ICE, ARTIFICIAL. 



429 



Evaporation of Liquids. All liquids are more 

 or less volatile. Those which are most vola- 

 tile are the most efficient agents for abstract- 

 ing heat. Such are ammonia, ether, etc. A 

 very large number of machines are in use 

 operated on this principle. That invented by 

 Ferdinand Carre was the pioneer, and its suc- 

 cessors have been mainly improvements, me- 

 chanical or chemical, upon his idea. Advantage 

 is taken of the property of water which ena- 

 bles it to absorb large quantities of ammoniacal 

 gas (700 times its own volume, at a moderate 

 temperature), capable of converting into ice 

 more than three times its own bulk. Aqua 

 ammonia is introduced into a boiler, and the 

 gas expelled by heat into a condenser, which 

 operates by a combination of cold and pressure. 

 From the condenser the ammonia passes in a 

 liquid state, and at a very low temperature to 

 a refrigerator in which vessels are placed con- 

 taining the water to be frozen. Carre claimed 

 that for every pound of coal consumed his ap- 

 paratus would make from eight to twelve 

 pounds of ice, according to the size of the ma- 

 chine. As the same ammonia is recovered, 

 returned to the boiler, and used over and over 

 indefinitely, the intrinsic economy of this and 

 similar methods is obvious. A machine capable 

 of producing 500 pounds of ice an hour will, in 

 the same time, liberate from solution, liquefy, 

 evaporate, and redissolve 100 pounds of pure 

 ammonia. The following table gives tempera- 

 tures on Fahrenheit's scale. The boiling-point 

 and latent heat of evaporation are at atmos- 

 pheric pressure : 



EVAPORATION OF LIQUIDS. 



Cold Air. These machines, now extensively 

 used, compress atmospheric air, partially cool 

 it while under compression, suffer it to ex- 

 pand, and lead it while at its reduced tem- 

 perature into a refrigerating-chamber. In the 

 best machines there are two or more com- 

 pressors to which the air is led through pipes 

 and powerfully compressed by steam-power. 



Thence it escapes, still compressed, into a series 

 of pipes, which are surrounded by moderately 

 cold water. Here the temperature of the com- 

 pressed air falls to within live or six degrees of 

 the water surrounding it, and then passes to 

 the expansion-cylinder, where by the simple 

 act of expansion it falls in some cases to 70 

 below zero. Many ingenious devices econo- 

 mize all the forces. In the Lightfoot machine, 

 for instance, the compressed air when it is lib- 

 erated returns about GO per cent, of the power 

 that has been expended in compressing it. It 

 is found that angles in the conduit- pipes re- 

 store the heat to air passing through. The 

 conduits therefore should be as straight as pos- 

 sible. Thirty to forty gallons of water are re- 

 quired to cool 1,000 cubic feet of air at the 

 normal pressure. Machines are constructed to 

 deliver from 25,000 to 285,000 cubic feet of 

 air per hour at about 25, varying somewhat 

 with the atmospheric conditions, but readily 

 maintaining a positively refrigerating tempera- 

 ture in properly ceiled rooms. Cooled air 

 gains about one degree of heat for every twenty 

 feet of conduit-pipe passed through ; therefore, 

 the refrigerating-chamber must be as near as 

 possible to the condensing machinery. It was 

 at first supposed that the cooled air would be 

 so heavily charged with moisture that some 

 artificial drying process would be necessary; 

 but it is found in practice that the moisture is 

 for the most part condensed and precipitated 

 in the form of snow before it reaches the de- 

 livery-pipes. This deposit of snow is so copi- 

 ous that special arrangements have to be made 

 for its periodical removal. The usual estimate 

 for keeping the temperature of a room down 

 to say 15 Fahr., is one and a half cubic feet of 

 cold air for every cubic foot of storage-room. 

 In practice this may often be reduced to one 

 cubic foot for each cubic foot of storage ; but 

 it is best to provide for the larger proportion. 

 The actual performance of a Haslam machine 

 intended to supply 60,000 cubic feet of cold 

 air in an hour was as follows : The storage- 

 chambers were sixteen in number, with a cubic 

 capacity of 48,000 feet, and capable of storing 

 1 1,000 sheep. The regular running time of the 

 engine was twenty hours a day, the four hours) 

 of rest being necessary to clear the snow-boxes, 

 valves, and air-trunks. The average speed of 

 the engine was 80 revolutions a minute at an 

 air-pressure of 44 pounds a square inch. This 

 gave a temperature of 57 Fahr. in the snow- 

 boxes, and kept the chambers at about +15 

 Fahr., that being found the best preservative 

 temperature for meat. About four and a half 

 tons of coal were used in twenty-four hours. 

 The chambers nearest the engine were easily 

 kept at a very low temperature, but the more 

 distant chambers required a more liberal sup- 

 ply of cold air. It has been estimated that, at 

 a comparatively moderate expenditure, passen- 

 ger-steamers on the regular sub-tropical lines 

 could be fitted with cold-air machines, which 

 would largely promote the comfort and health 



