873 



was given to the removal of nitric oxide (glowing copper) ; for 

 further details we refer to .a paper which will appear elsewhere. 



In this case, as well as in that of oxygen, the purity of the gas 

 was proved inter alia by the equality of the vapour-pressures obtained 

 at the beginning and at the end of condensation. 



e. Arc/on. 



For this gas we could start from the strongly argonous gas- 

 mixtures which the firm of Linde has recently brought into the. 

 market The final purification was eft'ected by means of Hempel's 

 mixture ') in a manner similar to that given by Crommej.in '). The 

 only modification which we applied in our apparatus conbisted in 

 each tube containing chemical substances wiiich might develop im- 

 purities, sucli as water-vapour etc., or conversely might react with 

 them, being flanked at each end by cooling tubes immersed in 

 liquid oxygen. Care was taken, moreover, that during the complete 

 circulation-process the gas should be at a higher pressure tlian the 

 atmosphere throughout the whole apparatus. 



Again in this case the final product was found to satisfy the test 

 of equal [)i'essures at the beginning and the end of condeusalion. 



d. I'he mixtures. 



The mixtures were prepared by adding nitrogen to Linde's argon- 

 nitrogen mixtures after these had been freed from oxygen by means 

 of glowing copper. The nitrogen had been obtained from air by 

 liberation from oxygen. We gladly acknowledge our indebtedness 

 to Mr. H. Fir.ippo Jzn. for his kind collaboration in this part of our 

 work. In a few cases use was made of the method of diminishing 

 the percentage of niti-ogen of Linde's mixtures (down to about 57o) 

 by means of a fraction-apparatus constructed by Mr. P'itjppo. 



2. Test of purity. 



The gases and mixtures were tested for the following impurities 

 ov. if necessary, simultaneously freed from theni. 



a. Water-vapour and carbon diomde were removed from the gases 

 wiiich were kept above water, freed from air by boiling, by passing 

 them previously to the measurements through a couple of cooling 

 tubes immersed in liquid oxygen. 



b. Hjidrocarbons. It was found that these were not present: a 

 thin spiral wire of tungsten which was made to glow in the gas 

 mixture was found not to change in resistance "). 



!■) W. M. Hempel. Gasanal. Methoden, 3nd edition p. 151. 



-) G. A. Grommelin. Dissert. Leiden 1910. 



'j Gonip. L. Hamburger, Ghem. Weekbl. 12, (1915) 62. 



56* 



