952 
unaccountable yet characteristic rashness, chose to 
consider as applicable to the communication of heat 
from particle to particle, as well as to its dissipa- 
tion from the surfaces of bodies. But the result was 
worthy of the looseness of the assumptions; the terms 
arising from the peculiarity of Dulong’s law are 
mostly dispensed with as the investigation proceeds ; 
and the whole work is to be regarded rather as a 
mathematical exercitation, than as a serious step in 
physics. Having elsewhere recorded a criticism on 
that part of Poisson’s work which treats of the Heat 
of the Globe, I shall not dwell farther on its defects.? 
(690.) Professor Kelland has published a work®—the only 
yess" one intended for students—on this subject, and he has 
Theory of likewise made a valuable report to the British Asso- 
Heat. ciation, on the best means of comparing the Mathe- 
matical Theory of Heat with Observation.? He has 
MATHEMATICAL AND PHYSICAL SCIENCE. 
(Diss. V1. 
suggested, that the results of the Newtonian law of 
cooling may be used in combination with such an 
hypothesis as to the relation between quantities of 
heat and the temperature shown by an air thermo- 
meter, as will reconcile it with Dulong’s law. But 
there is reason to think, that internal conductivity 
varies with temperature in the reverse manner of ex- 
ternal cooling ; in other words, that it diminishes as 
the absolute temperature increases.* 
MM. Duhamel and Stokes have considered the 691.) 
propagation of heat in bodies which do not conduct it Case of 
uniformly in all directions ; and their investigations (1:5 ao 
are very interesting in connexion with the beautiful notconduct 
observation of M. de Senarmont, that such is the case heat uni- 
in crystallized bodies. It is easily shown in a plate a » 
of gypsum coated with a thin layer of wax, to a small pret 
part of which heat is applied. 
§ 7. Dutone.—The Law of Cooling—Progress of the Science of Radiant Heat between Leslie’s 
and Melloni’s Discoveries ; transmission of Radiant Heat through Glass. 
Herschel ; De la 
Roche ; Professor Powell—Theory of Dew ; Wells. 
(692.) The researches of Leslie on Radiant Heat, though 
Subject of very generally appreciated both at home and abroad, 
aie were not, on their publication, immediately repeated 
sumed. or extended. On some points they were seriously 
controverted. But the labours of most of his imme- 
diate contemporaries, of whose names we have given 
a few at the head of this section, were rather prepa- 
ratory to the fuller developments of a later time—to 
be made with improved apparatus—than demon- 
strated discoveries. I make an exception, however, 
with regard to the experiments of Dulong and Petit 
on the laws of simple radiation and of the cooling 
of bodies by the contact of gases, not only because 
they established propositions quite new in an incon- 
trovertible manner, but also because they introduced 
into this branch of science a mode of investigation so 
delicate and precise, and methods of reduction and 
of physical analysis so beautiful and convincing, as 
placed the whole science of Heat on a new footing. 
The main credit is due, we believe, to Dulong, who 
was one of the most estimable and accomplished phi- 
losophers of his time. 
Pierre Louis Dutone was born in 1785, and 
showed precocious talents. He was admitted to the 
Polytechnic School at the age of 16, and rose through 
every grade of that celebrated institution until he 
became “ Directeur des Etudes.” It is superfluous 
to say that he was a good mathematician. He was 
(693.) 
Dulong— 
an eminent 
chemist. 
also a most excellent chemist; and the honour of 
being discoverer of the most terrific of fulminating 
compounds (chloruret of azote) was purchased by the 
loss of one of his eyes. But his experiments on 
Heat are those by which he will be longest remem- 
bered. The most important series of these was de- 
voted to a rigid examination of the amount of heat 
radiated under different circumstances, and of that 
dissipated by the contact of air. 
Thelaw of Cooling.—Ithad beensuggested byNew- 694.) 
ton, and tacitly admitted by nearly all writers on heat, Dulong 
including the most eminent, that simple radiation (or poe pet 
pulsation, in the language of Leslie) takes place in di- Newtonian 
rect proportion to the excess of temperature of the hot law of 
body above the surrounding space. Martine, and par- °oling- 
ticularly De la Roche, had indeed thrown doubt on 
the subject, and had rendered it probable that at high 
temperatures the velocity of cooling (which is propor- 
tional to the radiant energy) is considerably greater 
than Newton’s law supposes. Dulong and Petit, 
however, first demonstrated this. They disengaged 
the experiment as far as possible from the influence 
of the contact of air, by using the most perfect ex- 
haustion which the best air-pumps could produce ; 
and it appeared very plainly from the result of ob- 
servations made under successive degrees of rarefae- 
tion, that instead of there being the slightest appear- 
ance of radiation ceasing to take place when the vehicle 
1 See Second Report on Meteorology, Brit. Assoc. Reports, 1840. 
2 Theory of Heat. Camb,, 1837. 
3 Brit. Assoc. Reports, 1841. 
4 This at least is the result of an experimental investigation by myself on the conductivity of iron, executed on a principle 
which I believe to be new, but which I have not yet been able to publish. Mr Airy and Professor Kelland are each in possession 
of the outline of my method ; and the result noted in the text was briefly announced by me in the Reports of the British Association 
for 1852. 
If my health permits, I shall resume and publish these experiments, I. may here add, that I pointed out in 1833, 
from some experiments which I made at that time (Proceedings of the Royal Society of Edinburgh, vol. i. p. 5), that the metals 
range in the same order as conductors of Heat and of Electricity ; and this law appears to be confirmed by more recent obser- 
vations. 
