1894.] President's Address. 47 



all possible accuracy was attained by the comparison,* of Scott's 

 determination of the ratio of the volumes of hydrogen and oxygen 

 in the constitution of water, with "Rayleigh's determination of the 

 ratio of the densities. The result was 15 - 82, which is almost 1 per 

 cent. (O87 per cent.) less than the 16, which it would be according 

 to Prout's law. It is very slightly less (J per cent.) than Dittmar 

 and Henderson's value obtained by an investigationf for which 

 the Graham medal of the Glasgow Philosophical Society was 

 awarded in 1890. Values, not quite so small as these for the atomic 

 weight of oxygen, had been previously found by Cooke and Richards 

 (15-869) and by Leduc (15-876). There can be no doubt whatever 

 now that the true value is more than -J per cent, smaller than accord - 

 ing to Prout's law, and that in all probability it agrees exceedingly 

 closely with the results obtained by Rayleigh and Scott, and by 

 Dittmar and Henderson. The question of Prout's law being thus so 

 far set at rest, Rayleigh. persevering in the main object which he had 

 promised in 1882, "a redetermination of the densities of the principal 

 gases," attacked nitrogen resolutely and, stimulated by most disturb- 

 ing and unexpected difficulties in the way of obtaining concordant 

 results for the density of this gas as obtained from different sources, 

 discovered that the gas left by taking vapour of water, carbonic 

 acid, and oxygen from common air was denser J by 1/230 than 

 nitrogen obtained by chemical processes from nitric oxide or from 

 nitrous oxide, or from ammonium nitrite, thereby rendering it 

 probable that atmospheric air is a mixture of nitrogen and a small 

 proportion of some unknown and heavier gas. Rayleigh, and Ramsay 

 who happily joined in the work at this stage, have since succeeded in 

 isolating the new gas, both by removing nitrogen from common air 

 by Cavendish's old process of passing electric sparks through it, 

 and taking away the nitrous compounds thus produced by alkaline 

 liquor; and by absorption by metallic magnesium. Thus we have a 

 fresh and most interesting verification of a statement which I took 

 occasion to make in my Presidential address to the British Associa- 

 tion in 1871, "Accurate and minute measurement seems to the non- 

 scientific imagination a less lofty and dignified work than looking for 

 something new. But nearly all the grandest discoveries of science 

 have been but the rewards of accurate measurement and patient 

 long-continued labour in the minute sifting of numerical results." 

 The investigation is now being carried on vigorously, and has already 



* Scott, " On the Composition of Water by Volume," communicated by Lord 

 Kayleigh, 'Eoy. Soc. Proc.,' March 23, 1893. 



f ' Proceedings of the Philosophical Society of Glasgow/ 18901891. 



J "On an Anomaly encountered in Determinations of the Density of Nitrogen 

 Gas," ' Koy. Soc. Proc.,' April, 1894. 



Eepublished in Volume 2 of ' Popular Lectures and Addresses.' 



