520 Dr. Parks on the Thickness of the Liquid Film 



rods was '001)8 cub. mm. per. sq. mm., that is 80 x 10 -6 cub. 

 cm. per sq. cm. of surface. 



It is not improbable that in these experiments the sulphur 

 dioxide was coudensed by chemical combination with a film 

 of water previously existing on the surface of the glass, but 

 it may be noticed that the result obtained is of the same order 

 of magnitude as all the other results quoted in this paper. 



Martini * found that some precipitated silica exposed to 

 aqueous vapour increased in weight by 80 per cent, without 

 any alteration in appearance : on putting this moist silica 

 into water no heat was evolved, though the heat evolved on 

 wetting the dry silica amounted to 19 calories per gramme. 

 Martini does not state the artyi of surface exposed by the 

 pow^der, but the author has shown t that when dry silica is 

 wetted, the amount of heat evolved is about "00105 calorie 

 per sq. cm. ; and hence we may take the area of surface of 

 the powder used by Martini as about 18,000 sq. cm. per gm., 

 and the extreme thickness of the aqueous film w^ould therefore 



be l8feo =44xl0 " cm - 



It should be remarked here that when a powder is exposed 

 for a long time in an open tray it is likely to receive some 

 moisture by the ordinary process of condensation at tem- 

 peratures below^ the dew-point, and thus the more exposed 

 portions of the powder may receive excess of moisture. 



Bellati and Finazzi J have recently made an excellent 

 series of experiments showing the relation between the 

 amount of moisture absorbed by silica and the heat evolved 

 on putting the powder into water. The area of surface 

 exposed by the silica is not stated, but the authors consider 

 with good reason that if the powder were perfectly dry the 

 heat evolved on wetting it would amount to 26 calories per 

 gm., and hence w r e may assume the surface exposed by the 

 powder to amount to about 25,000 sq. cm. per. gm. 



In the following table the figures of the first two columns 

 have been selected from the original paper referred to, and 

 the other three columns have been introduced by the author 

 for the purposes of the present inquiry. An example wdll be 

 sufficient to explain the table and the diagram (p. 522) . When 

 the dry silica has absorbed, say, 2*38 per cent, of moisture it 



* Martini, Atti del JR. Istituto Veneto, Tonio lix. Parte Seconda. 

 p. 624 (1900). 



f Phil. Mag-. [6] iv. p. 247 (1902). 



\ Bellati e Finazzi, Atti del JR. Istituto Veneto, Tomo lxi. Parte 

 Seconda, p. 514 (1902). 



