n.

The choicest material of the builder and the sculptor is limestone baked
by the fires under the earth's crust into marble. The most enduring of
all the rocks are the foundation granite, and the metamorphic rocks that
lie next to them. Over these lie thick layers of sedimentary rocks laid
down by water. In them the record of life on the earth is written in
fossils.

THE AIR IN MOTION

Most of the beautiful things that surround us and make our lives full of
happiness appeal to one or more of our five senses. The green trees we
can see, the bird songs we hear, the perfume of honey-laden flowers we
smell, the velvety smoothness of a peach we feel, and its rich pulp we
taste. But over all and through all the things we see and feel and hear
and taste and smell, is the life-giving air, that lies like a blanket,
miles in depth, upon the earth. The substance which makes the life of
plants and animals possible is, when motionless, an invisible,
tasteless, odourless substance, which makes no sound and is not
perceptible to the touch.

Air fills the porous substance of the earth's crust for a considerable
distance, and even the water has so much air in it that fishes are able
to breathe without coming to the surface. It is not a simple element,
like gold, or carbon, or calcium, but is made up of several elements,
chief among which are nitrogen and oxygen. Four-fifths of its bulk is
nitrogen and one-fifth oxygen. There is present in air more or less of
watery vapour and of carbon dioxide, the gas which results from the
burning or decay of any substance. Although no more than one per cent.
of the air that surrounds us is water, yet this is a most important
element. It forms the clouds that bear water back from the ocean and
scatter it in rain upon the thirsty land. Solid matter in the form of
dust, and soot from chimneys, accumulates in the clouds and does a good
work in condensing the moisture and causing it to fall.

It is believed that the air reaches to a height of one hundred to two
hundred miles above the earth's surface. If a globe six feet in diameter
were furnished with an atmosphere proportionately as deep as ours, it
would be about an inch in depth. At the level of the sea the air reaches
its greatest density. Two miles above sea-level it is only two-thirds as
dense. On the tops of high mountains, four or five miles above sea
level, the air is so rarefied as to cause the blood to start from the
nostrils a