Liquefaction of Hydrogen. 421 



below —200° C. By closing the cock v, removing the rubber 

 cork, and opening the stopcock V, the liquid air flows out of H. 

 The rubber cap securing V is now removed, the tube L and 

 the vacuum- vessels M and K are replaced in position, and the 

 rubber sleeve s is secured to L with a single turn of copper 

 wire. 



Meanwhile the assistant in charge of the compressor has- 

 removed all air from the compression-apparatus by opening 

 the cock a (text-fig. 1), allowing the compressor to make a few 

 revolutions and then stopping it and opening the cock p. 

 This operation is repeated three or four times; the pressure 

 is then allowed to rise, the valve a being closed, and the gas 

 is allowed to escape, if necessary, through the cock ,i' into the 

 pipe Ni The arrangements are made so that the pressure 

 is raised to 200 atmospheres by the time the liquefier has been 

 cooled and the vacuum-vessels K, &c, replaced. 



The remaining operations may be shortly described : — 

 Communication is once more established between the 

 chamber C and the exhaust-pump, and the valve b is carefully 

 regulated so that the liquid air does not enter too fast ; a too 

 rapid flow is at once indicated by a fall of the mercury in the 

 vacuum-gauge. The expansion-valve is then opened by 

 turning the milled nut d, and the gas, passing upwards 

 through the coils, through the annular space F, through the 

 tubes G, w, and R, finds its way into P, and is allowed for a 

 few moments, in order to remove air from the apparatus, to 

 escape through the water in the tank e. The cock i is then 

 opened, and the gas is allowed to circulate through the system. 

 The chamber B and the vessel containing the coil A are 

 continually replenished with liquid air and solid carbonic acid 

 respectively. 



The whole difficulty in this part of the experiment lies in 

 properly regulating the escape of the hydrogen. The rate of 

 flow of the gas is roughly indicated by the height of the 

 glycerol in the gauge z, which shows the pressure in the 

 interior of the apparatus caused by the friction of the gas in 

 the tubes Gr, w, and P. It is intended in future experiments 

 to introduce in place of w a coil of lead pipe, and to connect 

 the top of the glycerol gauge with a tube leading into the 

 cylinder P, as it will then give an absolute reading of the rate 

 of flow of the gas. 



The reasons for which it is necessary to carefully regulate 

 the valve are twofold. Firstly, the hydrogen must not pass 

 too quickly through the refrigerating-coils else the gas is 

 insufficiently cooled; secondly, since liquid hydrogen has a 

 very low specific gravity, the gas and liquid do not separate 



