ON THE TRANSMUTATION OF SPECTRAL RAYS. 103 
as requiring the rays emitted by renovation to be of less or, at the utmost, of 
equal refrangibility to that of the incident*. If this law, of which, however, 
a different interpretation was proposed in Part I. Art. 6, held good with re- 
spect to ray-renovation upon the whole, itis evident that, of the transmutations 
enumerated in Art, 2 of the previous part, only the first six would be feasible, 
whilst the remainder would be impossible by the nature of things. Several 
experiments have been adduced in the preceding part, both to show the pro~ 
bability that phenomena the converse of fluorescence may occur, and to provide 
for their realization in a way analogous to the ordinary form of fluorescent 
appearances. The remainder of this paper is devoted to the detailed consi- 
deration of some of the circumstances relating to that of the proposed ex- 
periments which were designated before as the most hopeful. 
5. According to Prof, Draper’s experiments}, the incandescent state of 
metals, and the order of Newtonic rays which they emit, are strictly determined 
by their temperature, and independent of their nature. Other substances, 
however, such as chalk, marble, and fluor-spar, become luminous at different 
temperatures from the metals, which is also the case with gases. Now with 
respect to metals, if, as stated, their incandescent state is conditional upon a 
certain temperature alone, it is evident that, in whatsoever manner this tem- 
perature be imparted, the result will always be the same, viz. an emission 
of Newtonic rays. One means for the production of high temperatures is to 
be found in the concentrated rays of the sun, which produce an effect com- 
pounded of the aggregate effects of the different species of rays coexistent in 
each solar beam. The heating effect of any given kind of rays depends— 
1, on their amplitude; 2, on the absorptive power of the substance on which 
they are incident for the particular kind of rays. The calorific power of the 
solar rays as evolved by a non-absorbent prism and absorbed by lampblack, 
which has been found to absorb (Newtonic and Herschellic rays at least) more 
equably and completely than any other known substance, has been investigated 
in a masterly manner by Mellonit, and more recently again by Prof. Miiller, 
of Fribourg §. Both these philosophers agree in assigning the greatest calorific 
action to the rays situated in the Herschellie part of the spectrum, at some 
distance from the red border of the Newtonic; and though the greater 
erowding which dispersion causes in this part of the spectrum may partially 
account for this result, it is not liable to any doubt that, independently of 
that circumstance, the Herschellic bands of the spectrum separately, and still 
more in the aggregate, possess considerable heating power in comparison with 
the remainder of thespectrum. Again, the reflective power of the metals for 
Herschellic rays, though great, is not absolute; and, considering that the 
supply of Herschellic rays from the sun is almost unlimited, it cannot be 
doubtful that if the rays of the sun were concentrated by means of a very 
large mirror, but only their Herschellic components allowed to be incident 
on a piece of platinum-foil, for instance, placed in the focus, the platinum 
would be rendered incandescent, in the same way as a cone of unsifted solar 
beams reflected by a smaller mirror would make it. 
At the same time, if, instead of excluding all the Newtonic and Ritteric 
rays from the focus, some of the less refrangible among the Newtonic were 
sflowed to accompany the Herschellic rays which meet there, there is every 
reason to believe that, by a suitable adjustment, incandescence might be pro- 
* M.E: Becquerel (see Ann. de Chim. et de Phys. vol. lvii. p. 85, 1859) mentions an excep- 
tion to this law, which, however, as M. Becquerel also considers, is only an apparent one. 
+ Phil. Mag. vol. xxx. p. 347 (1847). ¢ Bibl. Univ. vol. xlix. p. 141 (1844). 
§ Pogg. Ann. vol. ey. p. 350 (1858) ; also Phil. Mag. vol. xvii. p. 233 (1859) : 
