formed by Condensation at the Surface of a Solid. 521 



is put into water and the heat evolved is found to be 18*29 

 calories per gm. 



But if the silica had been perfectly dry the heat evolved 

 would have been 26 calories, hence the heat due to 2*38 per 

 cent, of moisture must have been 26 — 18*29 = 7*71 calories 

 per gm. 



The thickness of the film of moisture is about 



•02,38 



= *95xl0 



m-6 



25000 



and the corresponding amount of heat per sq. cm. is 

 7*71 



25000 



= 3*08 Xl0- 4 calories. 



The results show that when the water film is only one 

 millionth of a centimetre in thickness, the heat evolved is 

 about one third of the whole amount, when the thickness of 

 film is two millionths the heat evolved is about one half of the 

 whole, and on further increasing the thickness of the film 

 the amount of heat evolved slowly approaches a maximum 

 which it reaches when the thickness is about 31*6 x 10" 6 , the 

 heat evolved being then *00105 cal. per sq. cm. 



Parts of 





Reduction of 





Heat evolved 



moisture 



Heat evolved 



the heat evolved 



Thickness of 



per sq. cm. by 



absorbed by 



per gramme 



by previous 



water film 



a film of this 



100 parts of 



of silica. 



absorption of 



TOO • a 



thickness 



drv silica. 



(?) 



moisture. 



25000 ' 



26-? 



' <*) 





(26-?) 





25000' 



2-38 



18-29 



7-71 



•95x10"! 

 2-14x10 I 



3-08x10"";! 



535 



12-23 



13-77 



5-51x10 , 



8-59 



9-17 



16-83 



3-44x10 fi 

 5-17x10 , 



7-53xio : 



10-94x10 I 



6-73x10 : 



12-92 



7-61 



18-39 



7-36x10 ; 



18-83 



6-50 



19-50 



780 X10 - ^ 



2736 



5-25 



20-75 



8-30x10 : 



39-95 



3-70 



22-30 



1598x10 : 



8*92x10 , 



46-35 



2-94 



23-06 



18-54 xl0"t 



9-22 xlO -4 



56-48 



1-66 



24-34 



22-59 x 10"^ 



9-74x10" J 



64-78 



•90 



25-10 



25-91 xl0 -b 



10-04 xlO -4 



76-94 



•19 



25-81 



30-78 xur° 



10-32xl0~ 4 



Dr. C. Barns * gives some valuable data on the size of the 

 water particles produced by condensation on a solid nucleus. 

 In the experiments described it seems that the condensation 

 must in the first place be caused by the surface action of the 



* Bams, Phil. Mag. [0] iv. pp. 21 to 29 and pp. 262 to 269 (1902) 



