172 Dulong and Petit on the Measure of Temperatures, [Maren, 
very just remark of Erxleben, as well as his memoir, seems to 
have escaped the attention of philosophers ; for in all posterior 
remarks on the same object, the law of Newton has been pre 
sented not as an approximation but as a rigorous and constant 
trath. 
Thus Mr. Leslie,* in his ingenious researches on heat, has 
made this law the base of several determinations, which, from 
that very cause, are inaccurate, as we shall prove in the sequel. 
Soon after the publication of Mr. Leslie’s book, Mr. Dalton 
made known, in his New System of Chemical Philosophy, 2 
series ef experiments on the cooling of bodies carried to a very 
hich temperature. The result of these experiments shows evi- 
dently that the law of Richmann is only an approximation at low 
temperatures, and that it is quite inaccurate at high tempera~ 
tures. Mr. Dalton, instead of seeking to represent his observa- 
tions by a new law, endeavoured to re-establish the law of 
Richmann by substituting for the usual thermometric scale the 
one which he founded on the notion that the dilatation of all 
Biquids is subjected to the same law ; an assertion which we have 
discussed in the first part of this memoir. But even supposing 
the accuracy of the principles of this new scale to have been 
constated, we should be under the necessity of acknowledging, 
_that it does not satisfy the condition of rendering the loss of 
heat m a body proportional to the excess of its temperature 
above that of the surrounding air; or in other words, that it does 
not re-establish the law of Richmann;, for before this could 
happen, it would be necessary that the law of cooling should be 
the same for all bodies, and our experiments prove the contrary. 
The last experiments undertaken on the subject which occupies 
our attention, arethose which Laroche has inserted in his memoir, 
relative to some properties of radiating heat. He establishes, 
among other propositions, that the quantity of heat which a hot 
body gives off in a given time by way of radiation to a cold body 
situated at a distance, increases, other things being equal, in @ 
progression more rapid than the excess of the temperature of the 
first above that of the second. 
This proposition is evidently for radiation the equivalent of 
that of Mr. Dalton for the totai cooling of a body in the air- 
But Laroche has only presented insulated facts, and has not 
sought for the law on which they depend. We shall see hereafter 
that the results are complicated by the action of particular 
eauses, from which it would be necessary to disengage them in 
erder to arrive at the law of cooling in a vacuum, which is not 
the same as raeiation. 
Thus the labours of philosophers. on the laws of cooling have 
been hitherto confined to showing that the law of Newton is 
® An Inquiry, &c. 
