252 Heat Evolved on contact of Liquid with Solid. 



experiments such as those made by Bellati. Suppose, for 

 example, p grammes of silica having a specific heat k, and w 

 grammes o£ water, are mixed and raised to a temperature t, 

 and the mixture is then put into a Bunsen's ice calorimeter 

 and cooled to 0° C, the heat given up is 



(pk + w +p .s.j s -p.s. ^ t = (pk + w-p .5 .-) . t, 



approximately, from equation v. 



It is necessary, therefore, to distinguish between the true 



or absolute variation and the apparent variation in the specific 



heat of water in contact with a solid. The true variation in 



dc 

 the specific heat is proportional to y, and is probably very 



small ; but since in any experiment it is impossible to prevent 

 the evolution or absorption of heat at the surface depending 



on the term -7-, the apparent variation in the specific heat, 

 at 



that is, the variation actually observed in any experiment, is 



proportional to the difference of the terms -7- and -y-, that 



is, approximately proportional to -. 



Hence the apparent specific heat of water in contact with 



a solid is approximately (1 ' - ), where A is the area of 



the surface of the water in contact with the solid, and w the 

 mass of the water. For example, in the earlier experi- 

 ments of the present investigation, the mass of water was 

 about 200 grammes and the area of surface exposed by 

 4 grammes of powder was 4x10900 = 43600 sq. cm., and 



the value of - was 37 x 10~ 7 : hence the apparent specific heat 

 of the water was equal to (l-.4|goa x 37 x 10 -7) = .9991.9. 

 It is evident that if the mass of water is small compared with 

 the mass of powder, the variation in the apparent specific 

 heat may be very great, so that it is not necessary to assume, 

 as Martini did, that some of the water is solidified on the 

 surface of the powder, in order to account for the apparent 

 variation in the specific heat. 



VI. Experiments with Mercury. 



Experiments were made to show a fall of temperature on 

 putting a finely divided solid into mercury. After several 

 fruitless attempts with silica, the following method was 

 adopted. 



