IT may interest some of your readers to know that the problem in thermodynamics, propounded by Mr. Blass in your number of August 29, has actually been put to the test. I pointed out Mr. Blass's letter to my brother, who is a freezing engineer, and he showed me a copy of the Zeitschrift fur die Gesammelte Kalte-Industrie (Munich) for August, in which an account is given of a machine on exactly the principle Mr. Blass suggests, by which Herr Linde has succeeded in liquefying air. It would appear, therefore, that the “theoretical minimum of temperature produced at C” would be determined by the point of liquefaction of the gas employed; with a perfect unliquefiable gas it would, I suppose, theoretically, be absolute zero.