1895.] on Argon. 527 



lightness in the chemically-derived nitrogenjbeing due to dissociation, 

 and to suggest strongly, as almost the only jDossible alternative, that 

 there must be in atmospheric nitrogen some constituent heavier than 

 true nitrogen. 



At that point the question arose, what was the evidence that all 

 the so-called nitrogen of the atmosphere was of one quality ? And I 

 remember — I think it was about this time last year, or a little earlier 

 — putting the question to my colleague, Professor Dewar. His 

 answer was that he doubted whether anything material had been 

 done upon the matter since the time of Cavendish, and that I had 

 better refer to Cavendish's original paper. That advice I quickly 

 followed, and I was rather surprised to find that Cavendish had him- 

 self put this question quite as sharply as I could put it. Translated 

 from the old-fashioned phraseology connected with the theory of 

 phlogiston, his question was whether the inert ingredient of the air 

 is really all of one kind ; whether all the nitrogen of the air is really 

 the same as the nitrogen of nitre. Cavendish not only asked himself 

 this question, but he endeavoured to answer it by an appeal to 

 experiment. 



I should like to show you Cavendish's experiment in something 

 like its original form. He inverted a U tube filled with mercury, 

 the legs standing in two separate mercury cups. He then passed up, 

 so as to stand above the mercury, a mixture of nitrogen, or of air, 

 and oxygen ; and he caused an electric current from a frictional 

 electrical machine like the one I have before me to jjass from the 

 mercury in the one leg to the mercury in the other, giving sparks 

 across the intervening column of air. I do not propose to use a 

 frictional machine to-night, but I will substitute for it one giving 

 electricity of the same quality of the construction introduced by 

 Mr. Wimshurst, of which we have a fine specimen in the Institution. 

 It stands just outside the door of the theatre, and will supply an 

 electric current along insulated wires, leading to the mercury cups ; 

 and, if we are successful, we shall cause sparks to pass through the 

 small length of air included above the columns of mercury. There 

 they are ; and after a little time you will notice that the mercury 

 rises, indicating that the gas is sensibly absorbed under the influence 

 of the sparks and of a piece of potash floating on the mercury. It 

 was by that means that Cavendish established his great discovery 

 of the nature of the inert ingredient in the atmosphere, which we 

 now call nitrogen ; and, as I have said, Cavendish himself proposed 

 the question, as distinctly as we can do, is this inert ingredient all 

 of one kind? and he proceeded to test that question. He found, 

 after days and weeks of protracted experiment, that, for the most 

 part, the nitrogen of the atmosphere was absorbed in this manner, 

 and converted into nitrous acid ; but that there was a small residue 

 remaining after prolonged treatment with sparks, and a final absorp- 

 tion of the residual oxygen. That residue amounted to about yl^ 

 part of the nitrogen taken; and Cavendish draws the conclusion 



