464 Mr. A. P. Chattock 



on an 



Molecular Charge. 



If, then, the number (?i) of molecular chains per sq. cm. 

 of cross section of a crystal were known, and if it were possible 

 to measure the alteration (p) in the density of the free charges 

 on its two electrified surfaces, due to an alteration (either 

 pyro- or piezo-electrically) from the state in which the mole- 

 cules are in absolute contact to that in which they are com- 

 pletely paired, it would be easy to calculate the value (q) of the 

 charge possessed by each molecule. For,by what has been said, 



2p 



n 



This it is not possible, with any accuracy, to do ; but neverthe- 

 less, both from pyro- and piezo-electric data, a rough estimate 

 of the value of q may be formed. 



J. and P. Curie * have measured the piezo-electric constant 

 for tourmaline and quartz : for the former they found it to be 

 5*4 x 10- 8 , and for the latter 6*4 x 10~ 8 E.S. units of electri- 

 city per dyne. 



Consider a cubic centimetre of one of these crystals, cut 

 with two faces perpendicular to the piezo-electric axis. It is 

 very probable that the molecules of a solid body occupy 

 from about \ to i the space occupied by the body itself f. 



1 — a/ — is therefore about the length by which a crystal 



1 centimetre long must be shortened in order to bring its 

 molecules into contact. Let M be the average value of 

 Young's Modulus for this compression, and the force in 

 dynes per sq. cm. which will effect the compression is 



( i --C / i L 5 ) M = (MM - 





This quantity added to the tenacity (T) represents the whole 

 range of stress to which the crystal can be subjected, and this 

 multiplied by the average value (k) of the piezo-electric 

 constant over the whole range of stress = (0*4 M + T) \ a 

 quantity which is less than p in the above equation for q. In 

 this expression T can be known accurately, but M and k are 

 only measurable under normal conditions; and it cannot be 

 doubted, of course, that even if the crystal could be kept from 

 crushing, the values of both M and k would undergo very 



* Comptes Rendus, xciii. p. 204 (1881). 



t Riicker, Journ. Chem. Soc. liii. p. 222 (1888). 



