524 Lord Rayleigh [April 5, 



WEEKLY EVENING MEETING, 



Friday, April 5, 1895. 



Sir Frederick Bramwell, Bart. D.C.L. LL.D. F.R.S. Honorary- 

 Secretary and Vice-President, in the Chair. 



The Eight Hon. Lord Rayleigh, M.A. D.C.L. LL D. F.R.S. 

 Professor of Natural Philosophy, R.I. 



Argon. 



It is some three or four years since I had the honour of lecturing 

 here one Friday evening upon the densities of oxygen and hydrogen 

 gases, and upon the conclusions that might be drawn from the results. 

 It is not necessary, therefore, that I should trouble you to-night 

 with any detail as to the method by which gases can be accu- 

 rately weighed. I must take that as known, merely mentioning 

 that it is substantially the same as is used by all investigators 

 nowadays, and introduced more than fifty years ago by Regnault. 

 It was not until after that lecture that I turned my attention to 

 nitrogen ; and in the first instance I employed a method of 

 preparing the gas which originated with Mr. Vernon Harcourt, of 

 Oxford. In this method the oxygen of ordinary atmospheric air is 

 got rid of with the aid of ammonia. Air is bubbled through liquid 

 ammonia, and then passed through a red-hot tube. In its passage 

 the oxygen of the air combines with the hydrogen of the ammonia, 

 all the oxygen being in that way burnt up and converted into water. 

 The excess of ammonia is subsequently absorbed with acid, and the 

 water by ordinary desiccating agents. That method is very con- 

 venient ; and, when 1 had obtained a few concordant results by 

 means of it, I thought that the work was complete, and that the 

 weight of nitrogen was satisfactorily determined. But then I 

 reflected that it is always advisable to employ more than one method, 

 and that the method that I had used — Mr. Vernon Harcourt's method 

 — was not that which had been used by any of those who had pre- 

 ceded me in weighing nitrogen. The usual method consists in 

 absorbing the oxygen of air by means of red-hot copper ; and I 

 thought that I ought at least to give that method a trial, fully 

 expecting to obtain forthwith a value in harmony with that already 

 afforded by the ammonia method. The result, however, proved 

 otherwise. The gas obtained by the copper method, as I may call it, 

 proved to be one- thousandth part heavier than that obtained by the 

 ammonia method; and, on repetition, that difference was only 

 brought out more clearly. This was about three years ago. Then, 

 in order, if possible, to get further light upon a discrepancy which 



