Electrolytic TJieory of Dielectrics. 467 



different from k measured at ordinary temperatures ; and in 

 the expression icT there is no compensating action, such as 



occurred in the corresponding expression JcM for piezoelec- 

 tricity. Hence it is inadmissible to substitute the measured 

 value of k for k. If, however, instead of T we write a quan- 

 tity corresponding to Young's Modulus — if, in other words, 

 we make pyro-electricity a question of change of length of 

 the crystal, like piezo-electricity, rather than of change of 

 temperature — the difficulty of not knowing the true value 

 of 1c vanishes to a great extent, as it did in the case of h. 

 Instead, therefore, of taking for T a range of temperature equal 

 to the absolute temperature of evaporation, calculate the 

 alteration of temperature required to bring the molecules into 

 contact (i. e. to shorten the crystal by 0*4 of its length) in 

 terms of the mean coefficient of expansion (a) during such an 

 alteration; add to this the temperature of evaporation (t) 

 reckoned from 0° C, and the result is T. q thus becomes 



»e?«) 



Now by precisely similar reasoning to that employed in the 

 case of the product TcM for piezo-electricity, it may be shown 

 that as k alters in one direction so 1/et alters in the other ; 



their product thus tending to constancy. — may therefore be 



fC 



represented by - (fc and a being for normal temperature) with 



the same sort of accuracy that kM. is represented by &M. 

 Unfortunately t cannot be considered negligible compared 



with -3-, as this is by definition equal to 273. If, however, 



we strike out t from the equation, we shall be calculating q 

 from the amount of electricity set free during an alteration of 

 the crystal from the condition in which its molecules are in 

 contact to the condition of normal temperature and pressure ; 

 that is to say, for precisely the same change that was used for 

 the piezo-electric calculation. The values of q obtained by 

 these two methods should therefore agree with one another, 

 at the same time that both are too small. 



According toFizeau, a for tourmaline =0*000009. Hence 



SSX1-3X °' 4 



>iXl ^ x 0-0Q000U 



1- <4xl0>« >dXlU ' 



