260 Sir James Deicar [Jan. 20, 



is plainly shown when they are made to boil by exhaustion in 

 jacketed glass tubes (Fig. 3) connected to a powerful pump. The 

 nitrogen and hydrogen turn into snowy solids ; but the oxygen 

 remains liquid. 



It seems strange that while liquid hydrogen or nitrogen can 

 be thus easily changed into the solid condition by evaporation under 

 exhaustion, yet the ordinary air-pump fails to effect the transition of 

 state in the case of liquid oxygen. This is due to the small vapour- 

 pressure of solid oxygen at its melting point, as compared with 

 hydrogen or nitrogen under similar conditions — together with the 

 greater need of very perfect heat-isolation in the arrangement of 

 the experiment. With the aid of charcoal as a gaseous condensing 

 agent at low temperatures, combined with the employment of proper 

 vacuum vessels, the change from the liquid into the solid can 

 be effected. Pure liquid oxygen, contained in a properly isolated 

 vessel, subjected to the exhaust produced by a quantity of charcoal 

 kept at about the temperature of boiling oxygen, has its pressure 

 lowered sufficiently to produce solidification to a transparent jelly. 

 The pressure at which solidification takes place is determined by 

 connecting a McLeod gauge to the vessel containing the soUd oxygen. 



About 75 grm. of good coconut charcoal is necessary to produce 

 and maintain the necessary conditions of exhaustion. From 5 to 

 10 c.cm. of liquid oxygen are employed, previously exhausted by 

 an air-pump. The oxygen exhibits considerable supercooling, and 

 a pressure of less than half the melting pressure can usually be 

 maintained on the liquid without producing solidification. These 

 experiments indicate that the melting-point pressure is between 

 1*115 and 1*125 mm. 



Fig. J: shows the arrangement of the apparatus. Fig. 5 shows a 

 modification, in which the oxygen is condensed in a bulb immersed in 

 the solid oxygen produced by the charcoal exhaust. 



In Fig. 4, A is a silvered vacuum-jacketed tube, with an uncoated 

 slit, containing the oxygen to be exhausted. This is further isolated 

 by a larger vacuum tube B, also silvered, and with a clear slit as 

 above, which contains Hquid air under an exhaust of less than 

 20 mm. A is sealed by the tube E on to a T-piece, whose limbs 

 connect (1) through a constricted portion to bulbs containing per- 

 manganate of potash crystals and phosphoric anhydride respectively, 

 (2) to a three-way cock C, to the arms of which are sealed {a) 

 the bulb D, containing 75 grm. of coconut charcoal [h) a T-piece 

 connecting to the mercury-pump stopcock and the McLeod gauge. 

 As soon as the charcoal vacuum is turned off the oxygen jelly 

 melts, to solidify once more when the vacuum is again turned on. 

 The reason of the peculiar behaviour of the oxygen will be under- 

 stood from the following table, giving the melting-point pressures 



