Laws of Evaporation and Absorption. 335 



rate of 6°*85 per minute; whilst that of water, in dry air at 

 the same temperature, produced a fall of 1 0, 33 per minute. 



Hence we have for the ratio of the velocities — = ,-- ? -- = 5'15. 



V l'QO 



Now the tension P of the vapour of ether at 12° is 12*243, and 

 that of water at the same temperature is jy = -4104. Moreover, 

 taking the latent heat of the vapours of ether and water for equal 

 weights to be 168 or L, and 1000 or / respectively, we have 



P_xL_ 12-243 x 168 



pxt ~"-4104x 1000 ~ ' ' 



, . ., V P x L . 



showing that — = = verv nearly. 



v px I 



Again, at the temperature of 5° C, it was found that V = 5°, 

 and v = 0°'So; 



V 5 „ . . PxL 9-058x168 a 



.•.-=—- =6 nearly, and T = -^— — ^r^ =6. 



v -85 J pxt *2ob x 1000 



Other things being the same, it may be presumed that the 

 cooling effect of evaporation varies inversely as the density or 

 pressure of the air. 



Heating Effects produced by the Absorption of Vapour by Dry 

 Substances, fyc. from an Atmosphere saturated with Vapour. 



2. As evaporation produces cold, so the condensation of vapour, 

 from whatever cause, produces heat. The force with which dry 

 substances (such as woollen, cotton, and other textile fabrics) 

 attract and condense the vapour of water is so great as, under 

 favourable circumstances, to raise the temperature of a thermo- 

 meter 30° F. Acting in this manner, I have found that the 

 heating power of dry woollen cloth is as great as any other sub- 

 stance, being about the same as that of fused chloride of calcium. 

 The heat developed not only depends on the quantity of the sub- 

 stance used, but also on the temperature of the air at the time, 

 as shown by the following experiments. 



The heat produced by the absorption of vapour, by dry substances, 

 is proportional to the tension of the vapour. 



Experiment V. A double roll of woollen cloth was fitted as a 

 cap to the bulb of the thermometer before described, and inserted 

 in the sulphuric-acid bottle described in Experiment III., where 

 it was allowed to remain for some hours. The cork of this ther- 

 mometer fitted the mouth of a large humid-air bottle containing 

 fifteen half-pints, in which a portion of water was placed. By 

 occasionally agitating the water of this bottle and allowing it to 



