ON THE TRANSMUTATION OF SPECTRAL RAYS. 99 
Cornelius and Marbach’s ‘ Physikal. Lexicon,’ showing that he then enter- 
tained the question of the possibility of phenomena the reverse of fluorescence, 
or of the transmutation of Herschellic into Newtonic rays. In the sequel, how= 
ever, M. Emsmann adduces facts which in his opinion exemplify phenomena 
of this kind, rendering thereby his estimation of what constitutes fluorescence, 
or its conyerse, of doubtful clearness. Iodide of mercury, which is commonly 
scarlet, upon sublimation becomes transformed into yellow crystals, which 
may be preserved for some time; several other substances exhibit similar 
changes of colour. Steel also alters its colour by heating. In all these in- 
stances, according to M. Emsmann, by the action of heating, that is to say, 
by heat-vibrations,” substances are made to reflect rays of higher refrangi- 
bility than would otherwise happen. Similarly, the rays emitted by incan- 
descent matter increase in refrangibility with the increase of temperature. 
Now it is easy to show that none of these facts in the least exemplify what 
they are intended for. In the first place, substances which change their 
colour in consequence of heating do so, generally, by selecting different rays 
for simple diffusion in their several states; if the incident beam was deficient 
in the rays which are reflexible, or consisted merely of invisible rays, then 
such a substance would turn black. ‘This is the case with the iodide of mer- 
cury for instance. Such substances, on the other hand, which, being self- 
luminous, assume different tints at different temperatures, as, for instance, 
incandescent metals, do so independently of incident rays, or, at any rate, not 
in a manner proving obviously or necessarily the transmutation of Herschellic 
into Newtonic rays. Nor does this fact show that, similarly as ‘‘ the dark 
chemical (or Ritteric) rays may produce modifications of one kind in the 
colour of the luminous (or Newtonic),” so also the dark heat (or Herschellic) 
rays may modify the colour of the same rays in an opposite direction—in 
which way M. Emsmann defines fluorescence and its converse in one place, 
As for steel, its coloration is generally supposed to be simply an instance of 
the coloration of thin plates; so that, upon the whole, none of the phenomena 
adduced can be considered as bearing any resemblance to those of fluorescence, 
or those which might be conceived as its counterpart. 
12, Another publication is by M. Dammer*, who observed in the winter of 
1862 a fact already noticed by Fusinieri, if not in exactly the same way, that 
ice beneath leaves, whether imbedded on the surface or in the midst of the crust, 
melts sooner than ice freely exposed to the rays of the sun. This, according 
to M. Dammer, is a phenomenon analogous to that adverted to by M. Salm. 
The fact, however, may be dependent rather on conduction than on radiation, 
and hence capable of explanation without the aid of assumed transmutations. 
It is, besides, to some extent similar to one of Franklin’s observations T, which, 
though directed to show differences dependent on colour, incidentally proved 
also that snow beneath strips of cloth melted more rapidly than if uncovered, 
13. In conclusion, it will be but just to state that Prof. Stokes, in his 
paper on fluorescencet, had adverted to the probability that transmutations of 
visible (Newtonic) into invisible (Herschellic) rays might account for the dis- 
appearance of light in cases of ray-absorption which cannot be classed under 
fluorescence. 
* Pogg. Ann. vol. cxy. p. 659 (1862), 
‘} ‘Letters and Papers on Philosophical Subjects’ (Appendix to ‘Exp. and Obs. on Elec- 
tricity’), London, 1769, p. 465. 
{ Phil. Trans. 1852, p. 554. 
H2 
