on Loio-Temperature Research, 1893-1900. 701 



transparent to thermal radiations. Its absorption-spectrum, too, is 

 virtually the same as that of the gas ; so that it could be inferred that 

 the molecular constitution of the element was little affected by change 

 of state. 



The practical difficulties impeding the preservation and observa- 

 tion of frigid liquids were, in great measure, overcome by Professor 

 Dewar's invention of vacuum-coated vessels for storing them. By 

 their means the access of heat, whether by convection or by radiation, 

 was so effectually checked that evaporation shrank at once to one- 

 fifth its former amount, and the deposition of a thin film of mercury 

 on the surface of the containing glass still further lessened the waste. 

 Added refinements of construction brought it down to a mere fraction 

 of what it had been in unprotected receptacles, and the durability of 

 volatile fluids thus obtained a thirty-fold extension. Tranquillity was 

 besides given to them, ebullition ceased, and they could be manipu- 

 lated with ease. This essential improvement was effected late in 

 1892. Thus, at the beginning of the septennial period we have now 

 to consider, a steady and continuous advance was in progress, and 

 much new territory had been annexed and explored. A tract, it is 

 true, lay beyond, not wide, but most arduous to penetrate ; yet there 

 was a hope of its thorough conquest through the amelioration of 

 methods and the experience acquired in their application. 



Liquefaction of Hydrogen. 



The main outstanding problem was the condensation of hydrogen. 

 This is the lightest of known substances, yet it resembles a metal in 

 being strongly electro-positive ; in being a conductor of heat and 

 electricity ; and in forming, with palladium, sodium and potassium, 

 compounds possessing some of the properties of alloys. Upon these 

 grounds Faraday based the forecast that solid hydrogen would show 

 metallic lustre. Its liquefaction was first demonstrated by M. Wrob- 

 lewski, of Cracow, in January 1884. A froth of hydrogen became 

 momentarily visible when the gas, cooled to the temperature of nitro- 

 gen boiling in vacuo, was suddenly released from a pressure of 

 180 atmospheres. This evasive appearance was reproduced by 

 M. Olszewski ; but neither experimenter succeeded in obtaining the 

 liquid in tangible form. This was accomplished at the Royal Insti- 

 tution, as the outcome of a long series of efforts, frequently baffled, 

 and persistently renewed. The conditions that had to be met were 

 approximately known through Wroblewski's determination, by the 

 aid of Van der Waal's formula, of the critical constants of hydrogen. 

 He fixed the temperature sine qua non (as it may be called) at 

 — 240° C, the corresponding pressure at 13 atmospheres, and the 

 boiling point at — 250°. With these conditions, Professor Dewar 

 made in 1894 a preliminary attempt to grapple, by mixing a small 

 percentage of air or nitrogen with hydrogen, and thus producing an 

 artificial gas capable of liquefaction by the use of liquid air. This 



