even gold left two hours on talc so heated left no spectrum, 

 permanent or temporary. So great is the effect of interposed sub- 

 stances, that even a slight tarnish on the metal exerts a very obvious 

 effectf. One shilling was left twenty-four hours on a polished part 

 of the plate, and another on a part of the same slightly tarnished 

 (but yet sufficiently bright to see oneself perfectly). A very slight 

 image only was left in the last case, that entirely disappeared when 

 breathed on twice, while that on the polished part of the plate re- 

 mained after being breathed on twelve or fourteen times. 



" A sovereign left twenty-four hours or above, tarnished, gave 

 scarcely a perceptible spectrum, and a sixpence none at all. On such 

 a surface a sovereign was left on two different occasions, under a 

 penny, for three hours at a heat of 1 60°, and barely left a spectrum of 

 its outer margin ; while on a well-polished surface, at same heat, the 

 outline of the impression also would have been left as a permanent 

 spectrum in an hour or two." 



Mr. Hunt considers that mass exercises an influence and increases 

 the effect, but Mr. Prater could not detect this in his own experi- 

 ments. 



" Does the thinness of the plate exert an influence ? A farthing (in 

 two experiments) pressed by twelve or fourteen pounds weight on a 

 polished piece of platinum foil, in thirty hours leaves no spectrum at 

 all ; neither did it on a fourpenny piece, or a sovereign, or half-sove- 

 reign, when kept three or four hours at 1 60° under the same weight. 

 I found a spectrum could be made on nearly equally thin zinc plates 

 (zinc foil), by leaving a sixpence on it an hour or two. Zinc not 

 being elastic, allows the pressure to be equal. The particular chemical 

 nature of platinum has however much to do with this effect, for I 

 found that when a fourpenny piece or another small brass metal ob- 

 ject was left on a highly polished lamina of steel — heated to 160° or 

 not — a spectrum was scarcely made. That elasticity and consequent 

 imperfect contact is not the sole cause of the incapacity of thin la- 

 minse of platinum and steel, for receiving spectral images, was to me 



* * With the glass the experiment was only continued forty-eight hours ; 

 with the paper, talc and cork, a fortnight, silver coin being used; with the 

 whalebone and gum, ten days, gold coin being used." 



f * One spectrum, however, may be made on another; thus after the talc 

 had remained eight hours on heated copper-plate and left a permanent 

 spectrum, a sovereign put on this an hour left a permanent spectrum." 



Q2 



