100 REPORT—1863. 
Part II. 
In the first part of this paper three experiments were described, having 
for purpose the production of the converse phenomenon of fluorescence. It 
is the object of this second part to discuss in greater detail one of the experi- 
ments proposed, viz. the second, in its relations especially to the subjects of 
phosphorescence and incandescence. 
1. The luminosity of matter, or the emission (in the language explained in 
the preceding part) of Newtonic rays—as well as radiation upon the whole— 
may arise in a twofold manner, which it seems important to distinguish. In 
the first case, there is a production of light by certain processes which do not 
imply pre-existing radiations ; whilst, in the other, only a reproduction and 
communication of rays actually takes place. Above and beyond these, a third 
case, of what, for the present at least, must be called spontaneous radiation, 
may be distinguished; to which has to be referred the luminosity of the sun 
for instance, and of the fixed stars. In these latter instances, no adequate 
cause can be, or has hitherto been, assigned for the ight emitted, except (if 
we suppose the radiascent state to indicate molecular vibrations) a certain 
velocity impressed on the molecules from all beginning and certain inter- 
molecular relations, corresponding in some degrce to the tangential tendency 
and gravitating force which rule the motions of the heavenly bodies*. 
2. The causes of production of light, as of rays generally, may be con- 
sidered as threefold, viz.—1, morphological and chemical; 2, electrical; and 
3, mechanical. The cases of reproduction, on the other hand, appear separable 
into two classes, according as the matter whose radiascence is considered is in 
immediate contact with the primarily radiating source, or not. The first kind 
of reproduction—to make our meaning clear—is exemplified principally in 
the phenomena of ignition exhibited by foreign matter, such as precipitated 
carbon-particles or certain vapours, mixed with comburescent gases or flames ; 
or by such instances as the incandescence of platinum wire in the common 
gas-flame, or of lime in the oxyhydrogen flame. The second kind of repro- 
duction, on the other hand, comprises all such appearances of light as are 
caused by the incidence of radiations emanating from distant sources, as, for 
instance, the sun, and presents to our consideration two different orders of 
phenomena, which require to be kept apart. The necessity,of this distinction 
is, first of all, suggested by the fact that the rays reproduced by the secondary 
radiator are sometimes identical with, but at others different from, those 
emitted by the primary radiator as to the characteristic of wave-length or 
refrangibility ; but there are cases which, without implying any such change, 
belong yet to the same class as those which do. As the operative cause of 
this distinction, the best authorities seem to be agreed in considering the 
compound nature of matter; the one kind of reproduction, ordinarily termed 
diffusion, being ascribed to the agency of ether, whilst the other kind, which 
is generally if not always accompanied by transmutation (in the sense of the 
word explained in the preceding part), and for which the term renovation 
might perhaps be suitably adopted, is assumed to arise from the intervention 
of the ponderable molecules of matter t. 
3. The mode of reproduction which has been noticed in the first place, and 
which occurs on the contact of radiascent substances of different natures, may 
* The above simile was employed already by Sir H. Davy, in his 7Essay on Heat, Light, 
and the Combination of Light’ (see Works, vol. ii. p. 15), though not quite consistently ap- 
plied throughout. : 
t Cf. (Prof. Stokes) Phil. Trans. 1852, p. 548; also (Dr. Young) Phil. Trans. 1802, 
p. 47. M. Angstrém (see Phil. Mag. xxiv. 2. 1862) seems to entertain a different opinion. 
