112 Prof. S. P. Thompson on the 



but a glance at; Mallet's figures will show that it must be 

 enormous. 



As I said at the beginning, this paper is but a summary of 

 methods and conclusions. Within its scope elaborate discus- 

 sions would not be admissible. I hope that my complete 

 memoir will soon be published ; and it is my intention to 

 discuss, in an appendix to it, both the bearing of the results 

 upon Prout's hypothesis, and the distribution of the variations 

 therefrom. I ought to say, that at the beginning of my 

 investigation I was strongly prejudiced against Prout's hypo- 

 thesis, and fully believed that it had been for ever overthrown. 

 My results have forced me to give it very respectful con- 

 sideration. 



XII. On the Opacity of Tourmaline Crystals, 

 By Professor Silvanus P. Thompson, B.A., B.Sc* 



Introduction. 



1. rriOURMALINE is distinguished amongst crystals for 

 J- its remarkable optical properties, particularly its 

 power of polarizing light. It is distinguished moreover by 

 possessing characteristic electric properties. It possesses also 

 a crystallographic interest as furnishing an eminent example 

 of non-superposable hemihedry. There can be little doubt 

 that these remarkable and characteristic qualities are closely 

 related to one another, though as yet very little is known of 

 the nature of this probable connexion. In the present paper 

 an attempt is made to connect the optical and electrical pro- 

 perties of the crystal, by showing that the opacity of the 

 crystal to light polarized in a principal plane of section can 

 be deduced from its electric conductivity. 



2. In a paper read before Section A of the British Associa- 

 tion at Dublin in 1878, by Dr. 0. J. Lodge and myselff, we 

 suggested, as a possible explanation of the phenomena of pyro- 

 electricity in tourmaline, that it might be found to possess 

 unilateral conductivity for electricity — and if for electricity, 

 for heat also. Our experiments on the electric conductivity, 

 however, led to negative results ; and in the case of heat-con- 

 ductivity, the only differences observed of a unilateral kind 

 were such as occurred while the temperature was either rising 

 or falling — not whilst it was constant. Our original sugges- 

 tion, therefore, was not confirmed, though I obtained instead 



* Communicated by the Physical Society, having been read at the 

 Meeting on June 11. 



t Report Brit. Assoc. (Dublin) 1878, p. 495 ; and Phil. Mag. July 

 lo/ 0. 



