678 - Physiography. [ October, 
But if when we have left the mountain top, we take up a with- 
ered apple of last year’s growth, the consideration of its surface 
may help us to understand that which before was so hard to com- 
prehend. When we plucked that apple, a year ago, its surface 
was smooth, and the skin was stretched tightly over the fruit be- 
neath. But since that time the apple has shrunk in size, the fruit 
having contracted within the skin, which, no longer css and 
glossy, is now wrinkled and puckered up. 
But just as in the apple, so too in our planet, there is an inner 
portion which is contracting, and an outer portion which does 
not shrink: and as surely as the earth is losing heat by radia- 
tion into space, her mass contracting and her size growing less, 
so surely must the outer portion become puckered up, the most 
prominent wrinkles forming what we call mountain ranges. 
While san-heat, therefore, enables rain, rivers, and the sea to de- 
nude the land and to combine in the formation of new continents, 
earth-heat causes a fresh supply of land to be raised above the 
waters. Were it not for this earth-heat, England, as already, 
mentioned, would during the course of geological time be en- 
tirely washed into the ocean of geological bankruptcy. All geo- 
logical action, except that due to the tides, is brought about by 
sun-heat or by earth-heat. — 
Before inquiring what is the cause of this sun-heat and this 
earth-heat, there is one more question to be answered. Of what 
`- does the air, the water, the cliff, ultimately consist? Are earth, 
air, and water, as the ancients believed, elements? No. The air 
is composed chiefly of a mixture of a gas called nitrogen with 
one-fifth of its volume of oxygen. It is not difficult, as will be 
seen in Professor Huxley’s book, for the chemist in his labora- 
tory to separate these two gases. Nor has he much difficulty in 
splitting up water into the two gases, oxygen and hydrogen; 
while the further task of ascertaining of what the solid crust of 
the earth is composed, though it requires more labor, is by no 
means beyond his powers. But whereas water contains but two 
elements, in the solid crust of the earth there are about sixty-five. 
= But what are these elements? They are simple bodies which 
os resist every effort of the chemist to decompose them into simpler 
_ bodies. Many chemists, however, believe that, though we can- 
not by any means at our disposal thus split them up, this is only 
ause me means at our a are limited, and that, at an 
