Owing to the fact that water in liquid form is nearly incompressible, it cannot be used to perform a cycle of operations such as take place in the steam engine. Theoretically, however, any substance having a temperature above its surroundings is capable of serving as a power generator. Disregarding the possibility of thermo-electric conversion of energy, useful mechanical work can be derived through the expansion of vapors of volatile substances.

Fig. 188 - A simple heat motor.

Many liquids are known to pass into vapor under a feeble heat, such as, for instance, the sun's rays.

If other media than water are used as heat carriers, it becomes indispensable to recover the original liquid by means of condensation for economic reasons.

A simple apparatus, which can be made by the Handy .Man, will demonstrate that a very small degree of heat is sufficient for performing light mechanical work.

A disk E, mounted on shaft D serves as support for six compartments, B1, B2, . . B6,, concentrically arranged about D. These as well as the disk can be made of tin soldered together. Two opposite chambers, for instance, B1 and B4 are connected by a small brass tube C, bent outward at the center to clear the shaft D and projecting through the chambers nearly to the outer cylindrical wall. Two uprights, F1, and F2, support the wheel at a proper distance over a small flame A or over a basin filled with hot water. One of each pair of chambers is filled with ether or acetone, while the other contains only the vapor of the liquid in an expanded state, but no air at all. The liquid is introduced into the chambers through small holes, and after the air has been blown out (by heating this liquid to the boiling point) each hole is sealed with a drop of solder.

Evidently, when one chamber passes through the hot zone, the liquid vaporizes and passes through the tube to the opposite chamber where it condenses. Thus the center of gravity is constantly changed, causing the wheel to revolve.