242 Mr. G. J. Parks on Beat Evolved or 



the influence of the substance in solution ; and in fact the 

 whole research was the outcome of another investigation in 

 which Dr. Gore showed that a powder has the property of 

 abstracting from a liquid part of the substance in solution. 

 One remark of Dr. Gore bears on the present investigation : 

 having made observations on precipitated silica and sand, he 

 states that the action is purely a surface one, and he suggests 

 that if the relation between the rise of temperature and the 

 area of the surface were known, the method could be employed 

 to obtain the area of the surface of a powder. 



Prof. Fitz Gerald* regarded the Pouillet effect as due to a 

 pressure at the surface of the powder and the liquid ; he 

 suggested the application of the laws of thermodynamics, and 

 he pointed out the need of further investigation and quanti- 

 tative treatment. 



Notwithstanding the large number of observations which 

 have been made in connexion with this phenomenon, there 

 are no data by means of which we are enabled to express the 

 quantity of heat evolved as a function of the area of the sur- 

 face. The experiments of Martini, Ercolini, and others show 

 that for the same powder and liquid the quantity of heat evolved 

 is proportional to the mass of the powder used in the experi- 

 ment, but no attempts have been made to calculate the area 

 of the surface exposed to the liquid. The equation used by 



Ercolini is M -\-pc — ?■ ^' = 0, where M is the mass of the 



water, including the water equivalent of the calorimeter, p is 

 the mass of the powder, and c the specific heat of the powder, 

 6 is the observed rise of temperature, k is a constant and 

 represents the amount of heat evolved on putting one gram of 

 the powder into water. 



The value of k is, however, not really constant, but di- 

 minishes very slightly as p increases. Martini explains this 

 on the supposition that some of the water is solidified on the 

 powder and its specific heat is thereby reduced to *5. Bellati 

 has, however, shown by direct experiment that this supposi- 

 tion is wrong. Some silica was well dried, and then exposed 

 to aqueous vapour so that it absorbed moisture, the mass of 

 which was determined by weighing the silica before and after 

 its exposure ; the specific heat of the water was then found 

 by means of a Bunsen's ice-calorimeter, an assumed value 

 being taken for the specific heat of silica. The specific heat 

 of the water was found to be much greater than *5, and it did 

 not differ very much from 1. 



* FitzGerald, 'Nature,' vol. xlix. pp. 293, 316 (1894). 



