398 
the natural limit to our conception of a movable heat- 
equilibrium, Suppose, for instance, that we have a large 
spherical chamber of the temperature of 100° C., and that 
this chamber is removed from all gravitating influence, 
so that a solid spherical body, also of the temperature of 
Ioo” C., may rotate on its axis in the centre of this cham- 
ber without requiring the support of an axle. The 
chamber may likewise be supposed to be void of air, so 
that there is nothing but the ether to bring the revolving 
body to rest. Now, if a sort of diaphragm or rim be in- 
troduced into the chamber, as in Fig. 9, the result will 
a 
—= 
& 
REVOLVING \ 
BODY / 
TosuRe 
encCLOSup, 
rs 
/ 
/ 
a i 
DIAPHRAGM 
a 
9. 
be that the particles of the enclosure to the left of the 
diaphragm will only receive heat from that portion of 
the revolving body which is approaching them, while 
those to the right of the diaphragm will only receive heat 
NATURE 
« 
| August 27, 1885 
visible spectrum. On the other hand, the rays which the 
body gives out are generally of a lower refrangibility than 
the exciting rays. Hence invisible rays may, by means 
of a phosphorescent or fluorescent body on which they 
fall, render themselves visible. This phrase, however, is 
perhaps not strictly correct, inasmuch as, before becominz 
visible, they have been changed into other rays of lower 
refrangibility. 
The object of introducing this subject here is rather, 
however, to discuss its bearing upon the theory of ex- 
changes than to treat it as a separate branch of inquiry ; 
and I may commence by remarking that at first sight it 
| seems to contradict the general law that the quantity and 
quality of the light and heat given out by a body depend 
upon its temperature, and upon this only. Thus, a ther- 
mometer at 100° C. is supposed to radiate from the sur- 
| face of its bulb heat which will be the same in quantity 
and quality whether the instrument has been heated by 
the sun’s rays or by plunging it into boiling water. Now 
| in such a body as luminous paint we have the light which 
from those portions of the same body which are receding | 
from them. 
But the wave-length of light is altered in one 
way by a body which is approaching us, and in 
another way by a body which is receding from us, 
so that the particles to the left of the diaphragm will, in 
reality, receive a different kind of radiation from those to 
the right. Here, then, we have something which upsets 
the temperature equilibrium, and we may even conceive 
that the particles to the left of the diaphragm will absorb | 
more heat, and therefore become hotter than those to the 
right. If so, we shall have the possibility of creating 
‘work out of this difference of temperature, or, in other 
words, of starting a kind of perpetual motion. 
We thus begin to see that, somehow, the revolving 
body must lose as much energy as we gain by means of | 
these differences of temperature, for otherwise we should 
have the transmutation of heat originally of the same 
temperature into work, which we cannot admit. But this 
means that a revolving body placed under these circum- 
stances must gradually part with its energy of visible 
motion, although it is not in contact with anything else 
than the ethereal medium. 
Before concluding this branch of my subject let me say 
a few words about phosphorescence and fluorescence. 
It is well known that certain substances remain lumin- 
ous—that is to say, continue to emit light for some time 
after they have been exposed to the light of the sun or of 
some other powerfully luminous body. Such substances 
are said to be phosphorescent. 
It is likewise known that other substances, more espe- 
cially certain liquids, emit light in a peculiar way while 
the luminous source acts upon them, but do not enjoy 
this property for an appreciable time after it has been 
withdrawn. Such bodies are said to be fuorescent. 
It is manifest that the difference between phosphor- 
escence and fluorescence is one of time, the bodies im- 
plied by the first term continuing to give out light for 
some time after the exciting source is withdrawn, while 
those implied by the second d»5 not retain this property 
for an appreciable time after the withdrawal of the lumin- 
ous source. Prof. Stokes, who has done much to advance 
this subject, has shown that the exciting cause of phos- 
phorescence and fluorescence is more especially the rays 
of high refrangibility—even rays beyond the violet of the 
we usually associate with a high temperature given out 
long after the sun has ceased to shine upon it, and when 
we know its real temperature to be that of the bodies 
around it. Do phosphorescent bodies form, therefore, an 
exception to the general law which represents the quality 
of the radiant heat as a function of the temperature ? 
I think we shall find, on examination, that in this gene- 
ral law it is taken for granted that no chemical change is 
taking place in the body in question, and no other molecu- 
lar change than that implied in the cooling of the body. In 
| a chemical action we have generally the transmutation of 
chemical energy into heat, and in molecular action we 
have generally the transmutation of molecular energy into 
heat likewise. That is to say, the body undergoing these 
changes becomes heated, and so gives out light and heat 
peculiar to the temperature to which it has been raised. 
But there seems to be no reason why molecular energy 
should not be somehow changed at once into radiant light 
and heat. In this case there would no doubt be an apparent 
breaking of the law above mentioned, which associates a 
certain temperature with a certain quantity and quality of 
radiant heat, but the exception would be only apparent. 
for, as we have stated, the law presupposes that no 
molecular change of this nature is taking place. 
In like manner our argument regarding an enclosure of 
a constant temperature and the theo1y of exchanges in 
general, while it allows of the greatest possible variety of 
substance and form in the enclosure, virtually assumes 
that no chemical or molecular change is going on 
amongst the substances introduced. We are, in fine, 
supposed to be dealing with radiant energy and absorbed 
heat, and with no other form of energy, and indeed we 
have just seen that if we have a body in visible motion in 
the enclosure, the equilibrium no longer holds. 
Thus we get rid of the difficulty by rejecting the bodies 
in question as not fulfilling stiictly our requirements. No 
doubt the phenomena of phosphorescence and fluor- 
escence are comparatively trivial exceptions, hut we may 
imagine an enclosure in which all the substances are at 
the temperature of 100°, while some one substance is 
gradually changing its molecular state, until at length we 
have a violent explosion accompanied with light and 
heat. Here the result is so obvious that we have no 
hesitation in recognising such a body as an exception not 
contemplated by the theory of exchanges. We are per- 
suaded that phosphorescent bodies are equally an excep- 
tion, the only difference being that the character of this 
exception is not nearly so pronounced. ‘ 
It has been pointed out by Prof. Tait that the 
conclusions of the theory of exchanges are only 
statistically true. That is to say, if we take a sens- 
ible time, such as a second, and a sensible quantity 
of any substance, such as a milligramme, then in 
an enclosure of constant temperature the absorption of 
