Production of solid oxygen by the evaporation of the liquid

Abstract
It seems strange that while liquid hydrogen or nitrogen can easily be changed into the solid condition by evaporating the respective liquids under exhaustion, yet the ordinary air pumps fail to effect the transition of state in the case of liquid oxygen. This is due to the small vapour pressure of solid oxygen at its melting point, as compared with hydrogen or nitrogen under similar conditions; together with the greater need of very perfect heat isolation in the arrangement of the experiment. By means of the use of charcoal as a gaseous condensing agent at low temperatures, combined with the employment of proper vacuum vessels, the change from the liquid into the solid can be easily effected. Pure liquid oxygen, contained in a properly isolated vessel, subjected to the exhaust produced by a quantity of charcoal kept at about the temperature of boiling oxygen, has its pressure lowered sufficiently to produce solidification to a transparent jelly. The pressure at which solidification takes place was determined by connecting a McLeod gauge to the vessel containing the solid oxygen. About 75 grm. of good coconut charcoal is necessary to produce and maintain the necessary conditions of exhaust. From 5 to 10 c. c. of liquid oxygen were employed, previously exhausted by an air pump. The oxygen exhibits considerable supercooling, and a pressure of less than half the melting pressure can usually be maintained on the liquid without producing solidification. These experiments indicate that the melting point pressure is between 1·115 and 1·125 mm.

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