IMPORTANT ELEMENTS AND MINERALS 25 
Of the gases, some are invisible like the oxygen and 
nitrogen of the air, which we are constantly breath- 
ing, while others, like chlorine, have distinct color 
and odor. | 
There is also a difference in their behavior: gold 
may be placed in the air, and indeed subjected to 
almost any conditions, and never change; silver tar- 
nishes when sulphur and some other elements reach 
it; iron rusts so readily, that in such structures as 
are exposed to the weather, it 1s commonly necessary 
to protect it with paint to exclude the oxygen. Ifa 
bar of steel is allowed to remain out of doors, it is 
first covered with a reddish or yellowish-brown rust, 
and finally becomes so weak that it crumbles. 
Since nearly all the elements are ready to form com- 
binations with others, we need not examine the earth’s 
crust with the expectation of findmg many elements 
uncombined. Very rarely, indeed, pure gold or silver, 
or some other element, is found in limited quantities ; 
but the law of change and combination is so strong, 
and is so constantly operating throughout the earth’s 
crust, that the elements are usually combined. one 
with another. 
How these combimations take place, the laws which 
they obey, and the results which are obtained, concern 
_ the mineralogist and must be omitted here. The point 
of importance to us is, that these changes are taking 
