944 
of surfaces to reflect and absorb it. He noticed the 
different heights at which a blackened and a bright 
thermometer stand even when exposed to common 
daylight ; but so far as I have observed, he did not 
distinguish the effect of colour in absorbing heat when 
that heat is accompanied by light or the reverse; and, 
indeed, this portion of his work stops short on the 
threshold of most interesting enquiries. He showed 
that radiant heat travels with great velocity, and ob- 
served the heating and cooling of thermometers in 
exhausted receivers. His work also contains obser- 
vations on hygrometry, on some points of meteoro- 
logy, and on the heat of friction. Indeed, its chief 
fault is embracing so many topics in so short a com- 
pass, thus preventing him from thoroughly examin- 
ing any one of them. To Pictet is due the establish- 
ment of meteorological observations at the convent 
of the Great St Bernard, which are amongst the 
most interesting which have ever been made, and 
which are still continued. He died in 1825, at the 
age of seventy-three. 
The interesting experiment of the reflection of cold 
moventie .¢d Prerrz Prevost, one of Pictet’s colleagues, to 
equili- devise the theory of “the Moveable Equilibrium of 
brium of Heat.” His idea is, that heat is a substance associated 
best. with bodies, of a highly elastic nature, and continu- 
ally given off from them in proportion to their tem- 
perature, which may represent the tension of the 
imaginary elastic fluid. When the temperature of a 
body is stationary, it is (according to this view) be- 
cause it receives by radiation from surrounding bodies 
exactly as much heat as it parts with in the same 
way. The general structure of this theory was sus- 
tained by the experiments of Leslie, and by some 
later ones on the law of cooling by Dulong and Petit, 
which, indeed, realize it in a remarkable manner, 
Prevost first published his ideas in 1791, in the 
Journal de Physique, and afterwards in a special 
work. Prevost was a man of an active and vigorous, 
rather than profound intellect. He was a foreign 
member of the Royal Society, and died in 1839, at 
the advanced age of eighty-eight. 
50 We now come to speak of Leslie’s important Essay 
maa on Heat which received from the Royal Society the 
Heat —Dif- distinction of the Rumford medals, and which pro- 
ferential cured for him a European reputation. It is a work 
one difficult to analyze, from the very fact, that its con- 
é struction is fragmentary, and its arrangement desul- 
tory and obscure. Our limits will only allow us to 
mention the methods of research and their chief re- 
sults. As a thermoscopic instrument, he used a 
modification of the common air thermometer (which 
last had been employed by Pictet), which, having two 
balls at a certain distance, connected by a bent tube 
containing a coloured liquid, showed the difference of 
temperature of the balls; and being hermetically 
closed, was free from the disturbing variation of at- 
(649.) 
Prevost— 
(650.) 
MATHEMATICAL AND PHYSICAL SCIENCE. 
(Diss. VI. 
mospheric pressure. This he called the differential 
thermometer. A similar instrument had been de- 
scribed by Sturmius in the seventeenth century. 
Whether Leslie had any previous knowledge of this 
does not appear; but, as Dr Young very correctly 
observes in one of his anonymous critical articles,— 
“The principle of the differential thermometer is too 
simple to be called an invention, and it is only by 
its ingenious application that Professor Leslie has 
made it an object of attention.” He usually em- 
ployed as a source of heat a canister of block tin, 
filled with boiling water, and having sides with dif- 
ferent surfaces. The radiating or emissive effect ofEmissive 
these surfaces was measured by the rise of the ther- eae 
mometer exposed to their successive influence in the surfaces. 
focus of a metallic reflector. The result showed a 
great variety of effect, varying from 100 when the 
surface was blackened, to 12 when it was of polished 
metal. The absorptive power of surfaces to non- 
luminous heat is also in exact proportion to their 
emissive power—a property which seems essential 
for preserving the equilibrium of like temperatures. 
Another and not less important law clearly esta-_ (651.) 
blished by Leslie was this, that the radiation of heat ¥@* wi 
from a plane surface takes place with unequal force r esaiant 
in different directions. When the specific heatingheat. 
power or density of the calorific rays is estimated in 
a direction perpendicular to the surface from which 
it emanates, it is found to be a maximum. At any 
other angle with the surface, it varies as the sine of 
the angle. This law (which Fourier showed later to 
be necessary for the equilibrium of temperature) 
obtains also in the case of light. Hence the appa- 
rent specific brightness or warmth of a surface is 
the same under whatever angle it is viewed with re- 
ference to the plane of the surface, which, when 
placed obliquely, contributes rays from a larger ex- 
tent of surface, owing to the foreshortening, but being 
weaker in the same proportion from every point, the 
aggregate effect is the same. Some interesting ex- 
periments were made on the number of coats of 
isinglass necessary to effect a complete transformation 
of the metallic into the gelatinous surface, which was 
found to be considerable; and, in like manner, the 
reflective character of metals was only very gradu- 
ally destroyed by varnishing. This observation, 
rightly interpreted, showed that some solids are per- 
meated by radiant heat, a conclusion which Mr Leslie 
utterly rejected. 
Another fundamental experiment less decisively (652.) 
proved was, that the law of radiation varies inversely 7 i) erge 
as the square of the distance, Perhaps the mostgquare of 
convincing, as well as the simplest proof of this has the dis- 
been given more recently by Melloni. If a delicate *"¢* 
thermometer or other apparatus for measuring radi- 
ant heat be confined in a case, so as only to admit 
rays coming within a definite angular space—and if 
1 This result had already been anticipated by Lambert; Pyrometrie, p. 197. 
oo 
