46 Anniversary Meeting. [Nov. 30, 



grand generosity of Mr. Ludwig Mond. The Royal Society interests 

 itself in all work contributing towards the object for which it was 

 founded the increase of natural knowledge ; and while gratefully 

 remembering the assistance so generously given to it in the humble but 

 highly valuable work of cataloguing papers which describe the results 

 of scientific investigations alread v made, it hails with delight this grand 

 foundation of a practical laboratory, of which the purpose is not the 

 teaching of scientific truths already discovered, but the conquer- 

 ing of fresh provinces from the great region of the unknown in 

 Nature. 



The greatest scientific event of the past year is, to my mind, un- 

 doubtedly the discovery of a new constituent of our atmosphere. If 

 anything could add to the interest which we must all feel in this 

 startling discovery, it is the consideration of the way by which it 

 was found. In his Presidential address to Section A of the meeting 

 of the British Association at Southampton in 1882, Lord Rayleigh, 

 after calling attention to Front's law, according to which the atomic 

 weights of the chemical elements stand in simple relationship to that 

 of hydrogen, said : " Some chemists have reprobated strongly the 

 importation of d priori views into the consideration of the ques- 

 tion, and maintain that the only numbers worthy of recognition are 

 the immediate 'results of experiment. Others, more impressed by 

 the argument that the close approximations to simple numbers cannot 

 be merely fortuitous, and more alive to the inevitable imperfections 

 of our measurements, consider that the experimental evidence against 

 the simple numbers is of a very slender character, balanced, if not 

 outweighed, by the a priori argument in favour of simplicity. The 

 subject is eminently one for further experiment; and as it is now 

 engaging the attention of chemists, we may look forward to the 

 settlement of the question by the present generation. The time has, 

 perhaps, come when a re- determination of the densities of the 

 principal gases may be desirable an undertaking for which I have 

 made some preparations." The arduous work thus commenced in 

 1882, has been continued for 12 years,* by Rayleigh, with unremit- 

 ting perseverance. After 11 years of it, a first important part of 

 the object, the determination of the atomic weight of oxygen with 



* " On the relative Densities of Hydrogen and Oxygen. Preliminary Notice," 



by Lord Rayleigh, February 2, 1888. 



lt On the Composition of Water," by Lord Rayleigh, February 26, 1889. 

 " On the relative Densities of Hydrogen and Oxygen. II." By Lord 



Rayleigh, February 5, 1892. 



" On the Densities of the principal Gases," by Lord Rayleigh, March 23, 1893. 

 ' On an Anomaly encountered in Determinations of the Density of Nitrogen 



Gas," by Lord Rayleigh, April 19, 1894. 

 All published in the ' Proceedings of the Royal Society.' 



