466 Mr. A. P. Chattock on an 



Now I think it will be admitted that if permanent charges 

 are to exist on the molecules of a body, the probabilities are 

 enormously in favour of their being ionic charges ; so that 

 the fact that the above line of argument leads to a value for q 

 so near 10 -11 is in itself evidence in favour of the truth of the 

 assumptions on which the reasoning was based. It was on 

 this ground that I concluded above that the pairing of the 

 molecules had taken place to an appreciable extent before the 

 limit of tenacity was reached. At first sight, it is true, this 

 method of justifying the means by the ends looks like a vicious 

 circle, but I believe it is in reality reliable. One obtains, as 

 it were, a new fact in the agreement between q and 10 — ll ; 

 and this affords a fresh point of departure for the reasoning. 



Coining now to the calculation of q from pyro-eleetric data, 

 Biecke * has measured what he calls the " specific electrical 

 moment " brought about by a change of temperature of 100° 

 in tourmaline ; i. e. the quantity of electricity set free at the 

 end of a rod of tourmaline of unit length parallel to the axis 

 and weighing one gramme. The mean value of this quantity 

 for twenty-two different specimens is 42 E.S. units, which, 

 when multiplied by the mean value of the densities of the 

 specimens (3*1) and divided by 100°, gives 1*3 as the number 

 of E.S. units liberated per sq. cm. of crystal surface normal 

 to the pyro-electric axis per degree alteration of temperature 

 ( = k) . The different specimens do not, however, agree very 

 closely with each other ; the maximum value of k for any 

 specimen being 1*8 while the minimum value is 0'5 |« 



Proceeding in the same manner as for piezo- electricity , 

 q will be given by the equation 



n 



where T is the range of temperature within which the mole- 

 cules pass from the state of absolute contact with each other 

 to the state of complete pairing (i. e. the range from the 

 absolute zero of temperature to the evaporation-point of the 

 crystal), and k is the mean value of k throughout that 

 range. 



Now k is necessarily unknown, and in all probability very 



* Wied. Ann. xl. p. 264. 



t It is perhaps worth mentioning that an exceedingly rough attempt 

 of my own to integrate the current from a crystal of tourmaline with time 

 during a known alteration of temperature corroborates Riecke's result so 

 far as the order of magnitude is concerned. It indicates a value of k 

 greater than any of his, but the maximum galvanometer reading was only 

 half a millimetre. 



