240 Mr. Gr. J. Parks on Heat Evolved or 



The enlargement necessary to carry the wheel may be 

 conveniently made circular (at least in the portion remote 

 from A) round the point B as centre. In this case it will be 

 easily moved along a ruling-edge set parallel to the motion 

 of B. 



The bar may be graduated in units of b from the point B 

 as zero, and the slotted pin-head may have a vernier for 

 reading cosh 6. Of course CB on the same scale will be 

 shin 0. 



XXVII. On the Heat Evolved or Absorbed when a Liquid is 

 brought in contact ivith a Finely Divided Solid. By 

 G. J. Parks *. 



I. Introduction. 



POUILLET | discovered the fact that when a powder is 

 put into a liquid which does not exert any solvent 

 or chemical action upon it, there is a rise of temperature. 

 In some of the experiments made by Pouillet with mineral 

 substances, the rise of temperature varied from *3° to 1° C. 

 This discovery was confirmed by several other investigators, 

 but nearly all the earlier observations were merely therm o- 

 metric, and are therefore of little value for purposes of com- 

 parison, since the rise of temperature must obviously depend 

 on the thermal capacity of the whole mass throughout which 

 the heat is distributed. In fact, by suitably varying the con- 

 ditions of the experiment it has been found possible to obtain 

 any rise of temperature up to 30° C. or more. 



Junck \ found that when sand is placed in water the tem- 

 perature of which is above 4° C. there is a rise of tempera- 

 ture, and when the temperature of the water is below 4° C. 

 there is a fall of temperature. This is quite in accordance 

 with what would be expected on the supposition that the 

 Pouillet effect is due to a pressure at the surface of the powder, 

 and the variation of pressure for a given variation of tem- 



J.cp 



perature can be calculated from the equation dp — - — '—.dr, 



where a is the coefficient of expansion of the liquid at con- 

 stant pressure, p the density, c the specific heat, t the absolute 

 temperature, and J the mechanical equivalent of heat. 



Meissner § showed that when certain powders are placed 



* Communicated by the Physical Society : read June 20, 1902. 

 t Pouillet, Ann. de Chim. el de Physique, xx. p. 141 (1822). 

 t Junck, Pogg. Ann. cxxv. p. 292 (I860). 

 § Meissner, Wied. Ann. xxix. p. 114 (1886). 



