64 On Polarization by Diffusion of Light. 



I do not hesitate to attribute these phenomena of polarization 

 to foreign particles. We know, indeed, from Dumas and Stas*, 

 that diamonds leave a spongy residue after combustion ; and 

 Sir D. Brewsterf recognized with the microscope cavities of 

 various shapes in their interior. 



Ice. — When very clear and compact, it exhibits very little 

 capacity of illumination. When floating in water in a large 

 glass vessel through which the pencil of rays passes, the trace is 

 very pronounced in the water, but almost invisible in the ice. 

 Cutting a piece of ice into a suitable shape, I had no difficulty 

 in distinguishing in it the trace, which, when viewed through a 

 lens, is seen to resolve itself into minute particles. It is much 

 less visible than that of the purest water I succeeded in obtaining 

 by distillation J; its polarization is complete §. 



Alum. — A crystal of alum, in which I had faces cut at right 

 angles, gives a trace in the parts where flaws are present, but in 

 other parts does not appear to have any capability of illumi- 

 nation. 



Gypsum. — I cite from memory a specimen of gypsum from 

 the collection of M. Alph. Favre. I examined it, a long time 

 ago, with the oxyhydrogen light only, and did not recognize in 

 it any trace. 



The whole of these observations appear to me to well 

 establish : — 



1st. That in non-fluorescent crystals the crystallizable matter 

 by itself is destitute of illuminability. 



2nd. That crystals which possess the faculty of being illumi- 

 nated by returning polarized light laterally, owe that property 

 either to the presence of foreign particles or to defects of crys- 

 tallization. 



of polarized light was thus obtained, which I caused to enter into the dia- 

 mond through the large front face of the brilliant, and to emerge through 

 the parallel facet (the collet) which generally terminates the rear of a 

 brilliant. The trace was then viewed with a Nicol, through one of the in- 

 clined faces that form the posterior pyramid. In these conditions, on 

 turning the Nicol, differences of intensity are observed if the trace is par- 

 tially polarized. 



* Annates de Chimie, 1844, vol. i. p. 15. 



t Trans, of Royal Society of Edinburgh, vol. xxiii. p. 1. 



X For the purification of water by distillation, see Archives, 1870, 

 vol. xxxvii. p. 146. 



§ I am here speaking of the naturally formed ice of ponds ; there are 

 great differences, according to the specimens. I recall here that Tyndall 

 indicated water proceeding from the melting of ice as the most exempt 

 from particles and illumination-power that he could obtain. 



