1 6 THE FAIRY-LAND OF SCIENCE. 
free; a gas will not retain either the same bulk or 
the same shape, but will spread over as large a space 
as it can find wherever it can penetrate. Such simple 
things as these you must learn from books and by ex- 
periment. 
Then you must understand what is meant by 
chemical attraction; and though I can explain this 
roughly here, you will have to make many interesting 
experiments before you will really learn to know this 
wonderful fairy power. If I dissolve sugar in water, 
though it disappears it still remains sugar, and does 
not join itself to the water. I have only to let the 
cup stand till the water dries, and the sugar will re- 
main at the bottom. There has been no chemical at- 
traction here. 
But now I will put something else in water which 
will call up the fairy power. Here is a little piece of 
the metal potassium, one 
of the simple substances 
of the earth; that is to 
say, we can not split it 
up into other substances, 
wherever we find it, it is 
_ always the same. Now if 
FIG. i. Piece of potassium in 
a basin of water. * P ut this P iece of P otas - 
sium on the water it does 
not disappear quietly like the sugar. See how it rolls 
round and round, fizzing violently, with a blue flame 
burning round it, and at last goes off with a pop. 
What has been happening here? 
You must first know that water is made of two 
substances, hydrogen and oxygen, and these are not 
