Chemistry. : Xi 
point was not undertaken by the authors of it. A little conside- 
ration is sufficient to show us that the equal expansibility of the 
gases cannot be considered as completely established. If 1 
recollect M. Gay-Lussac’s experiments (for I have them not at 
present at hand to consult) they were carried no higher than the 
temperature of 212°. Mr. Dalton’s were also limited by that 
temperature. Now it is obviously possible, as we see from the 
experiments contained in the paper before us, that the expan- 
sions of the different gases might have agreed with each other 
up to 212°, and yet have deviated from each other at higher 
temperatures. Thus the expansion of air and mercury follows 
the same law up to 212°, but at 392° a deviation is quite percep- 
tible, and at 582° it has become considerable. 
There is a method of deciding the question which has been 
long known to chemists, having been originally tried by Dr. 
Brook Taylor, and afterwards by Dr. Black, and by Dr. Crawford. 
I am rather surprised that such active experimenters as Dulon 
and Petit, who seem to have set out with the resolution of tak- 
ing nothing for granted, did not have recourse to it. To deter- 
a whether equal increments. of expansion were occasioned by 
equal increments of temperature, the philosophers above-men- 
tioned mixed together equal weights of water heated to different 
temperatures, and observed whether the heat of the mixture was 
the mean of the temperatures of the two portions of water before 
mixture. Suppose they had mixed one pound of water at 46° 
with one pound of water at 100°, and that they found the tem- 
perature of the mixed liquid to be 70°, they would have concluded 
that up to 100° equal increments of expansion were produced by 
equal increments of temperature. It is well known that Dr: 
Crawford, from experiments made in this way, concluded that 
up to 212° mercury expands equably when heated. This con- 
clusion is confirmed by the result of the experiments stated by 
Dulong and Petit. Now it would have been natural to have had 
recourse to a similar mode of measuring the expansion of air 
compared with the temperatures at heats considerably elevated 
above 212°. Water could not have been used for the purpose ; 
but the fixed oils would have answered sufficiently nearly to the 
temperature of 600°, and mercury could have been used for still 
higher temperatures. . Indeed a little ingenuity might have 
enabled them to carry the comparison up toa red heat by means 
of mixtures of lead or of tin; and thus to have settled a question 
which must still be considered as a desideratum of very material 
consequence, because it affects all our measurements of temper- 
ature, and all our conclusions respecting heat. ! 
The dilatation of several solid bodies were compared by Dulong 
and Petit with those of air and mercury. The method was simple 
and ingenious. Having determined the absolute dilatation of 
mercury by heat, they measured the dilatation of it in a glass 
tube. The difference gave them the absolute dilatation of the 
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