LXXIl. DISCUSSION. 
In a perfect process the nature of the working substance is 
immaterial, but in the actual process the efficiency depends other 
things being equal, upon the ratio of the specific heat to the latent 
heat of the volatile liquids such as ammonia and carbonic acid. 
The critical temperature of carbon dioxide is 88° F, when its 
latent heat is zero, hence these machines lose efficiency when the 
condensing water is about this temperature which must necessarily 
be the case in warm countries. In America they are using dry 
air machines on board ships in the Navy in which the air is 
initially compressed by means of a special pump—the Allen 
Dense Air Ice Machine—so that the weight of the substance 
circulated is much greater and the machinery less bulky. The 
air is under a pressure of 60 Ibs. per square inch, and is compressed 
up to about 210 Ibs. per square inch. 
Mr. Sere said that in the historical portion of the paper he 
had no intention to attempt to write a complete history of artificial 
refrigeration, or to record all the steps by which it had arrived 
at its present position. It was only because there are works 
dealing with the matter which entirely ignore the part which 
Australia has played in this connection, that the historical 
references were introduced at all, and he wished to place these 
Australian facts on record, before they could be contradicted or 
give rise to controversy. 
Mr. Statham gave some very interesting particulars regarding 
the falling off of efficiency in carbonic acid machines corroborating 
the statement made in the paper on that matter. The statement 
as to the valves on the Linde machine not affecting the pressures 
was not accompanied by diagrams, and it was not stated that they 
can do without springs at all or with as light springs as vertical 
valves all opening upwards—as in the Antarctic machines. With 
regard to oil in the cylinder, all good machines are now fitted with 
oil collectors as well as the « Linde.” The advantages of wet 
compression as advocated by Mr. Statham, can, if valid, be availed 
of in the first stage of the Antarctic machine as well as in the 
Linde system. 
