78 VAPORIZATION. 



quantity of any vapour rises into a vacuum at any particular 

 temperature ; the same quantity rises into air. Thus the vapour 

 from water, which rises into a vacuum at 80% depresses the 

 mercurial column one inch, or its tension is one thirtieth of the 

 usual tension of the air. Now, if water at 80" be admitted into 

 dry air, it will increase the tension of that air by 1 30th, if 

 the air be confined; or increase its bulk by 1 30th if the 

 air be allowed to expand. 



The spontaneous evaporation of water into air is much af- 

 fected by three circumstances : 1. the previous state of dry- 

 ness of the air, for a certain fixed quantity only of vapour can rise 

 into air, as much as into the same space if vacuous ; and if a 

 portion of that quantity be already present so much the less 

 will be taken up by the air ; and no evaporation whatever takes 

 place into air which contains this fixed quantity, and is already 

 saturated with humidity. 2. By warmth, for the higher the tem- 

 perature the more considerable is the quantity of vapour which 

 rises into any accessible space. Thus water emits so much 

 vapour at 40 as expands the air in contact with it 1 114th 

 part, and at 60 as much as expands air 1 57th part, or 

 double the quantity emitted at the lower temperature. Hence, 

 humid hot air contains a much greater portion of moisture than 

 humid cold air. 3. The evaporation of water is greatly quick- 

 ened by the removal of the incumbent air in proportion as it 

 becomes saturated ; and hence a current of air is exceedingly 

 favourable to evaporation. 



When air saturated with humidity at a high temperature is 

 cooled, it ceases to be able to sustain the large portion of 

 vapour which it possesses, and the excess assumes the liquid 

 form, and precipitates in drops. Many familiar appearances 

 depend upon the condensation of the vapour in the atmosphere. 

 When a glass of cold water, for instance, is brought into a 

 warm room, it is often quickly covered with moisture. The air 

 in contact with the glass is chilled, and its power to retain 

 vapour so much reduced as to occasion it to deposit a portion 

 upon the cold glass. It is from the same cause that water is often 

 seen in the morning running down in streams upon the inside 

 of the glass panes of bed-room windows. The glass has the low 

 temperature of the external air, and by contact cools the warm 

 and humid air of the apartment so as to occasion the precipi- 

 tation of its moisture. Hence also, when a warm thaw follows 



