Water is so familiar a substance, that it is unnecessary to dwell upon its properties. When pure, it has neither colour, taste, nor smell. At 32° of Fahrenheit’s[2] scale (the freezing point), it solidifies into ice, and at 212° it boils, and is converted into steam. There are two others of its properties which are especially interesting in connection with the growth of plants. [2] This is the scale of the common thermometer used in this country. _1st_, If sugar or salt be put into water, they disappear or are _dissolved_. Water has the power of thus dissolving numerous other substances in greater or less quantity. Hence, when the rain falls and sinks into the soil, it dissolves some of the soluble substances it meets in its way, and rarely reaches the roots of plants in a pure state. So waters that rise up in springs are rarely pure. They always contain earthy and saline substances in solution, and these they carry with them, when they are sucked in by the roots of plants. It has been above stated, that water absorbs (dissolves) its own bulk of carbonic acid; it dissolves also smaller quantities of the oxygen and nitrogen of the atmosphere; and hence, when it meets any of these gases in the soil, it becomes impregnated with them, and conveys them into the plant, there to serve as a portion of its food. _2d_, Water is composed of oxygen and hydrogen; by certain chemical processes it can readily be resolved or decomposed _artificially_ into these two gases. The same thing takes place _naturally_ in the interior of the living plant. The roots absorb the water, but if in any part of the plant hydrogen be required, to make up the substance which it is the function of that part to produce, a portion of the water is decomposed and worked up, while the oxygen is set free, or converted to some other use. So, also, in any case where oxygen is required water is decomposed, the oxygen made use of, and the hydrogen liberated. Water, therefore, which abounds in the vessels of all growing plants, if not directly converted into the substance of the plant, is yet a ready and ample source from which a supply of either of the elements of which it consists may at any time be obtained. It is a beautiful adaptation of the properties of this all-pervading compound (water), that its elements should be so fixedly bound together as rarely to separate in external nature, and yet to be at the command and easy disposal of the vital powers of the humblest order of living plants. 2. AMMONIA.—If the sal ammoniac of the shops be mixed with quicklime, a powerful odour is immediately perceived, and an invisible gas is given off which strongly affects the eyes. This gas is ammonia. Water dissolves or absorbs it in very large quantity, and this solution forms the common hartshorn of the shops. The white solid smelling-salts of the shops are a compound of ammonia with carbonic acid,—a solid formed by the union of two gases.