226 Intelligence and Miscellaneous Articles. 



elude also, that on the whole, " it seems right to admit that the effect 

 is greater when dissimilar metals are used ; " but he has not been able 

 to satisfy himself of the truth of the statement, that when the cop- 

 per coin is heated, and the plate of copper kept very cool, the effect 

 is increased. His experiments on the effect of heat generally show, 

 it is stated, — " 1st, that heat much increases the rapidity of the ra- 

 diation, even when the object is not in direct contact ; and 2nd, that it 

 takes place much more energetically from gold and silver than from 

 copper (a copper plate being used). They also show that a perma- 

 nent * spectrum is to be considered only as a higher degree of that 

 produced or rendered apparent by breathing. 



" Heat does not seem to increase the effect of metal coins on glass. 

 Neither did long contact." The polished surfaces not appearing 

 capable of receiving the impressions are "talc [talc-mica], and, among 

 the metals tried., steel to a certain extent, platinum and gold ;" and 

 experiments are described which, in the author's words, " seem al- 

 most sufficient to establish the important general principle, viz. that 

 the less metals are oxidable by exposure to the air, the less is their sus- 

 ceptibility to receive spectra." 



Mr. Prater draws the following conclusions from his experiments 

 as regards comparative polish in metals. " All these experiments 

 show that the dissimilarity of metals is not of such importance as 

 has been conceived : they show the difference wanted to produce 

 the effect is a difference in brightness or oxidation, i. e. as far as a 

 permanent and good impression, showing the lettering, fyc, is con- 

 cerned ; for I find when left on the plate half an hour or so, tarnished 

 or polished metals give equally good spectra. But in this case the 

 spectrum is only made apparent by breathing, and of course shows 

 nothing of the lettering, &c. However, even in this case, the spec- 

 trum of the tarnished sovereign disappeared less soon by breathing 

 on it than did that of the polished one ; so in reality the spectrum 

 of the former may be said to have been the most perfect. 



" The same remark applies to a glass plate. 



*' As regards the effect of interposed substances. As every sub- 

 stance tried left a spectrum, I did not much expect that the influence 

 would permeate any lamina, even of the thinnest description. Ac- 

 cordingly when a sovereign or shilling was left twenty-four or forty- 

 eight hours on a piece of stiff, though very thin, paper, it gave no 

 spectrum, but the mark of the paper was alone visible. The expe- 

 riment was repeated, half the coin resting on the copper plate and 

 half on the paper : and although it remained a fortnight in this po- 

 sition, the half only in contact with the plate was visible by breathing 

 on the paper, leaving its own spectral image just as if no coin had 

 rested on it at all. 



" The same experiment was repeated with the thinnest possible 

 layers of talc, gum, cork and whalebone, glass, plane and con- 



* " By a permanent spectrum," Mr. Prater observes, " is always meant, 

 in this essay, a spectrum that remains when the substances or coins are 

 removed — not a spectrum which cannot be rubbed off by gentle friction, 

 for all the above permanent spectra are yet soon effaced by friction." 



