954 MATHEMATICAL AND PHYSICAL SCIENCE. (Diss. VI. 
(701.) 
(702.) 
Observa- 
tions of Sir 
W. Her- 
schel on 
the heat of 
the solar 
spectrum, 
(703.) 
Other ex- 
periments. 
the necessity of introducing into physics a degree of 
precision then almost unthought of. Coulomb had 
done something of the kind in other branches ; but 
in Heat it was really new. It was speedily succeeded 
by a rapid advance of precision in almost every kind 
of delicate experimental research, Nowhere can 
the student of physics find a better model than in 
the celebrated memoir on the Law of Cooling, which, 
we may add, received, as a matter of course, from 
the Academy of Sciences, the prize in competition 
for which it had been composed.! It is only justice 
to the countrymen of Dulong to say that they retain 
the superiority in the deduction of numerical laws 
from observation, which Coulomb and he conspicu- 
ously exemplified. The industry of the Germans and 
of the English have indeed been great; but in this 
particular enquiry they have not equalled in address 
the members of the French Academy. 
In order to complete our sketch of the more im- 
portant steps connecting the discoveries of Leslie with 
those of Melloni, we will now, going back a little in 
point of date, trace the origin of correct experiments 
on the immediate transmission and refraction of heat 
by solid substances. 
Sir William Herschel having found it requisite in 
the course of his arduous observations on the sun to 
devise means for preventing the intense heat of its rays 
from reaching the eye, was naturally led to observe the 
effect of different coloured glasses, and even of coloured 
liquids in this respect, He also placed thermometers 
in different parts of the spectrum formed by a prism, 
in order to discover which of the rays it was most 
important to exclude. The singular result at which 
he arrived was this, that the intensity of heat accom- 
panying the light of the sun not only increases from 
the violet to the red end of the spectrum (as was 
already known), but is more intense quite beyond 
the red, and gradually diminishes in force for a long 
way farther. This result was keenly contested by Les- 
lie, but was confirmed by Englefield and Davy. Berard 
admitted the existence of invisible heating rays from 
the sun, but yet found the maximum effect within 
the red. Seebeck, by numerous experiments, proved 
that the position of the maximum depended on the 
nature of the prism, being found even in the yellow 
ray when a prism of water is employed, whilst with 
flint glass it always occurs in the space beyond the 
red, ‘The rationale of this curious result was first 
discovered by Melloni, whose labours will be detailed 
in another section ; and it was shown to depend on 
the different degree of absorption exercised on the heat 
of the several rays of the spectrum by the differing 
material of the prisms. 
Herschel made a very great number of experiments 
on the transmission both of solar and fire-heat through 
different kinds of glass and other bodies. But they 
were rough trials, giving a sort of practical test of 
this quality, rather than admitting of accurate esti- 
mations of the quantities of transmitted heat. It 
was impossible, for instance, to infer from them the 
degree in which the warmth induced in the glass or 
other medium by the heat which it absorbed, tended 
to raise the indications of the thermometer beyond; 
although such an effect was manifest from the re- 
sults of the experiments themselves. Hence it was 
open to an objector to deny the direct transmission 
of radiant heat through such bodies as glass, except 
in the cases of the sun and of brilliant combustion, 
when it cannot be doubted. 
Prevost had proved to his own satisfaction the (704.) 
immediate transmission of heat derived from bodies Important 
even when below the temperature of visible redness, ae Sein 
by using thin screens of glass, and renewing them pe ja 
frequently before they could have absorbed much Roche on 
heat. Maycock obtained a similar result; but to the Seip 
De xa Rocue is due not only the establishment of train... 
this fact beyond any reasonable doubt, but also the heat 
discovery of certain laws of its operation which are through 
inexplicable on any other supposition but that of 8 
immediate transmission, One of these laws, for in- 
stance, was this, that when a series of thin glasses 
are interposed between a source of heat and a ther- 
mometer, each successive glass transmits a larger pro- 
portion than the previous ones of the heat which falls 
upon it. De la Roche rightly accounted for this sig- 
nificant fact by assuming that heat is not homoge- 
neous, and that the heat which has once passed 
through glass has lost the rays which glass most 
easily intercepts. He farther found that the sus- 
ceptibility of heat to pass through glass increases 
rapidly with the temperature of its source. The 
experiments of De la Roche date from 1812. 
The next step was made by Professor Powell of 705.) 
Oxford (1825). He showed that the quality of the Professor 
heat transmitted by glass is not the same as that Powell. 
of the incident heat. This he proved by ascertaining 
the proportion of heat absorbed by a black relatively 
to a white surface. This proportion was invariably 
increased by the interposition of glass. Mr Powell 
concludes that heat consists of two kinds intimately 
mixed, hat of which the absorption depends on the 
colour of the surface on which it falls, is usually 
luminous, and is most easily transmitted by glass. 
That kind of heat which is equally absorbed by black 
and white surfaces is totally devoid of light, and is 
sometimes considered as pure radiant heat. 
It will be sufficient here to refer to an interesting (706.) 
Essay on Dew, published by Dr Wells in 1815, in Wells’ 
which he applies Leslie’s experiments on the radiating ries =e! 
1 As the original paper of Dulong and Petit, in the Memoirs of the Institute, or the Annales de Ohimie, is not always acces- 
sible, I may mention that it is translated almost or quite in extenso in Thomson’s Annals of Philosophy, vol. xiii., and in 
Mr Lunn’s excellent treatise on Heat in the Encyclopedia Metr 
~ 
lit, 
* 
