on Low-Temperature Research, 1893-1900, 711 



Lord Eayleigh showed that the refractivity of helium was very 

 small, being only ■ 1 238. On the same scale (refractivity of air 

 = 1*0), the value of the constant for hydrogen is 0*469, or nearly 

 four times greater, the opposite disparity of densities notwithstanding. 

 The monatomic constitution of all these gases was established by 

 finding 1*66 as the ratio between their specific heats at constant 

 pressure and at constant volume. While exercising no appreciable 

 absorption upon light, they glow brilliantly through the action of an 

 electric discharge. An excited neon-tube flames with an orange- 

 pink colour ; krypton shines pale violet ; xenon luminesces in 

 sky-blue. The corresponding spectra are extremely vivid and 

 characteristic. The progress of research at low temperatures led to 

 anticipatory partial disclosures of them. In a paper ' On the Spectra 

 of the Electric Discharge in Liquid Oxygen, Air and Nitrogen,' 

 published in 1894 in the Philosophical Magazine, Professors Liveing 

 and Dewar recorded the appearance, during the distillation and con- 

 centration in vacuo of liquid oxygen and air under diminished pres- 

 sure, of two unknown bright lines at wave-lengths 557 and 555, the 

 former coinciding approximately with the chief auroral ray. Both 

 were subsequently, by Professor Ramsay and Dr. Travers, associated 

 with krypton. Again, some strange bright lines, belonging to the 

 then unidentified spectrum of neon, were derived by Professor Dewar 

 in 1897 from a vacuum-tube filled with a residuum of gas from the 

 King's Well at Bath, collected by the kind permission of the Cor- 

 poration of that town ; this is one of the most valuable among the 

 available sources for the supply of scarce aerial constituents. 



Low Temperature as an Analytic Agent. 



What may almost be designated a new branch of pneumatic 

 chemistry, the analysis of gases by cold, was set on foot by Professor 

 Dewar in 1897. On the 4th of November in that year, he described 

 before the Chemical Society an apparatus for determining the propor- 

 tion of any atmospheric ingredient that remains uncondensed at 

 — 210° C. and is insoluble in liquid air under standard pressure. 

 Preliminary experiments showed that one part of hydrogen in a 

 thousand of air could just be detected by the newly devised method, 

 and that liquid air can dissolve one-fifth of its own volume of 

 hydrogen. Helium proved to be soluble, though in a less degree, in 

 liquid nitrogen. With the powerful aid of liquid hydrogen, these 

 researches were continued during four ensuing years. A striking 

 illustration of its extraordinary effectiveness in refrigeration is 

 afforded by the rapid production, through its means, of high vacua. 

 It was computed that the pressure of air in sealed tubes, frozen out 

 by immersion in liquid hydrogen, could not exceed one-millionth of 

 an atmosphere, apart from any that might result from the survival, in 

 minute quantities, of gases more refractory than oxygen or nitrogen. 

 The exhaustion, in other words, due to the frigorific treatment of air- 



