Absorbed on contact of Liquid with Solid. 243 



It seems highly probable that the specific heat of the water 

 close to the surface of the silica differs from the specific heat 

 of the water which is farther away from the surface, but any 

 attempt to distinguish clearly between the two must neces- 

 sarily lead to some doubtful assumptions. A. consideration 

 of the Pouillet effect is, however, incomplete if the possibility 

 of such a variation in the specific heat of the liquid is not 

 taken into account. There is also another possible source of 

 variation in the quantity of heat evolved, which none of the 

 investigators have considered, namely, the variation of the 

 heat evolved with the temperature. In many of the records 

 of experiments the initial temperature of the calorimeter is 

 not even stated, and in other cases, where the temperatures 

 are recorded, it is impossible to decide whether the variations 

 in the quantity of heat evolved depend upon differences 

 in the initial temperature of the liquid and powder, or upon 

 change of specific heat of the liquid, or upon some cause of 

 error in the experiment. 



The objects of the present investigation were to obtain a 

 relation between the quantity of heat evolved and the area of 

 the surface exposed, to find the rate of variation of heat 

 evolved with variation of temperature, and to apply to the 

 results the laws of thermodynamics. 



The nearest approach to a relation connecting the 

 quantity of heat evolved with area of surface is the state- 

 ment of Mr. Linebarger, that the finer the powder the greater 

 the heat effect, and in the case of water and silica the heat 

 effect is about proportional to the fineness of the powder. 

 This conclusion is based upon a very few experiments with 

 two samples of silica of different degrees of fineness, and it is 

 directly opposed to the views of Martini, who states that the 

 fineness of the powder does not influence the result to any 

 important extent *, 



II. 



On the Relation between the Area of Surface of Silica and 

 the Heat evolved on bringing the Surface in contact with 

 Water. 



In making any experiment on the Pouillet effect, it is 

 essential that the powder should be perfectly dry and that it 

 should be at exactly the same temperature as the liquid. 

 Very few of the investigators have succeeded in securing these 



* Martini, Atti del R. Istituto Veneto, Tomo lix. Parte Seconda 

 (]900), p. 622 (Non e dunque la sottigliezza dei o-ranuli della polvere eke 

 influisca in modo sostanziale sul fenomeno coine ce Jo provano le tre 

 qualita di carbonato di calce). 



R2 



