Water consists of an innumerable quantity of extremely minute particles called molecules. These particles have the property of being able to glide over, under, and to and from each other almost without resistance or friction. When water is heated in a boiler the action that takes place is this: As the heat is applied, the particles nearest the heated surfaces become expanded or swollen, and are so rendered lighter (bulk for bulk) than the colder particles, they are therefore compelled to rise to the highest point in the boiler.

This upward action is vividly shown by the illustration on page 242, and by Fig. 158, where the warmer particles are ascending and the cooler ones are descending by a process which is endless so long as heat is applied to the lower part of the containing vessel.

The cause of circulation is the result of an immutable law of nature (the law of gravitation), and is so simple that with moderate care in its manipulation failures in arranging steam heating apparatus are next to impossible. A very slight experience suffices to show that a pipe taken from the top of a boiler and given a direct or gradual rise to the point furthest from the boiler, and then returned and connected into it at the bottom will, upon the application of heat, cause the water to circulate. It is not necessary that the water should boil or even approach boiling point, to cause circulation, as in a properly constructed apparatus the circulation commences soon after the heat is applied and immediately the temperature is raised in the boiler. It is a very common error to suppose that the circulation commences in the flow or up pipe, whereas it is just the reverse. The circulation is caused by the water in the return pipe and can be described as a stream of heated particles flowing up one pipe from the boiler and a stream of cooler particles flowing down another pipe into the boiler; or it might be described as a means of automatically transporting heated water from the lower to the upper parts of a building, and providing a down flow of cold water to the boiler to be heated in turn.

Those having in charge the erection of hot-water systems for heating buildings, will do well to remember that the circulation they expect depends entirely upon the expansion of particles when heated, and that they must avoid as much as possible friction, exposure of flow pipes to very low temperature, and frequent or numerous short bends.

When properly arranged the action of “the steam loop” is a very good illustration of the circulation of hot water and steam, the flow is continuous, rapid and positive.

Note.—When the steam loop is properly connected, the stop valve at the boiler should always be left open and full pressure maintained in the steam pipe over night or over Sunday. The loop will keep up a powerful circulation, returning all water to the boiler as fast as condensed. On starting up in the morning, it is only necessary to open the waste cocks and blow out what little water may have condensed in the cylinders themselves. The throttle may then be opened and the engine started with the steam as dry as if it had been running continuously.