ARTIFICIAL REFRIGERATION AND ICE MAKING. XLI. 
pound of air possesses the intrinsic energy due to its specific heat 
multiplied by its absolute temperature, i.¢., 62 + 460 = 523 degrees 
and it occupies a volume of 13-141 cubic feet, which is represented 
by the horizontal length of the diagram. If such a volume of air 
is compressed to a density of four atmospheres, then between 47 
and 48,000. foot-pounds of energy will be required, and assuming 
a frictionless piston and a non-conducting cylinder, the air 
‘instead of following Mariotte’s law, and by an isothermal com- 
pression occupying one-fourth or twenty-five per cent. of its 
original volume at the original temperature will rise to a temper- 
ature of 320° and fill 37°3% of the original volume, the difference 
Tepresenting the work performed by the engine in the work of 
compression. Now while it is under this increased tension, which 
with cold-air machines seldom exceeds five atmospheres, the com- 
Pressed air may be passed through a condenser and have its 
temperature again brought to 62°, in which case the heat or 
energy of the engine will be communicated to the condensing 
water and for all practical purposes be lost. The air then only 
possesses the same intrinsic energy which it did before compression, 
but it is in a physical or mechanical condition which enables it to 
perform work by expanding again to atmospheric pressure. This 
expansion in practice is carried out in an engine similar to a steam 
engine which assists the working of the whole refrigerating 
Machine, and the final temperature of the air is found by simple 
Proportion thus—. 
~  Gredieee abs. tempera- | ; compressed abs. tempera- 
ture before condensation { ~~ } ture ite condensation 
or 461 + 320 = 781 : 461 + 62 = 523 
$0. is { original temperature \ i. { final eds 
re compression after expansion 
: 461 + 62 = 523 : $48 absolute = — 113 
or to make a simple proportion sum of it— . 
781 : 523 523 : 348 
and 348° ~ 461 = - 113° 
In actual practice this theoretical low temperature is never 
_ Teached, about—80° being the minimum and—-50° an ordinary tem- 
