on Low- Temperature Research, 1893-1900. 709 



bination of forces. M. Moissan, a specialist in the practical chemistry 

 of the element, brought his apparatus for its production to the Eoyal 

 Institution, for the purposes of his lecture, on May 28, 1897, and it 

 was used next day, in connexion with Professor Dewar's refrigerating 

 plant, to obtain the first specimen of liquid fluorine. This is a clear 

 yellow fluid of great mobility, boiling in open vessels at — 187° C, 

 and refusing to solidify at — 210°. The following are its chief 

 ascertained properties. It is soluble in liquid air and oxygen ; its 

 density is 1 • 14 that of water ; it has a capillarity less than that 

 of liquid hydrogen ; it gives no absorption-spectrum, and is devoid 

 of magnetic quality. The energetic chemical affinities characteristic 

 of the gas are almost entirely suppressed by the extreme cold needed 

 for its condensation. The liquid rests harmlessly in glass bulbs ; it is 

 indifferent to oxygen, water or mercury; only hydrogen and hydro- 

 carbons excite it to reaction with incandescence. 



Solid Hydrogen, Oxygen, and Air. 



The freezing of atmospheric air was first accomplished by Pro- 

 fessor Dewar in 1893. A litre of liquid air, subjected to exhaustion 

 in a silvered vacuum-vessel, yields about half that volume of colour- 

 less, transparent solid, which may last as such for half au hour. 

 Under magnetic compulsion, liquid oxygen is drawn to the poles 

 from the meshes of the " nitrogen-jelly," which forms the really solid 

 part of air-ice. This substance can only be examined in a vacuum 

 or in an atmosphere of hydrogen, since it instantly melts in con- 

 tact with the atmosphere, giving rise, at the same time, to a further 

 liquefaction of air. The interaction of the two processes can be 

 watched, and is curious to discriminate. Tbe difference between the 

 couditions of freezing of oxygen and nitrogen depends upon the 

 circumstauce that the vapour-pressure of the former substance, 

 boiling in an exhausted receiver, is inapjireciable ; that of the latter, 

 quite considerable. Solid oxygen can only be obtained through the 

 agency of liquid hydrogen. It is clear, blue ice. Hydrogen itself 

 was solidified by Professor Dewar, not without much difficulty, in 

 1899. This final product of refrigeration has a fusing-point of about 

 15° absolute, at a vapour-pressure of 55 millimetres. It has the 

 appearance of perfectly pure ice ; there is nothing metallic about it. 

 The exhibition to a crowded audience in the theatre of the Eoyal 

 Institution, April 6, 1900, of a form of matter representing so much 

 victorious exertion was a triumph tempered by the reflection that 

 the hydrogen-route towards the absolute zero stopped with its pro- 

 duction, leaving a trackless interval of narrow extent, but immense 

 importance. 



The era of "new gases" began in 1894, with the isolation of 

 argon ; helium was shortly afterwards extracted from clevite and 

 other rare minerals ; krypton, neon, and xenon were in 1898 

 spectroscopically identified as atmospheric constituents by Professor 



