SCIE NC i: 



575 



:..-t. li.m nf oiiri*, it becomta ari interesting inquiry to de- 

 t UMUMI.C. t, i mine its cause. If we take a hotnngeneotu and uni- 

 form piece of silver, anVl place it upon a heated iron, 

 its Miif.u-r will oxidate c(|iially, if all the parts of it are 

 exposed to the same degree of heat. A coin, however, 

 differs from a piece of .-livt-r ot' utiifnrni texture, a* it 

 has been struck with great force during the act of coin- 

 ing. In this process the sunk parts have obviously 

 bteen most comjH-essed by the prominent parts of the 

 die, and the elevated parts least compressed, the metal 

 being left as it were in its natural condition. A coin, 

 therefore, is a piece of metal in which the raised letters 

 and figures have less density than the other parts, and 

 consequently these parts oxidate sooner, or at a lower 

 temperature. When the letters themselves are rubbed, 

 off by use, the parts immediately below them have also 

 less density than the metal which surrounds them, und 

 consequently they receive from heat an oxidation and 

 a colour different from that of the surrounding surface. 

 Hence, the reason is obvious, why the invisible letters 

 are revived by oxidation. 



A similar effect take* place in the beautiful oxida- 

 tions which are produced on a surface of polished steel. 

 When the steel has hard portions, called pins by the 

 workmen, the uniform tint of the oxide stops near 

 these points, which always display colours different 

 from the rest of the mass. 



The smoking of the coin, the diminution of its oxf- 

 Uating power, by a repetition of the experiment, and 

 the recovery of that power by time, seem to indicate 

 that the softer parts of the metal absorb something 

 from the atmosphere which promotes oxidation. 



2. On the met hod of reading lite inscriptions on coins and 

 medals in the dark. 



On the me- Among the numerous experiments of a striking and 



thod ot' popular nature, with which science astonishes, and 



reading the sometimes even strikes terror into the ignorant, there 



inscriptions i s perhaps none more calculated to produce this effect 



' than the one which forms the subject of the present 



the dark. article, while it possesses at the same time the higher 



advantage of presenting to us some scientific facts, 



which we believe have not hitherto been observed. 



It has long been known that black surfaces radiate 

 heat, or throw it off more copiously than nhile sur- 

 Caces ; and that the same difference takes place between 

 rough and polished surfaces ; those which are rough 

 producing the same effect as those which are black ; 

 but in so far as we learn, no experiment lias been made 

 to prove, that the same results take place wrth radiant 

 light ; that is, that when bodies are heated to such a 

 degree as to radiate light, those which have htack or 

 rough surfaces discharge the light more copiously than 

 those which are white or pfilithed. 



This result respecting radiant light might no doubt 

 have been inferred from the experiments on the radia- 

 tion of heat, if these two substances had been previous- 

 ly found to comport themselves in a similar manner 

 when thrown off by bodies, or reflected from their sur- 

 faces. But this has never been established ; and there- 

 fore such an inference would have been premature in 

 the present state of our knowledge. If we confide in 

 experiments already made, indeed, we should be dis- 

 posed to conclu(k , that light and heat do not follow 

 the same laws, at least in their reflexion from bodies. 

 Brass, for example, is said by Mr. Leslie to have a re- 

 flective power for heat of TOO", while silver has only a 

 reflective power of <)0 ; but we know for certain, that 

 .silver reflects light much more copiously than brass. 

 Notwithstanding this apparent discrepancy* \\e would 

 rather question the accuracy .of the experiment, than 



the truth of law which hat been long considered u ^ 

 general. Com*"**. 



ng the experiments on the revival of the in- ***-_- 

 scription on coins by unequal oxidation, we bad occa- 

 sion to expose to a high degree of beat a coin, on which 

 the inscription had been rendered black by oxidation. 

 Upon taking the coin, while in a state of bright red 

 heat, into a dark room, we were eurprised to obferve 

 that the letters of the inscription were more luminous 

 than the real of the coin, in consequence of their oxi- 

 dated surface radiating the red light more copiously 

 than the other parts. Though the effect was not in 

 this case sufficiently striking to enable me to read tb 

 inscription, yet it occurred to me, that if by the action 

 of an acid, or any mechanical means, the general sur- 

 face of the coin should l>e made rough, or have its po- 

 lish removed, while the raised points which constituted 

 the inscription and the figure were polished, an effect 

 opposite to that in the preceding experiment should be 

 produced. ' accordingly took a French shilling of 

 Louis XV. and having roughened the depressed parts 

 of the surface, and heightened the j>oli-h of the in- 

 scription, &c. I placed it upon a red hot mass of iron, 

 and removed it into a dark room. When the shilling 

 began to radiate light, the inscription, BENKDICTCM 

 SIT XO.MKN DKI, appeared in obscure letters, while 

 the ground on which they stood ehone with a brilliant 

 light. By polishing the depressed parts of the surface, 

 and roughening the letters, I obtained, as might have 

 been expected, the opposite effect, the inscription be- 

 ing now legible, from its throwing off more light than 

 the surrounding surface. 



In order to perform this experiment with most suc- 

 cess, it is desirable to conceal from the observer's eye 

 the mass of red hot iron on which the coin is placed, 

 both for the purpose of rendering the eye fitter for ob- 

 serving the effect, and of removing all doubt that the 

 inscription is really red in the dark ; that is, without 

 the light of any other body either direct or reflected 

 falling upon it. 



The most striking form in which this experiment 

 can be exhibited, is to use a coin from which the in- 

 scription has either been wholly obliterated, "or oblite- 

 rated in such a degree as to be no longer legible. 

 When such a coin is laid upon the red hot iron, the 

 letters and figure become oxidated, as formerly de- 

 scribed ; and the black or brown film of oxide which 

 is found upon them radiating more powerfully than 

 the rest of the coin, th* letters will shine more bril- 

 liantly than the other parts, and may be read to the 

 great surprise of the observer, who had examined the 

 blank surface of the coin before it had been placed up- 

 on the iron. Ills surprise will not be diminished, 

 when he observes that the letters which he' saw more 

 luminous than the rest are now covered with a black 

 film of oxide. 



As the different radiating powers of parts of th 

 same surface, can neither be seen by the eye nor indi- 

 cated by any instrument, when the general tempera* 

 ture is below that of red heat, the principle of the pre- 

 ceding experiment may be employed to determine the 

 relative powers of radiation possessed by different me- 

 tals. Although the radiating power*, for example, of 

 gold, silver, copper, and tin p/ate, are, according to the 

 experiments of Mr. Leslie, em city the same, yet we are 

 persuaded that the preceding meihod will afford ocular 

 demonstration of the incorrectness of this result, when 

 these bodies are submitted to the experiment under 

 the same circumstances of magnitude, thickness, and 

 polish, in so far as this can be done. This opinion rs 

 founded on very unequivocal experiments on the reflet 1 - 



