Absorbed on contact of Liquid with Solid. 251 



V. 



On the Bate or Variation of the Specific Heat of Water with 

 Extension of the Water-Silica Surface. 



It has been shown that -j- = -j- , and hence from the 



ds t dt 



results stated above it follows that - r must be very small. 

 77 7 as J 



If the values of —r and - are correct to the second significant 



dt dc T 

 figure the value of -r is 2 x 10 — ', and this may be taken as 



. . ds . dc 



indicating the order of magnitude of the term — • 



dc 

 For an accurate determination of — , experiments would 



have to be made over a much wider range of temperature, 

 and it would be advisable also to obtain, if possible, a silica of 

 much finer and more uniform quality than that used in these 

 experiments. 



It is known that the surface-tension of a liquid in contact 

 with air can be represented approximately as a linear function 



dc 

 of the temperature, and hence, from equation (vii.), ~r is 



approximately zero. But it cannot be assumed that the 

 surface-pressure of water and silica follows the same law as the 

 surface-tension of a liquid in contact with air. 



In making any experiment to determine directly the 

 specific heat of water in contact with silica, we are met at 

 once with the difficulty that the specific heat of silica is not 

 accurately known. 



According to Joly* the specific heat of amorphous silica 

 is *2375. but it has been suggested f that this value is too 

 high, and that the error arose through neglect of the Pouillet 

 effect ; a similar question in regard to the specific heat of 

 carbon has been investigated by Kopp, Wullner & Bettendorf, 

 and "Weber %. 



Bellati §, in his attempt to determine the specific heat of 

 moisture absorbed by silica, took for the specific heat of the 

 silica the value '1993 as the mean of the various values 

 obtained for different forms of silica. 



The value of — cannot, however, be obtained by direct 



* Joly, Proc. R. Soc. xli. p. 250 (1886). 



t Martini, Atti del R. IstitutoVeneto, Tomo lix., Parte seconda, p. 637. 



% Weber, Pof/f/. Ami. t. 154, pp. 367-423 (1875;. 



§ Bellati, Atti del 11. Istituto Veneto, Tomo lix., Parte seconda, p. 945. 



