in vacuo and in Gases. 5 



2. The actual density of aqueous vapour in the air might 

 differ from that obtained by calculations founded upon Mariotte's 

 law and the theoretical density ; for this density was only verified 

 by Gay-Lussac at a temperature of 100° C, and under pressures 

 which differed but little from the ordinary pressure of the atmo- 

 sphere. 



I wished first of all to determine as exactly as possible the 

 weight of vapour which might be contained in air saturated with 

 humidity at different temperatures ; for it might be objected, 

 that in the experiments in which I derived the saturated air 

 directly from the atmosphere, there remained some uncertainty 

 as to the exact determination of the temperature. My new ex- 

 periments were free from this objection, for I drew the air, by 

 means of an aspirator, into a series of tubes filled with wet 

 sponge, and kept at a strictly invariable temperature during the 

 whole course of the experiment. Sixty-eight determinations 

 effected in this manner between limits of temperature from 0° 

 to 27° C. all gave the weight of vapour lower than that deduced 

 from calculation. The differences were, however, inconsiderable, 

 rarely rising to T \jth of the total weight. 



The fact being thus completely proved, its cause remained to 

 be discovered, and for this purpose I undertook experiments to 

 determine directly the density of aqueous vapour within limits 

 of temperature analogous to those existing in my first experi- 

 ments, as well as the elastic force of this vapour in the air. Un- 

 fortunately the direct determination of aqueous vapour in gases 

 at low temperatures presents difficulties which are almost insur- 

 mountable, the ponderable quantity of this vapour being too 

 small in proportion to that of the whole elastic fluid. I was 

 therefore compelled to confine myself to making experiments in 

 vacuo. As long as the fraction of saturation does not attain 

 T %ths, I found the density of aqueous vapour to be equal to that 

 deduced from the theoretical density, by the application of 

 Mariotte's law. But this density increases rapidly in proportion 

 as it approaches saturation. I think, nevertheless, that this 

 rapid increase is produced principally by a condensation of 

 liquid water upon the walls of the balloon in which my experi- 

 ments were made ; and this condensation commences long before 

 saturation. 



I could find no means of getting rid of this cause of error, and 

 I believe that the most exact method of determining the density 

 of a vapour in a gas still consists in condensing, by means 

 of absorbing substances, the vapour existing in a given volume 

 of gas, under known conditions of temperature and pressure. 

 But in this case the law of elastic forces must be thoroughly 

 settled. b 3 



