INTRODUCTION 5 



particles of copper and lemoc-yellow fragments of sulphur can 

 be distinguished lying side by side. Moreover, by throwing 

 some of this powder into water the constituents separate out 

 according to their relative specific gravities : the copper par- 

 ticles sink to the bottom, while the lighter sulphur particles 

 rest above the copper ; or complete separation may be effected 

 by treating a portion of the mixture with carbon disulphide, 

 when the sulphur is removed in solution and the copper left. 

 On allowing the carbon disulphide to evaporate, the sulphur is 

 recovered in small crystals. In this case a iJiere mixture of 

 sulphur and copper has been made, capable of separation by 

 mechanical means and partaking of the properties of its 

 constituents. 



If some of the mixture be heated in a test-tube a chemical 

 union between the copper and sulphur takes place, attended 

 by the production of heat and light, and there results a black 

 substance totally unlike, in all its properties, either of its 

 constituents. If this black substance be ground to powder and 

 examined under the microscope all the particles will be alike 

 black in colour, and neither sulphur nor copper can be detected. 

 If a portion of the mixture be treated with carbon disulphide 

 the substance is not changed in appearance, and the liquid, if 

 filtered off, deposits on evaporation no sulphur (or at most a 

 small trace, due to imperfect mixing or heating of the powder). 

 If the powder be thrown upon water it all sinks together. 

 The substance is evidently neither copper nor sulphur, but by 

 appropriate means it can be shown to contain both. If some 

 of the powder be treated with strong nitric acid a violent action 

 takes place, and a dirty yellow mass floats on the top, the 

 liquid becoming blue in colour. If the liquid be filtered and 

 brought into contact with a bright iron or steel blade or some 

 scrnp zinc, a deposit of red metallic copper will be obtained- 

 If the dirty yellow residue be heated in air, it will burn with 

 the blue flame and emit the characteristic odour of burning 

 sulphur. The black substance obtained by heating the mixture 

 of copper and sulphur is a chemical compound called sulphide 



