OBJECTS.] INTRODUCTORY. 71 



particles justify its extension to all forms of matter 

 whatever. 



The metal mercury or quicksilver, for instance, 

 may be supposed to be made up of distinct particles 

 of mercury of extreme minuteness, and according to 

 the temperature, these associate themselves in the 

 solid (frozen mercury), liquid (ordinary quicksilver), or 

 gaseous form (vapour of mercury). To whatever treat- 

 ment pure mercury may be subjected, we cannot get 

 anything but mercury out of it. The particles of 

 mercury have never been broken up. Hence they are 

 generally termed atoms, or particles that cannot be 

 divided ; and mercury is said to be an element, or 

 a substance which is not compounded of any other 

 substances. 



Here is a case in which it is very useful to dis- 

 tinguish between fact and hypothesis. The matter of 

 fact is that, up to the present time, no one has been 

 able to get out of pure mercury anything but pure 

 mercury. The statement that mercury is a simple 

 substance, and therefore never can be broken up into 

 any other substances, is a hypothesis which future 

 observation and experiment may or may not confirm. 



A hundred and fifty years ago it was universally 

 believed that water was as much an element as 

 mercury. But water is now well known to be a 

 compound. In fact, as has already been said, the 

 particles of water may be very readily broken up or 

 decomposed (in what way, you will learn when 

 you study chemistry) into two totally distinct sub- 

 stances, oxygen and hydrogen, which are gaseous 

 at all known temperatures, though by combining vast 

 pressure with extreme cold they have recently been 



