559 
ARTICLE XX. 
On the Elastic Forces of Aqueous Vapour. 
By M. V. Reenavutr*. 
[From the Annales de Chimie et de Physique for July 1844.] 
THE theoretical calculation of the work done by steam-engines 
requires a knowledge of a certain number of laws and physical 
data which have by no means up to the present time been esta- 
blished with sufficient accuracy. The chief of these data are :— 
Ist. The elastic forces of aqueous vapour at different tempe- 
ratures. 
2nd. The quantities of heat contained in a given weight of 
vapour when saturated under different pressures, or, more cor- 
rectly, the quantities of heat which 1 kilogramme of vapour in 
a state of saturation under different pressures will give off when 
it is converted into the state of liquid water at 0° Centig. 
3rd. The specific heat of liquid water at various temperatures. 
_ 4th. The density which the vapour of water in a state of sa- 
_ turation exhibits under different pressures. 
The first of these laws, that which treats of the elastic force 
of aqueous vapour at various temperatures, is the only one esta- 
blished in an exact manner since the beautiful experiments of 
MM. Dulong and Arago. The others may be considered as en- 
tirely unknown ; those which are adopted in practice have been 
admitted by simple induction, either by extending to vapours 
those laws which have been observed to apply approximatively to 
gases, or in slightly modifying those laws according to empirical 
results deduced from experiments nearly always incomplete and 
uncertain. 
The unquestionable utility attending the completion of our 
knowledge of the theory of vapours, induced me to undertake 
the solution of these various questions, well-aware of the great 
difficulties to be overcome, which however did not appear to me 
insurmountable. I should have been completely stopped in the 
execution of this project by the considerable expenses which the 
procuring and mounting of apparatus would have incurred, had 
not the Minister of Public Works, at the request of the Central 
* Translated by E. Ronalds, Ph.D. 
