Then, to complete the process, we let the gas expand again. Now just as compressing a gas heats it, letting it expand cools it. If we compressed it and then expanded it again we should be just as we were to commence with. But since, in between the two operations we extract a quantity of heat by means of the cooling water, we get at the end a very much lower temperature than that with which we started. We cannot cool the gas without compressing it, because heat will only flow from one body into another when the second is cooler than the first. But by making the gas hot temporarily by compression we enable the water to draw some heat from it, and then, allowing it to sink back to its original state, we get practically the old temperature, less what the water has extracted. The principle is really absurdly simple when one once gets to understand it. The application is not so simple as far as the designer of the machine is concerned, for he has to adjust the various parts to exactly the right shape and dimensions, so that they may work well with one another and produce the desired result with the minimum expenditure of power. To the observer, however, and to the user too, the finished machine is wonderful in its simplicity. The principle is illustrated diagrammatically in Fig. 5. In the centre is the compressor. Its action forces the gas along the pipe to the right and down into the condenser. As it flows downwards through the coil there cold water enters at the bottom of the tank, flows upward past the coil and escapes again at the top. Thus the coil is kept in contact with _cold_ water. Passing then through the bottom of the tank the gas travels from right to left through the "regulating valve" and into an arrangement almost exactly similar to the condenser but called the evaporator. Here the gas expands and suffers a great fall in temperature. This cold is communicated to liquid circulating in the tank which forms a part of the evaporator, and this liquid can be circulated through pipes into any rooms to be cooled or around vessels of water which it is desired to freeze. This liquid, which acts as the carrier of the cold, is called "brine," and is water to which is added calcium chloride to keep it from freezing. [Illustration: FIG. 5.--This diagram shows the working of the Refrigerating Machine. The pump compresses the gas and drives it through the coil in the condenser, where it is cooled by water. It passes thence through the coil in the evaporator, where it expands and cools the surrounding brine.]