Give some instances of its combinations
When wood is burned in a close vessel, or otherwise protected from the air, its carbon becomes charcoal. All plants contain this substance, it forming usually about one half of their dry weight. The remainder of their organic part consists of the three gases named above. By the word gas, we mean air. Oxygen, hydrogen and nitrogen, when pure, are always in the form of air. Oxygen has the power of uniting with many substances, forming compounds which are different from either of their constituents alone. Thus: oxygen unites with iron and forms oxide of iron or iron-rust, which does not resemble the gray metallic iron nor the gas oxygen; oxygen unites with carbon and forms carbonic acid, which is an invisible gas, but not at all like pure oxygen; oxygen combines with hydrogen and forms water. All of the water, ice, steam, etc., are composed of these two gases. We know this because we can artificially decompose, or separate, all water, and obtain as a result simply oxygen and hydrogen, or we can combine these two gases and thus form pure water; oxygen combines with nitrogen and forms nitric acid. These chemical changes and combinations take place only under certain circumstances, which, so far as they affect agriculture, will be considered in the following pages.
As the organic elements of plants are obtained from matters existing in the atmosphere which surrounds our globe, we will examine its constitution.
CHAPTER II.
ATMOSPHERE.
What is atmospheric air composed of?
In what proportions?
What is the use of nitrogen in air?
Does the atmosphere contain other matters useful to vegetation?
What are they?