ICE, ARTIFICIAL. 



427 



bed, table, and chair, and weigbs, complete, 

 about 2,500 pounds. It can be loaded upon an 

 ordinary double truck, and can be hauled by 

 a span of horses. 



The advantages of such buildings for army 

 purposes are readily appreciated. Two men 

 can set one of them up in a little more than 

 an hour, and four men in a correspondingly 

 shorter time. For hospital purposes, either as 

 temporary wards used in connection with per- 

 manent establishments, or as semi-permanent 

 field-hospitals, they are admirably contrived. 



By using a smaller number of sections independ- 

 ent structures, respectively, 18 by 22 feet, and 

 12 by 18 feet, may be made in the same man- 

 ner, suitable for camping parties or for any of 

 the purposes for which buildings of such di- 

 mensions are adapted. The completeness of 

 the whole outfit, containing in itself all essen- 

 tial furniture in the most compact shape, is ob- 

 vious at a glance. The barrack has been thor- 

 oughly tested in several very severe storms, 

 and it is said that it fully meets all reasonable 

 requirements. 



ICE, ARTIFICIAL. No material changes have 

 taken place for several years in the methods of 

 harvesting and storing for summer use the nat- 

 ural ice-crop of the Northern States. The de- 

 mand for consumption has, however, largely 

 increased, and the annual harvest is believed 

 to exceed 25,000,000 tons. The interests of the 

 industry are representated by the " Ice Trade 

 Journal," an eight page paper, published every 

 month in Philadelphia, and now in its tenth 

 volume. The business of manufacturing spe- 

 cial tools, wagons, and machinery has become 

 a special branch of industry, employing many 

 skilled artificers. 



With the increasing demand for ice under 

 conditions where the natural product can not 

 readily be utilized as, for instance, during long 

 sea-voyages or in tropical countries inventors 

 have turned their attention to the production 

 of ice, or its equivalent cold, by artificial means. 

 Such methods have long been employed by 

 Oriental nations. Plutarch, Theocritus, and 

 Aristotle refer to the preservation of ice and 

 snow, and to its use as a luxury during summer. 

 Aristotle doubts the healthfulness of ice-water 

 as a beverage. Solomon, however, may be 

 said to give it a quasi- approval, when he says 

 (Proverbs xxv, 13): "As the cold of snow in 

 the time of harvest, so is a faithful messenger 

 to them that send him : for he refreshes the 

 soul of his masters." 



The Hindoos have, probably for many cent- 

 uries, obtained ice by the following method : 

 In a wide, open field, unobstructed by trees or 

 buildings, pits are digged twenty or thirty feet 

 square and two feet deep. The floors of these 

 are covered with dry stalks of straw or cane, 

 upon which very shallow vessels of porous 

 clay are placed, and after nightfall are filled 

 with water. The evaporation is of course very 

 rapid, and in the morning, if the night has 

 been cloudless, thin sheets of ice are found in 

 the pans. This experiment has been success- 

 fully repeated in England and in France, but 

 the product is so small as to be unavailable for 

 practical purposes in temperate climates. Jars 

 of porous clay are also used in the tropics for 

 eooling water, and where they are carefully 

 tended ice is said sometimes to form in small 

 quantities. 



Probably the most familiar of all ice making 

 machines is the common ice-cream freezer, 

 namely, a metallic vessel surrounded by a 

 mixture of common snlt with snow or broken 

 ice, forming what is termed a u freezing-mix- 

 ture." In the metallic vessel is placed the 

 cream or other substance to be frozen, and an 

 interior arrangement of dashers keeps it agi- 

 tated while the ice and the salt pass from a 

 solid to a fluid condition melt and expand, 

 that is and by the process extract so much 

 heat from the cream that its temperature falls 

 below the freezing-point. An equally simple 

 method furnishes the Parisian restaurants with 

 the refreshing carafes Jrappees so familiar to 

 travelers. Decanters filled with fresh water 

 are set in shallow tanks filled with salt water, 

 which freezes more slowly than fresh. Con- 

 tiguous to the tanks are receivers, which can 

 be easily charged with vaporized ether. The 

 ether soon extracts the heat from the salt 

 water, which in turn absorbs that of the fresh 

 water, and the latter, after a suitable exposure, 

 solidifies within the decanter and is ready for 

 the table. 



In order to comprehend the operation of ice- 

 making machines, it is necessary to accept some 

 of the established phenomena of heat in its re- 

 lations to gases, fluids, and solids. Air, for 

 instance, the most common of the gases, be- 

 comes' heated when it is compressed. In this 

 compressed state its heat can be more readily 

 removed than when in its normal degree of ex- 

 pansion, and, when suffered to re-expand, it is 

 in a condition to absorb a large amount of heat 

 from any solid or fluid with which it is brought 

 in contact. Thus, three cubic feet of air at 

 the normal pressure of fifteen pounds per 

 square inch, with a temperature of 75 Fanr., 

 when compressed to one cubic foot will show 

 a temperature of about 225 Fahr. After it 

 has been allowed to cool until it has regained 

 its original temperature, 75 Fahr., it may be 

 allowed to re-expand to three cubic feet. 

 When its temperature will fall to about 25 

 Fahr., and if the expansion leads it into a re- 

 ceiver containing a vessel of water, the heat of 

 the water will pass to the air, and, unless the 

 volume of water be too great, it will soon 

 freeze. A greater degree of compression will 



