ARTIFICIAL REFRIGERATION AND ICE MAKING. XLII. 
attraction for scientists and is still being eagerly pursued, new 
triumphs in the way of simple apparatus for liquefying oxygen 
and atmospheric air being continually announced. Whenever a 
gas is vaporised from its liquid condition heat is taken up from 
some source of supply, and this is the property that is utilised in 
ordinary refrigerating machinery ; but whereas water at atmos- 
pheric pressure boils at 212° very much lower temperatures are 
sufficient for the evaporation of the four gases used for refriger- 
ation at atmospheric tension, viz :— 
Sulphuric Ether, Sulphur Dioxide, Ammonia, Carbonic Acid. 
+ 96° + 14° oe oe ee 
Now just as with water and steam, the boiling point of these . 
and other gases means the temperature at which such gases liquefy 
48 well as that at which their liquids pass to the gaseous condition ; 
in fact a temperature under which the material may be either 
liquid or gaseous, depending for its condition upon the heat units 
contained in or held by it; such temperature depends upon the 
pressure to which they are subjected at the time, and conversely 
the pressure under which any gas can be liquefied depends upon 
its temperature. For the practical purposes of artificial refriger- 
ation the lowest temperature to which heated gases under pressure 
can be reduced is limited by the temperature of the water used 
for condensation | this water may be as low as 45° or 50° in 
temperate countries, and in hot climates may exceed 90°. 
The diagram Fig. 5 shews in graphic form the vapour tensions 
of carbonic acid, ammonia, sulphurous acid, ether, and water 
under the temperatures met with in practical work, or their boil- 
ing points under widely varying conditions as to pressure. For 
instance it will be seen that carbonic acid, which under atmos- 
Pheric pressure will boil at 124° below zero, requires about 1080 bs. 
Per square inch to liquefy it at 96°, affording a great contrast to 
water, the boiling point of which at 14:7 Ibs. or one atmosphere is 
1 For experimental purposes to produce very low temperatures 
condensed gas may be cooled by a second refrigeration and a step by step 
Process adopted for steining the lowest extreme possik 
