56 THE FAIRY-LAND OF SCIENCE. 
lecture. Still, all these gases and vapours in the 
atmosphere are in very small quantities, and the bulk 
of the air is composed of oxygen and nitrogen. 
Having now learned what air is, the next question 
which presents itself is, Why does it stay round our 
earth ? You will remember we saw in the first lecture, 
that all the little atoms of a gas are trying to fly away 
from each other, so that if I turn on this gas-jet the 
atoms soon leave it, and reach you at the farther end 
of the room, and you can smell the gas. Why, then, 
do not all the atoms of oxygen and nitrogen fly away 
from our earth into space, and leave us without any air ? 
Ah 1 here you must look for another of our invisible 
forces. Have you forgotten our giant force, " gravita- 
tion," which draws things together from a distance ? 
This force draws together the earth and the atoms of 
oxygen and nitrogen ; and as the earth is very, big and 
heavy, and the atoms of air are light and easily 
moved, they are drawn down to the errth and held 
there by gravitation. But for all that, the atmosphere 
does not leave off trying to fly away ; it is always 
pressing upwards and outwards with all its might, 
while the earth is doing its best to hold it down. 
The effect of this is, that near the earth, where the 
pull downward is very strong, the air-atoms are drawn 
very closely together, because gravitation gets the 
best in the struggle. But as we get farther and farther 
from the earth, the pull downward becomes weaker, 
and then the air-atoms spring farther apart, and the 
air becomes thinner. Suppose that the lines in this 
diagram represent layers of air. Near the earth we 
