80 VAPORIZATION. 



more commonly used, acts on the same principle, but con- 

 sists of two similar and very delicate mercurial thermome- 

 ters, the bulb of one of which a is kept constantly moist, 

 while the bulb of the other b is dry. The wet thermometer 

 always indicates a lower temperature than the dry one, unless 

 when the air is fully saturated with moisture and no eva- 

 poration from the moist bulb takes place. In making an 

 observation, the instrument is generally placed, not in absolutely 

 still air, but in an open window where there is a slight draft. 



Srdly. The most simple mode of ascertaining the absolute quan- 

 tity of vapour in the air, is to cool the air gradually, and note the 

 degree of temperature at which it begins to deposit moisture, or 

 ceases to be capable of sustaining the whole quantity of vapour 

 which it possesses. The air is saturated with vapour, for this 

 particular degree of temperature, which is called its dew-point. 

 The saturating quantity of vapour for the degree of temperature 

 indicated, may then be learned by reference to a table of the 

 tension of the vapour of water at different temperatures.* It is 

 the absolute quantity of vapour which the air at the time of the 

 observation possesses. The dew-point may be ascertained 

 most accurately by exposing to the air a thin cup of silver or 

 tin-plate containing water so cold as to occasion the conden- 

 sation of dew upon the metallic surface. The water in the 

 cup is stirred with the bulb of a small thermometer, and as 

 the temperature gradually rises, the degree is noted at which 

 the dew disappears from the surface of the vessel. The tem- 

 perature at which this occurs may be taken as the dew-point. 

 Water may always be cooled sufficiently in summer, to answer 

 for an experiment of this kind by dissolving pounded sal-am- 

 moniac in it. 



The dew point may be observed much more quickly by means 

 of the elegant hygrometer of Professor Daniell.t This instru- 

 ment (see Figure) consists of two balls, connected by a syphon 

 and containing a quantity of ether, from which the air has 

 been expelled by the same means as in the cryophorus of 

 Dr. Wollaston, (page 63). One of the arms of the syphon 

 tube contains a small thermometer, with its scale, which 

 should be of white enamel ; the bulb of the thermometer de- 



* Such a table will be given in an Appendix, 

 f Daniell's Meteorological Essays, p. 147. 



