ploying those substances that are capable of producing the greatest degree of cold. But a difficulty is encountered in the high pressures of the gases produced in the pump, as there is no evading the physical fact that the cold-producing power of a gas is a concomitant of its tension, or pressure varying directly therewith. Thus, ammonia, with a pressure at rest of eight atmospheres, and at work of twelve to twenty atmospheres according to the temperature, is an excellent refrigerant, but the use of a gas with such a high pressure is attended with obvious drawbacks. At the other end of the scale is ether, which is manageable at a low pressure, viz., zero at rest, and ten to fifteen pounds per square inch at work; but this advantage has its corresponding drawback, in accordance with the law above mentioned, i. e., a comparatively low refrigerating power. It is, moreover, inflammable, and, in contact with any of the lubricants used on the pumppiston, there results an unintended product of soap, which, coating the parts of the mechanism, obstructs the passage of the latent heat from the circulating medium employed for freezing. Midway between these two agents, as regards its pressure, is sulphurous acid. This gives a high degree of cold, its pressure at work being three and one half to six atmospheres, and a little over two and one half atmospheres at rest. Aside from its rather high pressure, a serious objection to its use is the liability to corrosion of the parts on contact of the liquid with moisture, sulphuric acid being thereby produced, which rapidly wears away the more important parts of the mechanism employed.
The various defects enumerated, and others incident to the use of other agents not here particularized, viz., liability to explosion, inflammability, indifferent refrigerating capacity, high vacuum, high pressure involving rapid wear and tear and danger in use, and other more or less serious drawbacks, have made the attainment of a still better system than the best of those referred to imperative. The great desideratum, it will be seen, has been a process admitting of using some of the better cold-producing agents without the dangers or annoyances due to the high tensions of their gases, or to other peculiarities of their composition. The discovery of a method by which this object could be attained is due to the genius of the late C. M. Tessié du Motay.
This eminent French chemist, acting on the suggestion of one of his associates, M. Étienne Gillet, a gentleman who had made a close study of artificial ice-making, sought to combine two or more liquids which should have the property, in combination, of mutually neutraizing the defective features they exhibited when used separately, and which should at the same time retain their desirable qualities. He instituted experiments, in conjunction with M. Auguste Rossi, which resulted in the discovery that ether, when combined with sulphurous acid, furnished a compound absolutely free from any of the defects
