ICE-MAKING AND MACHINE REFRIGERATION. 25 



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 am- 

 monia. 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 con- 

 stantly 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 de- 

 scribed. 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 re- 

 turned 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 ma- 

 chines 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 wa- 

 ter, and it contains no soft snow-ice. It is, therefore, more com- 

 pact 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 



