334- Mr, T. Tate's Experimental Researches on the 



Table of results of Experiments, giving the rates of cooling, 

 &c. produced by the Evaporation of Water in a dry Air at 

 different Temperatures. 



Tempera- 



; Ratio of 

 cooling 



Maximum 

 depression, 



Corre- 

 sponding 



Corre- 

 sponding 



Corre- 

 sponding 



Value of v 



Value of D 



degrees 

 Cent., 



per minute, 



in degrees 



minimum 



tension of 



tension of 



bv formula 



by formula 



in degrees 



Cent. 



tempera- 



vapour at 



vapour at 



v = 3'5p. 



(1). 



T. 



Cent., 



D. 



ture, 



T temp., 



T 1 temp., 







.. 





T,- 



P- 



ft. 











1 



0-67 







4-5 







-3-5 



•194 



•133 



068 



450 



52 



0-85 



5-5 



-0-3 



•259 



•178 



0-90 



577 



10 



1-25 



6-8 



3-2 



•361 



•227 



1-26 



706 



14 



1-63 



8-0 



60 



•467 



•274 



1-63 



8-22 



17 



2-00 



90 



8-0 



■566 



•315 



1-98 



9-20 



241 



3-00 



11*5 



12-6 



•868 



•426 



304 



11-50 



Here the near coincidence of the results given in the second 

 and seventh columns shows that the rate of cooling, by the eva- 

 poration of water, varies as the tension of vapour at the given tem- 

 perature, the comparatively small differences of the latent heat 

 in this case being neglected. Again, the near coincidence of the 

 results given in the third and eighth columns shows that for- 

 mula (1) very nearly represents the maximum depression. This 

 formula is derived from theoretical reasoning, in the following 

 manner. 



At the point of maximum depression, the rate of cooling by 

 evaporation must be equal to the rate of heating due to radiation. 

 Now the rate of cooling varies as p v the tension of vapour at the 

 corresponding temperature; but by Dulong and Petit' s formula 

 the rate of radiation varies as 1*0077 T (1'0077 D — I) ; therefore 

 at the point of maximum depression we have 



l-0077 T (L-0077 D -l)=«p 1 , 



where a is a constant, which we find by elimination to be '265. 

 Whence we find 



R =i^ow^(roW T +1 )= 300 ^(i^+ 1 > (1) 



An approximate value of D may be found in the following man- 

 ner. Putting 1 + e for 10077, we find by development, and 

 neglecting the higher powers of e, (l+e) D — l=eD nearly; 

 hence we have a? x eD = ap l ; 



ap x _ 34-274^ ■ 



D = 



ea. 



1-01 T 



Experiment IV. When the temperature of the air was 12° C, 

 the evaporation of ether caused the thermometer to fall at the 



