SOLID HYDROGEN. 255 



other liquid gases might be used to replace the nitrogen in this experi- 

 ment. In making a selection, however, it is necessary to take only 

 those bodies that possess a reasonably high tension of vapor at the 

 melting point. The process would not succeed easily with a substance 

 like oxygen, that has no measurable tension of vapor in the solid 

 condition. 



In the autumn of 1898, after the production of liquid hydrogen 

 was possible on a small scale, its solidification was attempted b} T boil- 

 ing under reduced pressure. At this time, to make the isolation of 

 the hydrogen as effective as possible, the liquid was placed in a small 

 vacuum test-tube, placed in a larger vessel of the same kind. 

 Excess of hydrogen partly filled the annular space between the two 

 vacuum vessels. On diminishing the pressure by exhaustion the 

 evaporation was mainly thrown on the liquid hydrogen in the annular 

 space between the tubes. In this arrangement the outside surface of 

 the smaller tube was kept at the same temperature as the inside, so 

 that the liquid hydrogen for the time was effectually guarded from 

 influx of heat. With such a combination the liquid hydrogen was 

 evaporated under diminished pressure, yet no soliditication took place. 

 Seeing experiments of this kind required a large supply of the liquid, 

 other problems were attacked, and further attempts in the direction 

 of producing the solid for the time abandoned. During the course of 

 the present year many varieties of electric resistance thermometers 

 have been under observation, and with some of these the reduction of 

 temperature brought about by exhaustion was investigated. Ther- 

 mometers constructed of platinum and platinum-rhodium (alloy) were 

 only lowered 1^° C. by exhaustion of the liquid hydrogen, and they 

 all gave a boiling-point of —245° C, whereas the reduction in tem- 

 perature by evaporation in vacuo ought to be 5° C, and the true 

 boiling-point from —252° C. to —253° C. In the course of these 

 experiments it was noted that almost invariably a slight leak of air 

 occurred which became apparent by its being frozen into an air-snow 

 in the interior of the vessel, where it met the cold vapor of hydrogen. 

 When conducting wires covered with silk have to pass through india 

 rubber corks, it is very difficult at these excessively low temperatures 

 to prevent leaks, when corks get as hard as a stone and cements crack 

 in all directions. The effect of this slight air leak on the liquid 

 hydrogen when the pressure got reduced below 60 millimeters was 

 very remarkable, as it suddenly solidified into a white froth-like mass 

 like frozen foam. My first notion was that this bod}? - might be a sponge 

 of solid air containing liquid hydrogen. The ordinary solid air obtained 

 by evaporation in vacuo is a magma of solid nitrogen containing liquid 

 oxygen. The fact, however, that this white solid froth evaporated 

 completeby at the low pressure without leaving any substantial amount 

 of solid air led to the conclusion that the body after all must be solid 



