Meteorological observations and essays / By John Dalton.

  • John Dalton
Date:
1834
Catalogue details

Licence: Public Domain Mark

Credit: Meteorological observations and essays / By John Dalton. Source: Wellcome Collection.

    220/280 (page 194)
    air in the receiver admits of a diminution almost ad infintum, and vapour generated instantaneously supplies the place of the air withdrawn ; when air is let in, the vapour in the pear gauge is condensed, and there remains nothing but the small portion of air, with its saturating portion of vapour, at the top of the gauge.—The reason why the repetition of the experiment decreased the rarefaction by the pear gauge, was, that the frequent condensations of air and vapour in the barrel of the pump must have produced a deposition of water there, by which the effect was sooner at its ne plus ultra; for, when the vacuum of the barrel is not perfect, the quan- tity drawn from the receiver in a given time must be less than otherwise. I have no doubt if the experiments had been repeated often enough, the leather of the piston and the valves would have been in effect soaked with water, and the result as stated in the 4th fact: in this case, as soon as the spring of the air in the receiver is weakened to a certain degree, working the pump does not avail, because the vapour in the barrel, together with the resistance of the valves, is just sufficient to coun- teract the spring of the air within ; hence the rare- faction by the pear gauge is then scarcely greater than by the barometer gauge. Experience proves that spirit of wine rises sooner into vapour than water; consequently the rarefac- tion by the pear gauge, when the pump is wrought a given time, must be less than when water is used. Also, it follows, a priori, that the cooler the circum-