There is also a class of ammonia machines, that operate on what is called the "absorption system." In these machines the operation starts with ammonia water instead of anhydrous ammonia. The liquid is heated in a boiler, and a mixture of about nine parts ammonia gas and one part steam is driven off from it. The mixed vapors pass first into a rectifier, where most of the steam is condensed to water, which runs back into the boiler. The temperature in the rectifier is not low enough to condense the ammonia, which passes on, now nearly free from water, into the condenser. Here it is liquefied by the joint action of cold and pressure, only the pressure is not supplied by mechanical means, but by the expansive force of the stream of vapor that is constantly being driven out of the boiler. The liquid ammonia next passes into the expansion coils in the freezing tank, just as in the compression system. After doing its work the gas is led into an "absorber," which is very similar to the condensers already described. Here it is reabsorbed by the water that it was originally driven out of, this water ("poor liquor" it is called) having been forced out of the boiler by the pressure prevailing in it and cooled for the purpose. It is this operation that gives the name to the absorption system. The resulting solution of ammonia is returned to the boiler by a pump and begins again the same round of operations.
In hot climates natural ice is an expensive luxury, as it must be brought long distances, and suffers much loss from melting. In those regions the artificial product has a great advantage in respect to cost. Even where there is usually a cold winter, as in the northern United States, a failure of the ice-crop sometimes occurs in the fields usually depended upon, followed by a more or less necessary increase in price the following summer. Ice machines have now reached such a high degree of efficiency that their product can compete with natural ice in these latitudes. In the summer of 1890 the price of natural ice to families in New York was a dollar a hundred-weight, while artificial ice sold for fifty cents. No doubt further improvements in machinery and methods will be invented, which will make it possible to furnish ice at a still lower price than now, and will lead to a freer and more general use of this commodity. Not only can artificial ice be sold at a lower price than the natural in most markets, but it is more economical, for the reason that it does not melt so fast. This is because it is frozen without the interruptions that allow layers of bubbles to collect under natural ice formed on still water, and it contains no soft snow-ice. It is, therefore, more compact than any but the very best of the natural product.
Another advantage that is claimed for artificial ice is, that when made from distilled water it is free from the impurities
