xx.  Mistorical Sketch of the Physical Sciences, 1818. 
Irvine, Crawford, and even of Leslie. Dulong and Petit have at 
last undertaken to investigate the subject, and have published a 
most elaborate and intricate set of experiments, of which [ shall 
endeavour to give an account here. I cannot say that I am 
quite satisfied with these experiments. They seem to me to 
want the requisite simplicity ; nor am I quite convinced by the 
mode followed to establish some of the most important of the 
conclusions. I hope, therefore, that Mr. Leslie, or Mr. Dalton, 
will resume this most important subject, and endeavour to settle 
a preliminary question upon which the whole doctrine of heat, as 
far as it is the subject of experiment, in a great measure depends. 
Dulong and Petit employed the bulbs of mercurial thermome- 
ters as the hot bodies, which they allowed to cool in different 
circumstances. They previously proved, by a set of experiments, 
that the law of cooling continues the same, though the size ofthe 
body to be cooled varies, and that alterations in the shape of the 
hot body are immaterial, as they do not affect the law of cooling. 
It would appear from their experiments that the velocity of cool- 
ing is nearly inversely as the diameter of the bulb. This had 
been stated by Newton to be the casein his Principia. And Dr. 
Martine had verified the law by a set of experiments, which, 
though not very precise, were, however, sufficiently so to show 
that the rate of cooling was sensibly as the law established by 
Newton. 
They found that the law of cooling was the same when mercury, 
water, alcohol, and sulphuric acid, were employed. From this 
they have been led to conclude that all liquids cool according to 
the same law, and that the cooling of a liquid mass is subjected 
to the same law as a body of infinitely small dimensions. 
But when the vessel employed to hold the hot liquid is varied, 
the law of cooling varies along with it. Thus the law of cooling 
of a tin plate was found to be more rapid than that of a glass 
sphere. This had been observed by Mr. Leslie, and led him to 
conclude that the law of cooling is most rapid in those that 
radiate heat least. Dulong and Petit assure us, that this is the 
case within the limits of the thermometric scale in which Mr. 
Leslie’s experiments were made, but that the reverse takes place 
at high temperatures. 
The first. object to which our authors directed their attention, 
after these preliminary investigations, was the law of cooling m 
vacuo ; and as this law varies with the surface of the hot body, 
they investigated it when the surface of the bulb of the thermo- 
meter was glass, one of the substances which radiates heat best, 
and when it was coated with silver, one of the substances which 
radiates heat worst. As it was impossible to make experiments in 
a perfect vacuum, they endeavoured to determine the quantity of 
heat which was carried off by the small residuum of air remaining 
in the balloon. This was estimated by means of a set of experi- 
ments made in air of different degrees of rarity, estimating the 
