Meteorological observations and essays / By John Dalton.

  • Dalton, John, 1766-1844.
Date:
1834
Catalogue details

Licence: Public Domain Mark

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

    152/280 (page 126)
    are known to have that effect, from every one’s experience ; the last is proved to have such an effect, by the air-pump ; for, when the air is ex- hausted out of a receiver, a large quantity of vapour is raised from the wet leather upon the pump plate; this vapour is precipitated again when the air is let in, so as to appear falling like a shower.* Ifa quantity of warm water be placed under a receiver, when the air is rarefied to a suf- ficient degree, the water boils with great violence, and a large portion of it may in this manner be readily raised in vapour, which is as soon condensed by the cold of the surrounding medium, and falls upon the leather of the pump plate. The reason of this is, that the greatest heat water is susceptible of, or its boiling heat, depends upon the pressure of the air upon its surface ; the less the pressure, the less is the boiling heat ; and whenever it ar- rives at the boiling heat, the greater heat applied to augment its temperature, instead of doing so, converts a portion of it into vapour, which, as has been remarked, absorbs a great quantity of heat, without any increase of temperature.7 As this variation of temperature in boiling water according to the different pressure of the air, isa circumstance not foreign to the subject we are upon, and perhaps the quantity and mode of the variation * See a note upon this subject, page 129. + Hence we see the reason of the proviso, page 19, in determining the boiling point of thermometers.