HISTORY OF COLD AND THE ABSOLUTE ZERO. 221 



based on tlii.s princii)lo, capable of })r()diu'ino- li((ui(l air for industrial 

 purposes. These experimenters, had proved that, ovviny' to niolecuhir 

 attraction, compressed gas(es passing through a porous plug or small 

 aperture were lowered in temperature l)y an amount depending on the 

 difference of pressure and inversely as the square of the absohite tem- 

 perature. This means that for a steady difference of pressure the 

 cooling is greater the lower the temperature. The only gas that did 

 not show cooling under such conditions was hydrogen. Instead of 

 being cook'd it became actually hotter. The reason for this apparent 

 anomaly in the Kelvin-Joule effect is that every gas has a thermometric 

 point of inversion above which it is heated and lielow which it is 

 cooled. This inversion point, according to van der Waals, is six and 

 three-quarter times the critical point. The efficiency of the Linde 

 process depends on working with highly compressed gas well below 

 the inversion temperature, and in this res})ect this point may be said 

 to take the place of tiie critical one, when in the ordinary way direct 

 rK[uefaction is Ijeing effected by the use of si)eciffc liquid cooling 

 agents. The success of both processes depends upon working within 

 a certain temperature range, only the Linde method gives us a much 

 Avider range of temperature within which liijuefaction can be effected. 

 This is not the case if, instead of depending on getting cooling by the 

 internal work done ])y the attraction of the gas molecules, we force 

 the compressed gas to do external work as in the well-known air 

 machines of Kirk and Coleman. Both these inventors have pointed 

 out that there is no liujit of temperature, short of liquefaction of the 

 gas in use in the circuit, that such machines are not capable of giving. 

 While it is theoretically clear that such machines ought to be capable 

 of maintaining the lowest temperatures, and that with th(> least 

 ex})enditure of power, it is a very different matter to overcome the 

 l)ractical difficulties of working such machines under the conditions. 

 Coleman kei)t a machine didivering air at minus S?>'- for hours, but 

 he did not carry his experiments any further, Kecently M. Claude, 

 of I'aris, has, however, succeeded in working a machine of this tj])G 

 so efficiently that he has managed to produce 1 liter of li(|ui(l air per 

 horsejwwer expended per hour in the running of the engine. This 

 ()u(])ut is twice as good as that givtMi l)y thv Linde machine, and there 

 is no reason to doubt that tlie yiehl will be still furtluM' improved. It 

 is clear, therefore, that in the innncHliate t'utui'c the production of 

 liciuid air and hydrogen will b(> effected most economically by the use 

 of machines producing cold by the expenditure of mechanical work. 



LiyUH> HYDROCEN AND HELILTM. 



To the physicist the copious ])ro(luction of li(|uid aii- by the methods 

 described was of peculiar intei'cst and value as affording the means of 

 attacking the far iilore dithcult |)roblem of the liquefaction of hydro- 

 gen, and even as encouraging th(> hope that liquid hydrogen might in 



