584 REGNAULT ON THE 
equally heated strata, that a constant agitation is required to pre- 
vent this separation from taking place ; these methods become on 
that account quite impracticable for temperatures above 100°. 
I had recourse for elevated temperatures to a well-known mode 
of procedure, which has been adopted by several philosophers, 
and especially by MM. Arago and Dulong. This method con- 
sists in ascertaining the temperature at which water boils under 
known pressures; it possesses the advantage of being appli- 
cable under the most elevated pressures, and gives very accu- 
rate results when properly conducted. 
In the apparatus of MM. Arago and Dulong the water did 
not actually boil (Annales de Chimie et de Physique, 2me Series, 
vol. xliii. p. 74) ; its elastic force increased by the action of heat 
up to a certain maximum, which was determined by an air ma- 
nometer, at the same time that the thermometers immersed in 
the vapour and in the liquid of the heating vessel were observed ; 
at other times the manometer and thermometers were simulta- 
neously observed whilst the temperature was still ascending and 
approached its maximum. It is to be feared that by this mode 
of operating, the thermometers, which are necessarily a little 
behind the temperature of the vapour, do not attain the same 
degree which it possesses at. the moment when their maximum 
is noted. 
The error which could arise from this cause is probably very 
small, or even imperceptible in experiments made under great 
pressures, because in this case a very small difference of tempe- 
rature corresponds to a great alteration in the elastic force; but 
the error would probably not be imperceptible at feeble pres- 
sures, for instance at such as are inferior to the pressure of the 
atmosphere. But it is easy to render this mode of proceeding 
unobjectionable, by so arranging the experiment that the con- 
ditions are quite identical with those under which water is boiled 
under the ordinary pressure of the atmosphere for fixing the 
point of 100° in thermometers; and the temperature at which 
water boils under different pressures can be determined with the 
same precision. For this purpose water is made to boil in a 
vessel communicating with a pretty considerable space, in which 
the air may be compressed or dilated at: pleasure; this air forms 
an artificial atmosphere, exercising pressure upon the surface of 
the heated liquid. Thus a boiling temperature is obtained as _ 
perfectly constant as that which water presents when boiling 
