XL. NORMAN SELFE. 
is alternate liquefaction and volatilization. Although many 
different media have been tried, each of which has some special 
quality to recommend it, the principal ones to which reference 
will be made, are sulphuric ether, sulphurous acid, ammonia, and 
carbonic acid. In the system introduced by Carre a solution of 
ammonia in water is employed, the gas is driven off by the direct 
application of heat, and is again reabsorbed by the water after 
fulfilling its functions in the circuit of the apparatus; this is 
known as an “absorption system.” The greater number of 
refrigerating engineers now adopt the compression system, but 
some well known authorities still advocate the advantages of the 
absorption system. 
COLD AIR MACHINES. 
These machines, coming under the first or simpler system already 
referred to, operate by virtue of the law that all mechanical work 
has a thermal equivalent. The diagram Fig. 4, illustrates the 
action of such a machine in dealing with a pound weight of air. 
At atmospheric density or etl Ibs. and a cenpereet of 62° one 
COMPRESSION AND EXPANSION 
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