36 Scientific Proceedings, Royal Dublin Society. 



The combination of mercury with sulphur may thus he repre- 

 sented : — 



Hg + S = HgS + 4-9 heat-units (Nernst, 1888). 



That is, 200 grammes of mercury combine with 32 of sulphur to 

 form 232 grammes of mercury sulphide, with an evolution of heat 

 capable of raising 4-9 kilogrammes of water VQ. 



The physical state of the reacting substances must be taken into 

 account, and likewise the physical state— whether solid, liquid, or 

 gas — of the products. 



A reaction in which heat is evolved is termed exothermic ; a re- 

 action in which heat is absorbed is called endothermic. 



In the former the compound formed differs from its constituents 

 by something less than the energy associated with them. In the 

 latter case the compound differs by containing more energy than 

 its constituents. But in the former reaction the heat disturbance is 

 positive, and is denoted by a x>lu8 sign (+) ; in the latter it is a 

 negative quantity, and is indicated by a minus sign (-). 



For example, when hydrogen combines with oxygen to form 

 water, much heat is evolved, and this is explained by the following 

 equation :~ 



H2 + = H2O (gaseous) + 58 '2 heat-units. 



But when oxygen combines with oxygen to form ozone there is 

 much heat absorbed, thus : — 



O2 + O = O3 - 30 heat-units. 



Conditions of Chemical Change in Smelting Operations. 



All chemical reactions take jolace [only within certain limits of 

 temperature. At low temperatures chemical combinations generally 

 cannot take place ; at very high temperatures chemical combinations 

 cannot exist. 



Ores of the useful metals are for the most part oxides, carbon- 

 ates, or sulphides. The carbonates and sulphides are generally 

 converted into oxides by the process of roasting. 



Smelting operations are conducted either in reverberatory 

 furnaces or blast furnaces. The fuel is first burnt to carbon 



