The production of a low temperature by the alternate compression and expansion of air is perhaps best accomplished by the Bell and Coleman apparatus, which has been described in the article on Food Preservation, its primary application being for preserving fresh meat on long voyages.
The "binary absorption" system of Tessie du Mothay and A. I. Rossi is the most recent development of the science of producing artificial cold. Experiments on ethers indicated that those formed by the acids, as well as their alcoholic radicals, possess the property of absorbing sulphur ous anhydride (sulphur dioxide), some of them to the extent of 300 times their volume of gas in certain conditions, ordinary ether standing first. Upon this fact, the new system is founded. The liquid employed is ethylo-sulphurous dioxide, obtained from ordinary ether by saturating with sulphurous oxide gas. This liquid, at a temperature of 15°-18° (60°-65° F.), has no pressure, and can be readily kept in glass bottles at 27°-32° (80°-90° F.); its tension is only 2-5 lb. Thus a machine charged with it, when stopped, will show no pressure on the gauges, and even a vacuum at rest, if the temperature is low ; while with other liquids, even the stoppage of the machine does not prevent the pressure of the vapours inside soon reaching its point of equilibrium with the tem perature outside, and even at as low a temperature as 0° (32° F.) sulphur dioxide (sulphurous oxide) alone, as used in the Pictet machine, has still 15 lb. a sq. in. of pressure, exerting a constant and increasing pressure on the vessels containing it, and, in case of a small leak starting, causing the entire loss of the charge. What is said here of sulphurous oxide applies with still more force to liquid ammonia, methyl chloride, and methylic ether.
Such a binary liquid as that just mentioned, when evaporated under a vacuum, is resolved into its two constituents, the mixed vapours entering the pump together ; then, under a small compression, ether liquefies first, a few lb. pressure being sufficient for it, even with such
waters as are met with in tropical climates. The ether thus liquefied absorbs in the condenser the vapours of sulphurous oxide, reconstituting the " binary liquid," and thereby avoiding the excess of mechanical compression, which would otherwise have been necessary to effect this liquefaction of the oxide. Thus for the work of compression of the pump, is substituted a power of chemical affinity, and absorption of the less volatile absorbent for the vapours of the more volatile. With the advantage of the low pressure of the ether, is combined the advantage of the intensity of cold pro duced by the volatilization of the sulphurous oxide, avoiding its drawbacks. In presence of water and the ether, the 'sulphurous oxide is transformed, not into sulphuric acid, as before, but into " sulphorinic " acid, the action of which acid upon metals ie insignificant, if not absolutely nil. The sulphurous acid being an extinctor relieves the ether of one of the drawbacks to its use, and acting as self-lubricant, renders the greasing of the working parts unnecessary. In a machine making 6 tons of ice daily, the pressures in the condenser in normal and regular working have been 14-15 lb., descending to 10-11 lb. under most favourable conditions, and reaching 20-23 lb. under least favourable conditions. The water used for condensation has been but i-A of that needed by a Pictet machine of the same capacity. The smallness of pressure required renders the machine much simpler, ordinary valves, &c., sufficing, The New York Ice Machine Co. are working very successfully with the system in the United States.
Bibliography.—B. H. Paul, Artificial Freezing and Refrigeration' (Tour. Soc. Arts, vol. xvii., No. 839, London : 1868); M. Ledoux, Ice-making Machines' (New York : 1879).