Take a glass tube filled with oxygen gas, and put in it a lump of charcoal, cork the ends of the tube tightly, and pass through the corks the wires of an electrical battery. By passing a stream of electrical fluid over the charcoal it may be ignited, when it will burn with great brilliancy. In burning it is dissolved in the oxygen forming carbonic acid, and disappears. It is no more lost, however, than is the carbon of wood which is burned in a stove; although invisible, it is still in the tube, and may be detected by careful weighing. A more satisfactory proof of its presence may be obtained by _decomposing_ the carbonic acid by drawing the wires a short distance apart, and giving a _spark_ of electricity. This immediately separates the oxygen from the carbon which forms a dense black smoke in the tube. By pushing the corks together we may obtain a wafer of charcoal of the same weight as the piece introduced. In this experiment we have changed carbon from its solid form to an invisible gas and back again to a solid, thus fully representing the continual changes of this substance in the destruction of organic matter and the growth of plants. CHAPTER III. HYDROGEN, OXYGEN AND NITROGEN. HYDROGEN AND OXYGEN. [What is water composed of? If analyzed, what does it yield? How do plants obtain their hydrogen and oxygen?] Let us now consider the three gases, _hydrogen_, _oxygen_ and _nitrogen_, which constitute the remainder of the organic part of plants. Hydrogen and oxygen compose _water_, which, if analyzed, yields simply these two gases. Plants perform such analysis, and in this way are able to obtain a sufficient supply of these materials, as their sap is composed chiefly of water. Whenever vegetable matter is destroyed by burning, decay, or otherwise, its hydrogen and oxygen unite and form water, which is parted with usually in the form of an invisible vapor. The atmosphere of course contains greater or less quantities of watery vapor arising from this cause and from the evaporation of liquid water. This vapor condenses, forming rains, etc. Hydrogen and oxygen are never taken into consideration in manuring lands, as they are so readily obtained from the water constituting the sap of the plant, and consequently should not occupy our attention in this book. NITROGEN. [If vegetable matter be destroyed, what becomes of these constituents? What is the remaining organic constituent? Why is it worthy of close attention? Do plants appropriate the nitrogen of the atmosphere?] _Nitrogen_, the only remaining _organic_ constituent of vegetable matter, is for many reasons worthy of close attention. 1. It is necessary to the growth and perfection of all cultivated plants. 2. It is necessary to the formation of animal muscle. 3. It is often deficient in the soil. 4. It is liable to be easily lost from manures.