170 Dulong and Petit on the Measure of Temperatures, [Mancn, 
not escaped the penetration of this celebrated philosopher. 
Most of the phenomena, whose irregularity he had perceived, 
vary in the way which he had pointed out; but he wanted the 
data necessary to verify his ingenious theory. The researches, 
of which we have given an account, enable us to present much 
more certain notions on the measurement of temperatures, and 
to explain several difficulties which had been started on the 
subject. It is evident, by what we have said respecting the 
variation of the capacities, that no thermometric scale can indi- 
cate immediately the increments of heat corresponding to a 
determinate elevation of temperature ; for supposing we found 
one which possessed that property relatively to a peculiar sub- 
stance, it could not be applied to others, because the capacities 
of all bodies do not vary in the same way. 
By comparing together all the thermometric scales, we may 
make ourselves equally certain that no one exists in which the 
dilatations of all bodies can be expressed by simple laws. These 
laws would vary according to the scale which we adopted. Thus, 
if we take the air thermometer, the law of the dilatation of all 
bodies would be increasing. If we chose iron for our thermo- 
meter, all bodies would follow a decreasing law of dilatation. If 
we took the mercurial thermometer, corrected from the complica- 
tion occasioned by its envelope, iron and copper would follow an 
mcreasing law of dilatation, while platinum and the gases would 
follow one continually decreasing. 
In the state to which the question is now reduced, we cannot 
allege any peremptory reason for adopting one of these scales 
exclusively. We may say, however, that the well-known 
uniformity in the principal physical properties of all the gases, 
and particularly the perfect identity in the laws of their dilata~ 
tion, renders it very probable, that in this class of bodies the 
disturbing causes have not the same influence as in solids and 
liquids, and that consequently the changes in volume produced 
by the action of heat upon them are more immediately dependant 
upon the force which produces them. It is, therefore, very 
probable, that the greatest number of the phenomena relating to 
heat will present themselves under a more simple form, if we 
measure the temperatures by an air thermometer. It is at least 
by these considerations that we have been determined to employ 
this scale constantly in all the researches which constitute the 
object of the second part of this memoir. The success which 
we have obtained may be stated as an additional motive in 
favour of the opinion which we have given. But we do not 
pretend that the other scales ought to be excluded in all circum- 
stances. It is possible, for example, that certain phenomena 
may present themselves in a more simple manner, by reckoning 
the temperatures on the thermometrical scales deduced from 
the dilatation of each of the bodies whose dilatations were 
observed. It was this indeed which led us to follow with soa 
