Chromogenetic Properties of Sulphur . 261 



associated with the energy of combination, and more parti- 

 cularly with that energy left unabsorbed from the gross 

 energy of the system, and which may be terme 1 the residual 

 energy. 



That this is the case appears to be indicated when com- 

 parisons are instituted between normal potentials of the 

 various metals, also the heats of formation of the compounds. 



As would be expected, if a comparative series of oxides or 

 sulphides be taken, the heats of formation ran parallel with 

 the potentials of the combining substances. It will be seen 

 that in the oxides, colour appears with those members having 

 the lower heat of formation, similarly with sulphides, but 

 that comparison between oxides and sulphides shows that 

 often a colourless oxide has a corresponding but coloured 

 sulphide having a smaller beat of formation. 



Heat of Heat of formation 



Oxide. formation of sulphide. 



Nd 2 3 435-1 285-9 



A1 2 0, 380-2 126-4 



B 2 3 ' 272-6 75-5 



As 2 0, 219-34 — 



Sb 2 3 163-0 34-4 



As 2 3 159-6 — 



Cab 145 90-8 



MgO 143-3 79-4 



Si-0 141-2 99-3 



Bi 2 3 1378 — 



BaO 126-3 102-5 



K 2 98-2 103-5 



ZnO 85 3957 



CdO 65-65 34-35 



CoO 641 21-9 



NiO 615 195 



PbO 50-3 20-4 



Cu 2 43-8 18-26 



T1 2 42 8 21-6 



CuO 37-7 8-67 



HgO 20-7 10-7 



It will be seen from the accompanying table that oxides 

 having heats of formation of over 70 are colourless, while 

 the limit of colour for sulphides appears to be in the neigh- 

 bourhood of a heat of formation of 40. It is further 

 interesting to note that oxides and sulphides having heats 

 of formation about the above numbers, develop colour on 

 heating, e. g. ZnS, ZnO, Sb 2 3 . It could be stated with truth 



