ON THE TRANSMUTATION OF SPECTRAL RAYS. 101 
with great probability be considered as coming under the head of renovation. 
The most remarkable instances of this kind are those in which light is en- 
gendered by the contact of two non-luminous substances, generally of different 
temperatures. The earliest well-substantiated instance of this description 
seems to be afforded by Boyle’s experiments on the celebrated Clayton diamond, 
which became luminous in a dark room by contact with an iron plate heated 
to a temperature below redness, or with warm parts of the human body *. 
A similar though perhaps not quite the same phenomenon was noticed by 
Canton, whose artificial phosphorus, after exposure to light and subsidence 
into apparent darkness, had its light restored by the application of heated 
non-luminous mattert. In the case of Canton’s phosphorus, the necessity of 
a previous exposure to light in order to produce the phenomenon just described 
seems to be rigorously established, but with regard to diamonds it is perhaps 
still doubtful. It is equally undecided whether some kind of morphological 
change, or combustion, or nothing of the kind, causes or accompanies this 
evolution of light upon the contact of dark unequably heated bodies. The 
observations of Sir D. Brewster on the loss of colour which green fluor-spar 
exhibits after calcination, simultaneously with the loss of ability to shine by 
subsequent exposure to light or to high temperatures§, at first sight would 
indicate that colouring-matter or its combustion are the cause of the lumi- 
nosity observed before calcination, which ceases of course after the expulsion 
of the colouring-matter, which takes place at the higher temperatures. But 
the observations of Dessaignes|| on the revival of the phosphorescent power 
through electrical shocks, which, according to Pearsall J, is attended by a resto- 
ration of colour, do not seem to countenance such an opinion, but rather to 
point to molecular disarrangement as the cause both of the phosphorescence 
and its destruction. 
Other though less clear examples of ray-renovation on the contact of two 
radiators of different descriptions have been alluded to in that part of the 
preceding paragraph which refers to the phenomena of flame. These appear 
to show that matter, whether in the solid or gaseous state, introduced into 
gaseous comburescent substances, may change the rays emitted by the same, 
as it were in statu nascenti; or take upon itself, seemingly, the function of 
principal radiator. One of these phenomena has suggested the speculations 
contained in the present paper, and serves as foundation for one of the three 
experiments proposed in the preceding part ; a full consideration of the whole 
subject, however, is reserved for a future occasion**, 
4, The renovation and transmutation of rays incident from distant radi- 
* Appendix to ‘ Considerations, &c., touching Colours,’ London, 1764, p.416. The phos- 
phorescence of diamonds, consequent on insolation, was first noticed by Dr. Wall (see Phil. 
Trans. 1704-5, p. 69). 
+ Phil. Trans. 1768, p. 342. 
¢ Cf. Priestley’s ‘History, &c., of Light, &.,’ p. 373; and (M. O. Fiebig) Pogg. Ann. 
vol. exiv. p. 292 (1861). 
§ Edinb. Phil. Journ. vol. i. p. 286 (1819). 
|| Journ. de Phys. vol. lxxi. p. 67 (1810); also bd. vol. Ixviii. p. 465 (1809). 
{| Journ. Roy. Inst. vol. i. p. 277 (1831). It should be observed that the colour given 
to fluor-spar by electricity is not generally the same as possessed by the mineral before 
calcination. 
** A remarkable example of a phenomenon in many respects similar to those of flames, 
adverted to in the text, is exhibited in the ingenious experiment performed by Mr. Wedg- 
wood (see Phil. Trans. 1792, p. 271), in which, by a hot stream of non-luminous air, a piece 
of gold-foil was rendered incandescent. In the same paper, Wedgwood advances also the 
question, remarkable for its time, ‘ Whether a body can be made red-hot by concentrated 
rays of other colours.” It should be recalled, however, that Wedgwood’s views on the 
nature of, and relation between, light and heat are not those now prevailing. 
