REGNAULT’S HYGROMETRICAL RESEARCHES. 607 
Part I. 
Of the Elastic Forces of Aqueous Vapour in the Air. 
The maximum elastic forces of aqueous vapour in vacuo have 
been determined by a great number of experimenters ; but the 
results of their experiments differ far too much to establish the 
law of the elastic force of aqueous vapour in vacuo with any de- 
gree of certainty. I have myself lately made a great number of 
experiments on this subject by various methods, and, I believe, 
with all the means of precision which science at present offers. 
As the results which have been obtained have always agreed, I 
am certainly justified in regarding the table calculated from these 
experiments as preferable to all those which have been published 
previously, and in the following pages I shall adopt it exclu- 
sively for the calculation of the maximum elastic forces of 
aqueous vapour @7 vacuo. 
In hygrometrical observations, we require to know the elastic 
force of aqueous vapour, not in vacuo, but in the air under the 
pressure of the atmosphere. It is admitted that these elastic 
forces are absolutely the same as those which exist in vacuo. I 
have in vain sought in the annals of science the experiments by 
which this identity has been established, and I do not think that 
by means of the apparatus which are described in the elemen- 
tary treatises on Physics we can institute experiments suffi- 
ciently precise to leave no doubt on this subject. 
It appeared to me necessary to make some new experiments 
to decide this delicate question, employing apparatus exactly si- 
milar to those which I used to determine the tensions of aqueous 
vapour in vacuo, in order that the results of the two series of 
experiments should allow of more strict comparison. 
The tensions of aqueous vapour in saturated air may be deter- 
mined by means of the apparatus described in my memoir ‘ On 
the Elastic Force of Aqueous Vapour’. Only the apparatus con- 
sisting of the two barometers is replaced by a system of two 
communicating tubes, arranged as in fig. 8, Pl. VII., and care 
is taken in each experiment to restore the level of the mercury 
to one and the same line, marked on the tube pg, in order 
that the volume of air may always remain the same, and that its 
elastic force alone may vary. 
A small glass bulb, filled with water and sealed, is previously 
placed in the balloon. The balloon is perfectly dried, and finally 
