114 Dulong and Petit on the Measure of Temperatures, (Fes. 
quantities of heat produced always the same increase of volume, 
such a body would possess all the requisites which philosophers 
have judged necessary and sufficient to constitute a perfect 
thermometer. 
_ Such an instrument, however, might not offer all the advan- 
tages which it appears at first to promise. If the specific heat 
of all other bodies, for example, when referred to this thermo- 
meter, were variable, and unequally variable, in each of them, 
it is very evident that we could conclude nothing, @ priori, 
from the indication of this thermometer relative to the quan- 
tities of heat acquired or lost by a determinate variation of 
temperature. 
We see then that the first step to be taken in this research is. 
to ascertain if the capacity of a great number of bodies, taken 
with the same scale, vary in the same manner; and if the dila- 
tations of bodies, which differ most in their nature, are- subjected 
to the same laws. This last comparison, with which we shall 
begin, being susceptible of a greater degree of precision than the- 
first, we have extended it much further, and we think that we 
have: taken every possible care to secure the accuracy of the 
results. 
Of the Dilatation of the Gases. 
When we have no other object but to establish a general: 
comparison between the dilatations of all bodies, the thermome- 
tric substance to which all the measures are referred may be’ 
chosen in an arbitrary manner. The construction of the mereu- 
rial thermometer being easier, and its use more convenient, we 
have employed it in almost all our experiments. 
The comparison of this thermometer with the air thermometer 
has been made long ago by Gay-Lussac, between the limits of 
freezing and boiling water. It results from the experiments of 
this celebrated philosopher that the two instruments do not pre- 
sent any sensible discordance within that interval of temperature. 
Mr. Dalton thinks, on the contrary, that the mercurial ther- 
mometer would be about 1° higher than the air thermometer 
towards the middle of the scale, where the difference would 
obviously be the greatest, since the two instruments agree at 0° 
and at 100°.* : | 
We see from this, that if there exist really a difference 
between the dilatabilities of air and mercury, it must be very’ 
small between the limits of freezing and boiling water. 
We at first pursued this comparison for inferior temperattres. 
In a first experiment made at — 20°, we found a perfect identity 
between the two instruments ; and by a great number of obser- 
vations made from — 30° to — 36°, we observed slight differ- 
ences ; but sometimes positive, and sometimes negative, so that. 
the mean of all the measures taken simultaneously on, the two, 
*- Of the Centigrade Scale, 
