“The process of liquefying oxygen, briefly speaking,” says the professor, “is this. Into the outer chamber of that double compressor I introduce, through a pipe, liquid nitrous oxide gas, under a pressure of about 1,400 pounds to the square inch. I then allow it to evaporate rapidly, and thus obtain a temperature around the inner chamber of −90° C. (−130° F.). Into this cooled inner chamber I introduce liquid ethylene, which is a gas at ordinary temperatures, under a pressure of 1,800 pounds to the square inch. When the inner chamber is full of ethylene, its rapid evaporation under exhaustion reduces the temperature to −145° C. (−229° F.). Running through this inner chamber is a tube containing oxygen gas under a pressure of 750 pounds to the square inch. The ‘critical point’ of oxygen gas, that is, the point above which no amount of pressure will reduce it to a liquid, is −115° C., but this pressure, at the temperature of −145° C., is amply sufficient to cause it to liquefy rapidly. In drawing off the liquid under this pressure, I lose nine-tenths of it by evaporation, and I have not yet seen how to diminish that loss. Every pint of it which I collect therefore represents ten pints manufactured. In all, I have thus far collected and used about fifty gallons, and the cost of machinery and experiments, very generously met by subscription among members of the Royal Institution and others, has been about five thousand pounds sterling.” It should be here stated that one of the most generous contributors to the fund has been Professor Dewar himself, a large fraction of the sum having come out of his own pocket. [Illustration: THE LECTURE-ROOM OF THE ROYAL INSTITUTION.]