THE LIQUEFACTION OF GASES 57 



measurably small, and does not altogether exceed 

 the four-hundredth part of the argon present. 



The spectrum of the sun shows some lines which 

 do not coincide with those of any chemical element 

 in conditions usually known on the earth. Among 

 these lines many are due to terrestrial elements 

 in solar circumstances, but a bright line in the 

 yellow part was detected in the spectrum of a 

 solar prominence, and was examined carefully 

 by Frankland and Lockyer during the eclipse of 

 August 1868. To explain its presence they called 

 into existence a hypothetical element, placed it in 

 the sun, and gave to it the name helium. For 

 many years the line in the sun's spectrum was 

 the only evidence for the existence of helium ; 

 but in 1895 its presence on the earth was an- 

 nounced by Ramsay, who had detected it in the 

 spectroscopic analysis of the gases dissolved in 

 the mineral clevite, together with the other new 

 gases krypton and neon. Since this discovery, 

 helium has been isolated and collected in appreci- 

 able quantities, and its physical and chemical 

 properties are now well known. Of all substances 

 investigated, helium has proved the most difficult to 

 liquefy. But in July 1908, Professor Kamerlingh 

 Onnes, of Leyden, obtained liquid helium for the 

 first time by the use of a regenerative apparatus 

 and a plentiful supply of liquid hydrogen. 



It will be seen from the foregoing account that 

 the difficulty of obtaining these low temperatures 

 is very great. While a temperature of many 

 hundred degrees above the freezing-point of 

 water is easily reached in a common fire or gas 

 flame, to cool hydrogen to 250° below that point 



