THE AERIAL OCEAN 2tt WHICH WE LIVE. 59 
the air we breathe is now found by no means the simple 
mixture of oxygen and nitrogen, with a little car- 
bonic acid and still less ammonia, which were all that 
science had discovered in it till within the last few 
years. We must add to the invisible mixture, not 
only argon, whose presence in the atmosphere was 
detected about three years ago, and crypton, a more 
recent discovery, but two more constituents which 
are believed to be simple or elementary substances, 
neon and metargon. Still, all these gases and va- 
pours 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 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 ! 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 earth and held there by 
gravitation. But for all that, the atmosphere does not 
