COLD STORAGE 167 



based upon the vaporization of ammonia, and kept con- 

 stant in spite of great changes in the temperature of the 

 air outside. Cold-storage plants are a modern device, and- 

 only within comparatively recent years have they come 

 to be used to any considerable extent for the preservation 

 of food. They are now found in all our large cities, and 

 they are being utilized more and more each year, pro- 

 ducing profound modifications of the conditions of civilized 

 life. By means of them a large variety of foods can be 

 preserved for many months without any tendency toward 

 putrefaction and decay, and may be used at any time with 

 the confidence that they have been kept in a perfectly 

 good condition. The cold-storage plants make it possible 

 to keep fresh for winter or spring use a large quantity 

 of the perishable products which previously, if not capable 

 of preservation by canning, it was necessary to throw away 

 because of the certainty of putrefaction and decay. Such 

 devices are being used more and more, and are produc- 

 ing a far more stable condition in the food supply. It is 

 now possible to have fresh at any season of the year the 

 perishable food products produced at almost any other 

 season, • provided we take the trouble to preserve them in 

 cold-storage plants, and our city markets can furnish fresh 

 fruits at almost any time. 



The length of time during which food can be preserved 

 by cold storage depends upon the temperature. If actually 

 frozen, as is commonly the case with fish, fowl, and flesh, 

 it makes little difference how long it is kept. Frozen 

 food in the arctic regions keeps for years, and will indeed 

 keep as long as it remains frozen. The same would be 

 true of frozen food in cold storage. But some kinds of 



