[ 517 ] 



LIII. On the Thickness of the Liquid Film formed by 

 Condensation at the Surface of a Solid. By Dr. G. J. Pakks *. 



IT was known more than half a century ago f that when a 

 solid is placed in a gas or vapour there is a condensation 

 of the latter on the surface o£ the solid, and in particular 

 that glass has the power of condensing water-vapour at 

 temperatures above the dew-point. 



Aragof proposed to measure the amount of condensation 

 by the optical method of interference, and quite recently 

 Lord Kelvin § has suggested a method depending upon 

 electrical conductivity. 



In almost every department of physical research glass 

 bulbs or tubes are used, and the presence of moisture on the 

 surface of the glass is a continual source of trouble. Prof. J. 

 Trowbridge || has lately called attention to this matter in 

 connexion with spectrum analysis. 



There can be little doubt also that many of the standard 

 results for the specific heats of finely divided or porous solids 

 are incorrect, for if a solid is perfectly dry, heat will be 

 evolved on wetting it 1" when it is placed in the calorimeter, 

 and if it is not dry then the specific heat obtained is not the 

 true specific heat of the solid ; in either case the specific heat 

 obtained will be too high. Thus, from the most recent deter- 

 mination of the specific heat of pure precipitated silica, well 

 dried and sealed in a bulb, the value appears to be *1808 **, 

 but the values previously obtained for amorphous silica are 

 much higher than this. 



The present brief inquiry, which is not intended to be by 

 any means exhaustive of the subject, arose out of another 

 investigation, not yet completed, in which the author has 

 attempted to determine by direct experiment the surface- 

 pressure of water and other liquids in contact with glass. 



It was found that everything depended on keeping the 

 surface of the glass perfectly free from moisture until the 

 moment of the experiment ; and the author was thus led to 

 consider the quantity of moisture concerned in surface- 

 pressure, in the Pouillet effect, and in surface-action generally. 



* Communicated by the Physical Society : read February 27, 1903. 



t Jamin et Bertrand. Phil. Mag. [_4J vi. p. 157 (1863); Con^tes 

 Rendus, June 1853, p. 994. 



% See Phil. Mag. [4] vi. p. 157 (1853). 



§ Lord Kelvin, Phil. Mag. [6] iv. p. 181 (1902). 



|| Trowbridge, Phil. Mag. [6] iv. p. 150 (1902), 



IT Parks, Phil. Mag. [6] iv. pp. 240, 251 (1902). 

 ** Bellati e Finazzi, Atti del JR. Istituto Veneto, Tomo xi. Parte 

 Seconda, p. 507 (1902). 



