i88o.] 
On Water and A iv. 
181 
more easily to a<5t upon the metal. Other metals will a Ct 
in the same way. For instance, here I have a curious 
metal which was discovered in this place by the famous Sir 
Humphry Davy. It is a metal called potassium, which is 
Fig. 16. 
i 
extracted from ordinary potash, and the attraction of this 
metal for oxygen is so powerful that, no doubt, we shall be 
able to burn the metal even upon ice : for ice, as you know, 
is solid water. If we place a little piece of this metal upon 
the ice, you see that it actually burns upon the ice. How 
is it that it burns ? It burns because it seizes upon the 
oxygen of the water and allows the hydrogen to go free ; 
and that purple flame which is produced is due to the 
liberated hydrogen which has been ignited by the heat pro- 
duced by the chemical union between the potassium and the 
oxygen. 
Thereis another thing with which I should like, at all 
events, to begin to familiarise your minds, and that is the 
nature of what we call heat. You saw it developed here by 
this combination. It is a notion universally accepted at the 
present day that this thing which we call heat is a brisk 
motion or vibration — a quivering, so to say — a tremor of the 
