2 Prof. R. Threlfall on the Electrical 



repetition of the experiments with a view to testing our con- 

 clusions by an extended examination of the phenomena in 

 question. This investigation has proved a most laborious 

 task; and it was not until December 1891 that I finally 

 satisfied myself that the effect was due to the combination of 

 nitrogen with the mercury of the pressure-gauge under the 

 influence of the electric discharge, and in the presence, as I 

 think, of a minute trace of some other substance whose nature 

 I have not been able to determine. 



The experiments to be described leave little doubt that pure 

 nitrogen at ordinary temperatures does not condense in the 

 same way that oxygen condenses, whether the discharge be 

 by means of external electrodes or by means of wires fused 

 into the tube. Since these experiments form the starting- 

 point of other researches, I will begin by an account of a 

 method by which pure nitrogen may be most advantageously 

 prepared. Singularly enough this apparently simple chemical 

 problem has not, so far as I know, been solved hitherto. Stas, 

 in his researches, makes the remark that nitrogen is easily 

 obtained pure (Bulletin de V Academie Roy ale des Sciences de 

 Belgique, 1860, ser. 2, t. x. p. 254); and this remark may 

 very possibly have produced misunderstanding — everything 

 depends on what is meant by the word pure. The fundamental 

 principles in all processes of purification are : — 1st, that 

 reagents used to remove any impurity must really keep the 

 impurity when they have got it, and not liberate it or its 

 equivalent through any instability of the compounds funned ; 

 and 2nd, that the reagents themselves must not give rise to 

 impurities. As an example of a common violation of the first 

 principle, 1 will mention the absorption of sulphuretted hydro- 

 gen, sulphur dioxide, or the oxides of nitrogen, by potash or 

 soda ; and of the second, the use of imperfectly prepared phos- 

 phorus pentoxide. Trouble arising from layers of air or 

 other gases condensed on the surfaces of glass vessels can 

 only be relieved by arranging the method of production of 

 the gas required in such a way that the glass surfaces may 

 remain for weeks or months in contact with the gas under 

 experiment, otherwise completely purified. If mercury be in 

 contact with glass in any part of the apparatus, then, as I 

 believe, it is impossible by ordinary means to be sure that the 

 glass is ever completely denuded of its primitive layers of gas. 

 I append a list of some of the various ways in which nitrogen 

 has been prepared for careful work, and shall, I think, be able 

 to show that all these processes are open to objection, at all 

 events where a continuous supply of gas is necessary. The 

 list is short, because nearly all experimenters simply state that 



