136 Mr. A. E. Tutton [May 2, 



WEEKLY EVENING MEETING, 



Friday, May 2, 1902. 



Sib William Crookes, F.R.S., Honorary Secretary and 

 Vice-President, in the Chair. 



A. E. Tutton, Esq. B.Sc. F.R.S. F.C.S. 



Experimental Researches on the Constitution of Crystals. 



Experimental work in connection with the study of crystals offers 

 attractions of a more than usually fascinating kind. For, in the first 

 place, crystals themselves are such wonderfully beautiful objects ; 

 they are, indeed, unquestionably the most beautiful of all the inani- 

 mate productions of Nature. It is even doubtful whether we are 

 right in considering them inanimate, for the force of crystallisation, 

 though it may lie dormant for thousands of years, is ever ready, 

 when a suitable environment offers, to re-assert itself. In the second 

 place, the study of crystals involves the investigation of that most 

 exquisite of all the forms of energy, light — that extraordinary effect 

 of wave-motion in the ethereal all-pervading medium, which we now 

 believe to be due to an exceedingly rapid, periodic, oscillating change 

 in the electrical condition of the atoms and molecules of the incan- 

 descent light-giving source. 



We will allow the beam of light from an electric lantern to fall 

 on this cluster of diamonds, carbon in its most exquisite crystalline 

 form. The diamonds are arranged, as you see, in the shape of a 

 crown, and were generously lent for this lecture by Mr. Streeter. 

 You observe a magnificent play of light waves rippling towards 

 you in every variety of colour and scintillation. The object of 

 the experiment is that you may distinguish the two distinct types 

 of emanation, namely, exterior reflections of white light from the 

 facets and coloured rays due to penetration of the light into the 

 interior structure of the diamonds, and subsequent refraction and 

 reflection outwards again. The former kind you observe best in 

 the innumerable images in white light of the carbon points of the 

 lanterns, reflected on the ceiling and screen. 



It is by the determination of the direction of the exterior reflec- 

 tions that we are enabled to ascertain the wonderfully regular angular 

 relations of the various faces of the crystals ; and it is by the study 

 of the light which has penetrated that we gain the best information 

 concerning the internal structure. 



We shall consider this evening some of the results of a study of 

 the crystals of certain series of definitely related chemical salts. 



