CHAPTER XXII. 

 ELEMENTS. COMPOUNDS. LAWS. 



62. ELEMENTS. 



WHEN studying the action of heat on substances, one of three obser- 

 vations is made. 



I. Certain substances do not permanently change in appearance 

 or weight, even after prolonged heating, e.g. platinum and 

 silver. 

 II. Certain substances when heated increase in weight and change 



in properties, e.g. magnesium, tin and copper. 



III. Other substances change in appearance and decrease in weight, 

 e.g. mercury oxide and blue vitriol. 



In the first case it is evident that the platinum and silver have re- 

 mained unchanged, and that no new substance has been obtained 

 from them. 



In the second case an oxide has been formed. This is a new substance 

 which "differs greatly from both the metal and the oxygen which go 

 to form it, and is more complex than its constituents. 



In the third case new substances are obtained from the original 

 substance, each of which may be regarded as being simpler than the 

 substance from which it was obtained. Mercury oxide might be called 

 a complex substance, and mercury and oxygen simple substances; unless, 

 indeed, still more simple bodies are subsequently obtained from them. 



A simple substance, or, as it is more commonly called, an element, may 

 be defined as a distinct kind of matter from which no other kind of matter 

 has been obtained except by addition to it. 



But it must be borne in mind that as the methods which chemists 

 adopt become more refined it is likely that some of these may be found 

 to be wrongly regarded as elements. Up to the time of Davy (1807) 

 the substances soda, potash arci lime were regarded as elements. He 



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