( 884 ) 
and?) by means of a liquid-air (oxygen) separator to such an extent, 
that, when passed through the hydrogen liquefier it gave a quantity 
of liquid hydrogen (although comparatively small) before the 
liquefier became choked, and this liquid, by vaporisation, gave pure 
hydrogen. When the hydrogen liquefier had again been put in a 
workable condition, the operations were repeated with a new quantity 
of gas which had undergone preliminary purification in the liquid-air 
separator, and the various yields of pure hydrogen thus obtained 
were carefully collected and united, until the required quantity was 
available. 
This method of operating was rather troublesome, and when once 
I was in possession of a sufficient quantity of pure hydrogen to 
keep the eyele in continuous action it was an obvious advantage 
to avail myself of this cycle for the purification of commercial 
hydrogen. In Suppl. N°. 19 the communication was already made 
that an apparatus was being constructed, in which the purification of 
the hydrogen was effected by means of its liquefaction, while another 
apparatus had already been constructed in which the impurities were 
frozen out of the gaseous hydrogen to be purified, by means of the 
pure liquid hydrogen of the cycle. 
The suitability of the latter apparatus has been proved by long 
use; it is represented in Pl. I and a description of it is here given. 
The chief portion of the apparatus is the spiral a,, in the lower 
end of which the liquid hydrogen is vaporised; it is placed in a 
vacuumvessel 6, which is closed by means of a cap g. The hydrogen 
which is to be purified flows through the tube c,, between the 
vacuum glass 6, and the cylinder d/,, and along the cooled spiral in 
the opposite direction to that in which the gaseous hydrogen flows 
away, which is formed by vaporisation inside the spiral. By this 
means the air contained in the hydrogen is deposited on the windings 
of the spiral. The purified hydrogen escapes through the paper 
eylinder d, the copper tubes d, and d,;, and the regenerator d,. 
The liquid hydrogen is supplied through «,, and the insulated 
tube a,a,, to the lower end of the vaporising spiral a,. To ascertain 
how much liquid hydrogen must be supplied, the temperature of 
the purified gas as it enters the cylinder d, is determined by means 
1) The air separator usually works at a pressure of 60 atm. and with a velocity 
of 2M. per hour; the impurities remaining do not then amount to more than 
1/,°/, and the quantity of liquid air used is about 2 litres per hour. In Comm. 
94 the value }/5)"/,, was given: to reach this value, however, the velocity must 
be much smaller, in order that none of the liquid that separates out should be 
carried along with the current. 
