10 GRAVITATION. 



water]. What a wonderful thing it is to see 

 that it requires so much water as that [a half- 

 pint vessel full] to fall towards the earth, com- 

 pared with the little mass of substance I have 

 here ! And again, if I take this metal [a bar of 

 aluminium ( 3 ) about eight times the bulk of the 

 platinum] we find the water will balance that as 

 well as it did the platinum ; so that we get even 

 in the very outset, an example of what we want 

 to understand by the words forces or powers. 



I have spoken of water, and first of all of its 

 property of falling downwards : you know very 

 well how the oceans surround the globe how 

 they fall round the surface, giving roundness to 

 it, clothing it like a garment ; but, besides that, 

 there are other properties of water. Here, 

 for instance, is some quicklime, and if I add 

 some water to it, you will find another power or 

 property in the water. ( 4 ) It is now very hot, 

 it is steaming up, and I could perhaps light 

 phosphorus or a lucifer-match with it. Now, 

 that could not happen without a force in the 

 water to produce the result ; but that force is 

 entirely distinct from its power of falling to the 

 earth. Again, here is another substance [some 

 anhydrous sulphate of copper ( 5 )] which will il- 



