This knowledge can do us no good except by the assistance of chemistry, which explains the properties of each part, and teaches us where it is to be found. It is not necessary for farmers to become chemists. All that is required is, that they should know enough of chemistry to understand the nature of the materials of which their crops are composed, and how those materials are to be used to the best advantage. This amount of knowledge may be easily acquired, and should be possessed by every person, old or young, whether actually engaged in the cultivation of the soil or not. All are dependent on vegetable productions, not only for food, but for every comfort and convenience of life. It is the object of this book to teach children the first principles of agriculture: and it contains all that is absolutely necessary to an understanding of the practical operations of cultivation, etc. [Is organic matter lost after combustion? Of what does it consist? How large a part of plants is carbon?] We will first examine the _organic_ part of plants, or that which is driven away during combustion or burning. This matter, though apparently lost, is only changed in form. It consists of one solid substance, _carbon_ (or charcoal), and three gases, _oxygen_, _hydrogen_ and _nitrogen_. These four kinds of matter constitute nearly the whole of most plants, the ashes forming often less than one part in one hundred of their dry weight. [What do we mean by gas? Does oxygen unite with other substances? 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.