ve manner than was possible in a paper devoted to the
consideration of a number of different systems and apparatus. In the
United States and in Germany, as well as to some extent elsewhere,
ammonia has been very generally employed for refrigerating purposes
during the last ten years or so. In this country, however, its
application has been extremely limited; and even at the present time
there are but few ammonia machines successfully at work in Great
Britain. No doubt this is, to a large extent, due to the fact that in
the United States and in Germany there existed certain stimulating
causes, both as regards climate and manufactures, while in this
country, on the other hand, these causes were present only in a
modified degree, or were absent altogether. The consequence was that
up to a comparatively recent date the only machine manufactured on
anything like a commercial scale was the original Harrison's ether
machine, first produced by Siebe, about the year 1857--a machine
which, though answering its purpose as a refrigerator, was both costly
to make and costly to work. In 1878 the desirability of supplementing
our then existing meat supply by means of the large stocks in our
colonies and abroad led to the rapid development of the special class
of refrigerating apparatus commonly known as the dry air refrigerator,
which, in the first instance, was specially designed for use on board
ship, where it was considered undesirable to employ chemical
refrigerants. Owing to their simplicity, and perhaps also to their
novelty, these cold air machines have very frequently been applied on
land, under circumstances in which the same result could have been
obtained with much greater economy by the use of ammonia or some other
chemical agent. Recently, however, more attention has been directed to
the question of economy, and consideration is now being given to the
applicability of certain machines to certain special purposes, with
the result that ammonia--which is the agent that, in our present state
of knowledge, gives as a rule the best results for large
installations, while on land at any rate its application for all
refrigerating purposes presents no unusual difficulties--promises to
become largely adopted. It is hoped, therefore, that the following
paper respecting its use will be of interest.

In all cases where a liquid is employed, the refrigerating action is
produced by the change in physical state from the liquid to the
vap