10 M. V. Regnault on the Elastic Forces of Vapours 



cathetometer, the surface of the mercury in the measuring tube 

 being brought successively to each of the divisions marked upon 

 the tube. 



This done, a certain quantity of the volatile liquid was intro- 

 duced into a test tube, which had been previously filled with 

 dry mercury, exhausted in vacuo ; then, after the communication 

 between the two tubes had been established, the gas was passed 

 from the measuring tube into the other, under a pressure suffi- 

 ciently low to allow the whole of the liquid introduced to vola- 

 tilize in the gaseous space thus afforded it. Lastly, the gas 

 mixed with vapour was repassed into the measuring tube. 



A new series of determinations as to the elastic force of the 

 gaseous mixture was then commenced, by causing this to occupy 

 successively the same volumes that had been occupied by the dry 

 gas in the first series. It is clear that by deducting from the 

 numbers of the second series the corresponding numbers of the 

 first, we get the elastic forces of the vapour at a constant tem- 

 perature, but under variable pressures and different volumes, and 

 consequently in variable states of saturation. 



From these different observations it may be deduced : — 



1. From the first series, whether the dry gas follows the law 

 of Mariotte. I shall give in another memoir the observations 

 which I have made in this manner upon the compressibility of a 

 great number of gases. The method, when executed with the 

 necessary care, allows us to prove that atmospheric air itself 

 departs sensibly from this law under the slightest variations of 

 volume. The differences belong to the order of those deduced 

 from the curve which I have already published, and which in- 

 cludes a great interval of pressure. By the same method I have 

 determined the law of compressibility followed by mixtures, in 

 known proportions, of atmospheric air with a gas, which, like 

 carbonic acid, departs greatly from Mariotte's law. 



2. By combining the two series, we may ascertain how far the 

 gas, more or less charged with vapour, departs from Mariotte's 

 law, when it is more or less distant from the state of saturation. 



3. We may determine how the elastic force of the vapour 

 varies from the moment when the liquid begins to be deposited 

 in drops upon the walls of the tube, and consequently ascertain 

 whether the elastic force of this vapour varies in proportion to 

 the quantity of liquid condensed. 



I give here the series of experiments which I have made upon 

 vapour of sether in atmospheric air and hydrogen gas. I have 

 subjoined other experiments by which I have determined the 

 elastic force of the same vapour in carbonic acid gas, although I 

 do not think that the latter can be employed with confidence in 

 the determination of the tension of the vapour, in consequence 



