on Low-Temperature Research, 1893-1900. 703 



tubes with solidified atmospheric air forms a constantly-recurring 

 embarrassment in the use and management of liquid hydrogen. 



Low-Temperature Thermometers. 



This brilliant result, achieved after a long series of discourage- 

 ments and failures, was only the prelude to fresh enterprises. There 

 is no finality in science. Truth still flies before, enticing its 

 votaries to enter upon ground more and more difficult and broken. 

 This emphatically applies to researches on the production of artificial 

 cold. With every downward step the obstacles become more formid- 

 able, the circumstances more critical. Even the exact determination 

 of the temperatures attained is encompassed with difficulties. Ordi- 

 nary methods of heat-measurement collapse under extraordinary 

 conditions. Hence the choice of a thermometer for fixing the 

 boiling-point of hydrogen was a matter of the nicest delicacy. 

 Those giving changes of temperature in terms of electrical resistance 

 were the most readily available ; but the law of their construction, 

 being empirical, could scarcely be expected to hold good far beyond 

 the limit of experience. Kecourse was then had to gas-thermometers 

 of the " constant volume " form, filled severally with hydrogen, 

 helium, oxygen, and carbonic acid. The two latter were employed 

 to resolve the doubt whether the rule of equable contraction with 

 cold continued valid down nearly to the boiling-points of the fiduciary 

 substances ; and they gave reassuring replies. It appeared that either 

 a simple or a compound gas might be depended upon for the deter- 

 mination of temperatures until liquefaction set in. The results 

 obtained with the hydrogen and helium thermometers might accord- 

 ingly be accepted with confidence, more especially in view of their 

 close agreement. From them it was concluded that hydrogen boils 

 under atmospheric pressure at — 252*5° C, or 20 '5° above absolute 

 zero ; and that its critical temperature is —241° C.,the critical pres- 

 sure being about 15 atmospheres. Liquid hydrogen shows no 

 metallic affinities. It is a non-conductor of electricity, and freezes 

 into an ice-like solid. Its lightness is extraordinary ; water is 

 fourteen times more dense. Perfectly transparent, it gives no 

 absorption-spectrum, and is entirely colourless. Its specific heat, 

 although not very different from that of liquid oxygen when volumes 

 are compared, is twelve times greater for equal weights, and is six 

 times that of water. Its scientific value depends, however, not only 

 upon its exceptional qualities, but upon its power as a frigorific 

 agent. In this capacity, it was rendered serviceable only after some 

 months of severe experience, abnormally cold substances being as 

 troublesome to deal with as abnormally hot ones. The obstacles 

 surmounted in the case of liquid air were presented in an aggravated 

 form by liquid hydrogen, with the further complications due to the 

 solidification of all the ambient air. Its preservation thus offered 

 a twofold problem. Not only the access to it of heat had to be 



