428 



ICE, ARTIFICIAL. 



cause a correspondingly lower temperature in 

 the re-expanded air. Other gases are far more 

 powerful refrigerating agents than air. Water, 

 the most abundant of the fluids, and the one 

 that is popularly inferred whenever the term 

 "ice" is used, holds a large amount of heat 

 and yields it slowly, as witness the length of 

 time for which a hot-water bottle will remain 

 warm. To reduce water to the form of ice, a 

 very large amount of heat must be abstracted 

 from it, and all the operations of compression 

 and expansion of gases (of course including 

 air) aim at this result. 



Since 1861 the use of machines for the re- 

 frigerating processes of manufacture and trans- 

 portation has become very general. The prin- 

 ciple of heating a gas by compression, suffering 

 it first to cool, and then to expand, as described 

 in the case of atmospheric air, is the basis of 

 the operation. This calls for powerful com- 

 pressing machinery, a cooling reservoir capable 

 of sustaining a tremendous pressure from with- 

 in, and proper conduits for the expanded and 

 very cold gas. 



The most extensive application of dry-air 

 refrigerators is in the preservation of meat and 

 other perishable foods. Since the successful 

 introduction of the process in 1878, it has be- 

 come possible to send fresh meat to any part 

 of the world, or to preserve it for use on ship- 

 board. Previous attempts to refrigerate on a 

 large scale by means of ice had failed, owing 

 to the amount of moisture carried by the air 

 cooled in this manner, but the cold-air ma- 

 chines deliver the air practically without moist- 

 ure. To prepare a room or the hold of a 

 ship for the refrigerating process, it should be 

 lined with outer and inner courses of tongued- 

 and-grooved boards one inch or one and a 

 quarter inch thick, having an interspace of 

 nine inches, which is filled with charcoal or 

 silicate cotton, or some non-conducting sub- 

 stance. Especial care should be taken in con- 

 struction so that the chamber shall be per- 

 fectly tight. The cold air from the machine 

 is usually led into the refrigerating chamber 

 through dncts placed near the ceiling, and 

 after circulating through the room is led back 

 to the machine to be used over again, with the 

 addition of a small amount of fresh air. By 

 this means the temperature may be kept at 

 about 10 Fahr., which allows an ample mar- 

 gin for accident to the machinery, necessitat- 

 ing a temporary stoppage for repairs this, of 

 course, if the chamber is air-tight, or nearly 

 so. Improvements are constantly reducing the 

 expense of the refrigerating process, and in- 

 creasing the efficiency of the machines ; and a 

 reduction of temperature on board cattle-ships 

 has been effected in climates where the heat 

 threatened to prove fatal to the living cargo. 

 The different methods of abstracting heat 

 which is equivalent to producing cold may, 

 as indicated by the foregoing, be divided into 

 three distinct classes, which will be described 

 under separate heads : 



Freezing-Mixtures, that is, Melting Solids. The 

 familiar ice-cream freezer is the simplest form 

 of this type. In the more complicated de- 

 vices the same principle is merely elaborated 

 by using different mixtures and more economi- 

 cal mechanical appliances. The salts used can 

 be recovered by evaporation of the water, and 

 used over again indefinitely. In hot climates 

 the heat of the sun may be used to effect this 

 evaporation, but elsewhere artificial heat must 

 be employed, with, of course, increased ex- 

 pense. The usual method is to cause the 

 freezing-mixture to circulate around the cans 

 or molds in which the ice is to be formed, 

 pumping it, or causing it to flow back to be 

 refrigerated before it has parted with all of its 

 heat. The cost of producing fifteen tons of 

 ice in twenty-four hours by the Siemens ma- 

 chine is estimated at about $1.50 a ton, with- 

 out making allowance for wear of machinery 

 and repairs. Nearly all the artificial heat is 

 applied to recovering the salt, the proportions 

 being estimated at two and a half tons of coal 

 to fifteen tons of ice. The following table 

 gives a list of freezing-mixtures, with their re- 

 spective efficiency in reducing temperatures : 



TABLE OF FREEZING-SIIXTUKES. 



