MECHAXICAL REFRIGERATIOX 353 



Mechanical Refrigeration. — The underlying principle of 

 mechanical refrigeration, that of the consumption of heat in the 

 vaporization of a liquid, is not a new one. In the hot climates of 

 many eastern countries, water for drinking purposes is kept in 

 porous earthen vessels so that the wind may e\"aporate the 

 moisture as it oozes through the pores of the vessel and so cool 

 the water. In Arizona, and to some extent in Oklahoma, the 

 fanners cool their cream by wrapping the cream can with a 

 suitable cloth which acts as a wick to carry up moisture to be 

 evaporated and, in so doing, absorbs heat from the cream in 

 the can or, in other words, cools the cream. 



A more refined and very much more effective application of 

 this principle of cooling, through the ^'aporization of a liquid, 

 is made under the modern system of mechanical refrigeration. 

 Under the Compression system, which is explained in the chap- 

 ter entitled, " CooUng Facilities for Creameries " the two sub- 

 stances most commonly used are anh\-drous ammonia and car- 

 bonic acid gas. Professor Carl Linde of ^Munich invented the first 

 ammonia compression machine in 1S73. The carbonic acid 

 macliine, also compression, and copied after Linde's designs, 

 appeared in 1880. In large creameries and central cold storage 

 houses in this country, ammonia plants are much more common 

 than carbonic acid plants, although on ships the latter are prob- 

 ably in more general use. In another system of mechanical 

 refrigeration, what is known as the Absorption process is used. 

 Under this system two substances are used, one of which remains 

 a liquid and absorbs tlie other at ordinary temperatures. One 

 combination is that of sulphuric acid and water, and another, 

 water and ammonia. 



Benefits of Cold Storage. — Mechanical refrigeration was first 

 introduced into the United States about rS88. Its general effect, 

 however, upon the storing of food and upon the market was not 

 appreciably felt until about 1902. Cold storage is of great bene- 

 fit to the pubhc as a whole. As to the consuming public, it 

 enables them to get perishable food products held over in manv 

 cases from the time of high production to the time of scarcity, 

 thus establishing a greater uniformity of price throughout the 



