Fusion is a subject connected with that of heat, which has been already slightly touched upon. Fusion is the passage of a solid into the liquid state. Most substances have the power of fusion. Exceptions are to be noted, however, in such substances as wood, paper, coal, etc., which instead of fusing, decompose under high temperatures. Many substances have long been considered impossible to fuse, but as it has become possible to produce higher temperatures, their number has steadily decreased. Such hard substances as rock crystal have been fused. Every fusible substance begins to fuse at a certain temperature, which is invariable for each substance if the pressure be the same. Regardless of the intensity of the heat applied, from the instant that fusion begins the temperature of the body ceases to rise, and remains constant until the fusion is complete.

Solidification is the reverse of fusion, it being the passage of a body from the liquid to the solid state. Every body, under the same pressure, solidifies at a fixed temperature, which is the same as that of fusion of the same substance. From the commencement to the end of solidification, the temperature of the substance remains the same.

In connection with this chapter, the subject of gravity, though not of the direct, practical importance to the reader that other subjects considered may be, should not be overlooked.

Briefly stated, gravity is the tendency of any body to fall toward the earth. When a gravity supply of water is spoken of, what is meant is that the supply is due not to any outside force, but. simply to its tendency to fall. The same significance is at-tached to the gravity heating system. Weight is the common meaning of the term gravity. Thus, in the gravity supply system, water is delivered by gravity, that is, by virtue of its weight. In drainage work, the common system is known as the gravity system as distinguished from systems in which the drainage is forced along by special ejectors or pumps.

A few remarks concerning the properties of water will not be out of place. Water is composed of hydrogen and oxygen, two parts of the former to one of the latter. Water readily absorbs gases, this action occurring to the greatest extent when the pressure of the gas upon the water is greatest, and at low temperatures. The reason for this is that when under pressure and at low temperatures, the elastic force of the gas is less than under the reverse conditions. The absorption of gases by water renders it necessary to cover storage tanks for water when located in such positions as to be liable to such danger.

Water is hard or soft according to the relative amount of certain salts which are present in it. These salts are generally taken up by the water from the earth, the most common being those in which lime is present. A water containing these salts is said to be hard. Rain water, being free from salts of this nature, is soft, and in addition is the purest water obtainable. Rain water or any other pure water, however, often attacks certain metals more strongly than other more impure waters. This is due to the fact that the elements of which it is composed are more free to unite with other substances than water which already contains more or less matter of a chemical nature.

Air is composed of oxygen, nitrogen, and a small amount of carbonic-acid gas. The proportions are about one part of oxygen to four parts of nitrogen. Nitrogen is a poisonous element, very destructive to life. Upon the presence of oxygen all combustion and chemical action depends. While nitrogen is destructive of life, oxygen gives life, although the former is necessary to dilute the latter, for otherwise the process of oxidation would be so severe that life could not be maintained.

Other subjects might be taken up to advantage if space allowed, but for lack of that, the author has devoted himself entirely to those subjects which have a more or less practical application in the work of plumbing. Much other material, such as the subject of siphonage, will be found at different points, but which for various reasons are considered under other headings.