Going more into detail, he makes clear some of the mechanical and chemical difficulties which beset him in the work. “The secret of my success,” he continues, “has been the mechanical arrangements combined with the use of ethylene. This is a volatile hydrocarbon, and is the chief illuminating constituent of coal gas. The only means of keeping it liquid for any length of time is to surround it with solid carbonic acid. Faraday was the first to call attention to the properties of ethylene, and we manufacture it by heating sulphuric acid in a glass retort protected by an iron cover to 160° C. Alcohol heated to 160° C. is allowed to drip into it and ethylene results, passing off as a gas, which is stored, after being purified. It is then compressed by two pumps, the first with a six-inch plunger and six-inch stroke, and the second a two-inch plunger and six-inch stroke. This liquefies it under the pressure stated. It is nasty stuff to handle, as, whenever it becomes mixed, by leakage or otherwise, with nitrous oxide or air, an explosion is imminent, and we have had not a few explosions in the course of the work. It liquefies at −103° C. (−152.4° F.), and when boiled in a partial vacuum absorbs a large amount of latent heat. The failure of preceding attempts to liquefy oxygen is due to lack of knowledge of its ‘critical point,’ and the law which that phrase describes. As long ago as 1851, Natterer subjected oxygen to a pressure of 2,800 atmospheres, or over thirty tons to the square inch. He obtained no result, because, as I have said, no amount of pressure will affect it above −115° C. I liquefy it at −145° C. for two reasons. The lower the temperature at which it is liquefied, the less is the pressure required upon the oxygen and the greater is the amount of latent heat which it absorbs in evaporating. By evaporating, under exhaustion, oxygen liquefied at −145° C., I get as low as −200° C., which I could not do were it liquefied at a higher temperature. [Illustration: PROFESSOR DEWAR’S LECTURE-TABLE. FROM A PHOTOGRAPH TAKEN OF PROFESSOR DEWAR’S FIRST LECTURE ON THE LIQUEFACTION OF OXYGEN.]