formed by Condensation at the Surface of a Solid. 519 



Temperature during experiment about 12° C. 

 Weight of silicate 1*10 gm. 



Total area o£ surface of silicate and tube about 1000 sq. cm. 

 Initial weight of tube and silicate 7'9517 gm. 

 Weight after 4 days 7'9650 gm. 



6 „ 7-9650 „ 

 Increase of weight -0133 „ 

 Estimated thickness of film 13*3 x 10~ 6 cm. 



Fig. 1. 

 lilt 1 I I ■ I I 1 



- t » I 1 1 1 I I I 1 1 1 1 1 I I I 



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 

 Time, in days. 



The cotton silicate thus covered with a film of moisture 

 showed no alteration in appearance even when examined 

 under the highest power of the microscope, but when the 

 silicate was placed in water no heat was evolved, though 

 when the same substance was thoroughly dried and placed in 

 water the heat evolved amounted to *0011 calorie per sq. cm. 

 Hence it may be inferred that the Pouillet effect for water 

 in contact with glass at 12° C. is confined to a film of 

 moisture the thickness of which is about 13'3 x 10 -6 cm. 



It will be interesting to compare with this result the results 

 obtained by other experimenters with different substances 

 and under widely different conditions. The earliest measure- 

 ment of surface condensation of which the author has been 

 able to find an account, is that of Magnus *, who, from experi- 

 ments on the expansion of sulphurous acid gas, Bound that 

 the amount of gas condensed on the surface of smooth glass 



* Magnus, Phil. Mag. [4] vi. p. 836 (1853), 



