260 Dr. 0. C. M. Davis and Dr. F. W. Bixon on the 



oxygen. In other words, the thio-compounds are less stable 

 than the oxy-compounds. A similar idea is brought forward 

 by Thiele *, who considers that in unsaturated compounds 

 the combining energy is not completely absorbed. 



Peters j" in his work on residual affinity and additivity 

 found that one gram-molecule of silver cyanate absorbed 

 one gram- molecule of ammonia gas, whereas one gram- 

 molecule of silver thiocyanate absorbed two gram-molecules 

 of ammonia gas — further evidence that the substitution of 

 sulphur for oxygen profoundly modifies the reactivity of the 

 molecule. 



Weimar J, who experimented on the effect of solvents on 

 the colour of dissolved sulphur, considers that certain colours 

 observed occur when the linking of the sulphur in the 

 compounds is weakened, and thus the sulphur atoms approach 

 a condition of freedom from combination. Schutz § suggests 

 that in compounds including one colour element an increase 

 in the atomic weight o£ the colour element leads to intensifi- 

 cation of colour, giving as examples ZnS, CMS, HgS, and 

 P 2 S 3 , As 2 S 3 , Sb 2 S 3 . He further states that by increasing the 

 atomic weight of the elements attached to a colouring- 

 element, the colour is deepened, instancing the series 

 P 2 3 , P 2 0^, P 2 Se 3 , P 2 Se 5 . It will be noted that sulphur 

 is not chosen here as a colour element. A critical com- 

 parison of the various sulphides reveals many interesting 

 facts. Thus, while the lower sulphides of sodium, potassium, 

 rubidium, and caesium are colourless, the power of absorbing 

 light is developed in the more complex sulphides, the higher 

 members possessing, as do the higher sulphides of barium 

 and calcium, various colours. 



The metallic oxides are in some instances white substances, 

 while the corresponding sulphides are coloured, e.g. Sb 2 3 , 

 Sb 2 S 3 , As 2 3 , As 2 S 3 , Y,0„ Y 2 S 3 , La,O s , La 2 S 3 . 



Similar effects arise in the case of compounds other than 

 sulphides ; the oxides vary in colour, and iodides may be 

 coloured where the corresponding chlorides are white, e.g. Hg, 

 Bi, Pb. 



It would seem that the colour, of: a combination ol two 

 elements, in addition to the determining effect of the nature 

 of the elements themselves, may possibly in a measure be 



* Thiele, Annalen, 1899. 



T Peters, Berichte, 1908. 



\ Weimar, J. Russ. Chem. Soc. 1915. 



§ Schutz, Z. Phys. Chem. 1892. 



