THE LIQUEFACTION OF GASES 6i 



fourth the boiling-point of hydrogen, and it has 

 proved at least as hard to pass the interval 

 between hydrogen and helium as it was to pass 

 from air to hydrogen. 



But, as was foreseen, the liquefaction of helium 

 was effected by an extension of methods previously 

 successful with other gases. A preliminary study 

 of its properties showed that, after cooling in 

 liquid hydrogen, it should cool further when sub- 

 jected to a regenerative process. After attempts 

 by several investigators had failed. Professor 

 Onnes succeeded, and the year 1908 saw the last 

 known refractory gas reduced to the state of liquid. 



The liquefaction of helium gives command of 

 a steady temperature of about 4°. 5 absolute, its 

 boiling-point in open vessels. That temperature, 

 within 5° of the absolute zero, is thus possible, 

 and Onnes has reached perhaps a degree lower 

 by working under low pressure ; but there, with 

 our present methods and materials, seems to 

 come the end of any probable advance. 



We may now pass to a brief account of the 

 methods of measuring these very low temperatures. 



Mercury freezes at a temperature of — 40°C., 

 and, at such temperatures as those now under 

 consideration, a mercury thermometer clearly is 

 useless. The resistance of a metallic wire to the 

 passage of an electric current is a quantity which 

 can be measured easily and accurately. This 

 resistance, diminishing as the wire is cooled, 

 depends on the temperature. With some alloys 

 the diminution of resistance with temperature is 

 very small, but with pure metals it is consider- 

 able, and roughly, at any rate, proportional to the 



