166 Dulong and Petit on the Measure of Teniperatures, [MAncn, 
disposed, could preserve a stationary temperature for about a 
quarter of an hour. % 
Finally, to avoid the error which would have been occasioned 
by the unequal temperature of different points of the mass, we 
continually agitated the liquid, when at the maximum tempera- 
ture ; and, by means of a thermometer of a constant volume, we 
had exactly the mean temperature, which must be likewise that 
of the body. The fixed oils acquiring, as is known, a very great 
fluidity, when they are very hot ; the coat of them which remains 
attached to the body plunged into them is very thin. However, 
we did not neglect. to take into view the heat coming from this 
addition of matter, though in most cases the correction was 
exceedingly small indeed.* 
When the body subjected to experiment had been raised to a 
certain temperature, measured by the means just stated, it was 
plunged as rapidly as possible into a great mass of water, and 
the temperature of this liquid was observed as soon as the equi- 
librium was established. It was in the measurement of this 
temperature that it was necessary to apply the greatest precau- 
tions, in order to obtain results on which dependance could be 
put. We always employed so great a quantity of water that the 
imcrease of temperature never exceeded five or six centigrade 
degrees. To measure it, a thermometer was employed whose 
divisions corresponded to the hundredth part of a degree. The 
water was contained in a very thin vessel of tin plate, placed 
insulated upon three points. This vessel participated in every 
case of the heat; but as its weight and its specific heat were 
accurately known, it was easy in all the calculations to make 
allowance for it. 
In most of the experiments, the water was cooled beforehand 
such a number of degrees, that, after the immersion of the body, 
it was raised to the temperature of the surrounding atmosphere, 
In other cases, it was of the temperature of the air before the 
experiment. The first method appeared to us most accurate, and 
it did not require any correction; for the water, immediately 
after the body is plunged into it, acquiring a temperature very 
little different from that which it has when the equilibrium is 
established, the external air must produce only an imperceptible 
effect on it. In the second method, on the contrary, we must 
take into the account the loss of heat which the mass experiences 
in consequence of its excess of temperature and the duration of 
the experiment, This correction may be determined with a 
sufficient degree of precision by a subsequent observation on the 
rate of cooling of the water employed. The large size of the 
* his correction is deduced from the weight of oil, carried off by the metallic 
plate. Toascertain it, we were obliged in each case to make a preliminary expe- 
riment, in which we ascertained the increase of the weight of the plate when it 
came out of the oil bath, At 300° this increase never exceeded three or four decie 
grammes. 
