On the Piezo-electric Property of Quartz. 331 



Further observations with the liquid air when a good deal of 

 nitrogen had evaporated gave //,= 1*215 ; and when still 

 more nitrogen had gone we found /^ = 1*218. 



The value found above for the refractive index of liquid 

 nitrogen at —190° C. when it has, according to Prof. Dewar's 

 observations, a density of *89, gives for the refraction-constant 



= 0*225, and for the refraction-equivalent 3*153. 



Qj 



If we take Mascart's value for /a for gaseous nitrogen, 

 namely 1*000298, and the density at '001256, we get for 



—j— the value 0*237, which agrees well with that found 



above for the liquid. 



If we take Lorenz's formula (% — T ) T for the constant of 



\fi 2 + 2J a 



refraction, we get the value for liquid nitrogen 0*1474 and 

 the refraction-equivalent 2*063. 



Briihl's values of the refraction-equivalent, derived from 

 observations of the compounds, were for free nitrogen 2*21, 

 for nitrogen as in nitrous oxide 2*27 at least, and for nitrogen 

 as in ammonia 2*50. These numbers are somewhat larger 

 than ours, but not a great deal. On the other hand, Glad- 

 stone's value for nitrogen in nitriles was 4*1 and in ammonia 

 5*1, which differ a good deal from our value, using Glad- 

 stone's formula, namely 3*153. 



We hope in a short tima to give the refractive indices for 

 other rays, and the dispersive powers, of liquid oxygen, 

 nitrogen, and air at a temperature as low as —200° C. 

 August 14, 1893. 



XXXIII. On the Piezo-electric Property of Quartz. 

 By Lord Kelvin *. 



§ 1. TN the present communication we are not concerned 

 JL with the six-sided pyramid, or planes parallel to the 

 sides of a six-sided pyramid, seen at the ends of a quartz 

 crystal farthest from the matrix. Nor are we concerned with 

 the transverse striae generally seen on the sides of the prism, 

 which are undoubtedly sk-ps (probably having faces parallel 

 to the faces of the terminal pyramid) by which the prism 

 becomes less in transverse section from the matrix outwards. 



* Communicated by the Author, having" "been read before Section A of 

 the British Association at its recent meeting in Nottingham. 



Z 2 



