262 Mr. R. B. Wilsey on the Crystal 



thai; the heat of formation appears to be one of the important 

 determining factors for colour, or rather, as previously stated, 

 the higher residual energy of the molecule leads to production 

 of colour. 



This connexion between energy and colour has been 

 instanced by Baly*, who has developed the idea that mole- 

 cular frequencies are definitely related to the frequencies 

 of the combining atoms, and has calculated several illus- 

 trative examples, basing his computations on the quantum 

 theory. 



The data available for any such extended calculations are 

 too scanty to be of much use, but it seems within the bounds of 

 possibility that a definite relationship between colour and 

 residual energy might be established. 



There is a fair amount of scattered evidence in support of 

 the view put forward — the colour of the polysulphides of 

 elements such as sodium, potassium, rubidium, caesium, 

 barium, and calcium ; the tendency to colour of unsaturated 

 compounds ; groups of compounds such as AgCl (29'38), 

 AgBr (22-7), Agl (14-2), PbCl 2 (82'77), Pbl 9 (39'8), HgCI a 

 (53-3), Hr-I 2 (24-3); the colour of N0 2 and N 2 4 . 



The visible spectrum only has been dealt with, and it is 

 understood that other factors very difficult of differentiation 

 may have effect on colour, but there does appear to be a 

 very close connexion between colour and what has been 

 termed residual affinity. 



University of Bristol. 

 March 2nd, 1921. 



XXXI. The Crystal Structure of the Silver Halides. 

 ByU. B. Wilsey f. 



THE crystal structures of the silver halides have been 

 studied by the powder method t. Following the pro- 

 cedure of Hull, the Ka radiation of molybdenum (\ = *712 A) 

 was isolated with the aid of a zircon filter, and allowed to 

 fall on the powdered sample spread in a thin layer upon a 

 ribbon of dry mounting tissue. The diffraction pattern was 

 received upon a film bent in the form of a semicircle. 



* Baly, Phil. Mag. 1920. 



f Communicated by Dr. L. Silberstein. 



X P. Debve & P. Scherrer, Phys. Zeits. xvii. pp. 277-283 (1916). 

 A. W. Hull, Phvs. Rev. ix. pp. 84-87 (1917), and Phys. Rev. x. 

 pp. 661-696 (1917). 



