NOTES 289 



details of an object and the wave-length of the light by which 

 that object is seen. The article " Optics " in the ninth edition 

 of the Encyclopedia Britannica, which appeared in the year 1875, 

 is by him, and in it will be found much of his optical work. 



The Cambridge period came to an end in 1884, and Lord 

 Rayleigh returned to Terling, but previously to this, in an 

 address in 1882 as President of the Mathematical and Physical 

 Section of the British Association, he had indicated his next line 

 of work. 



He wrote : " The time has perhaps come when a redetermina- 

 tion of the densities of the principal gases may be desirable — 

 an undertaking for which I have made some preparations." 

 The discovery of Argon was heralded in 1892 by a letter 

 to Nature which begins : "I am much puzzled by some 

 recent results as to the density of Nitrogen " — he had pub- 

 lished in 1887 and 1889 papers on the densities of hydrogen and 

 oxygen, and the composition of water — " and shall be much 

 obliged if any of your chemical readers can offer suggestions as 

 to the cause. According to two methods of preparation I 

 obtain quite distinct values. The relative difference, amount- 

 ing to about one-thousandth part, is small in itself " — the 

 difference ultimately found was one-two-hundredth — " but it 

 lies entirely outside the errors of experiment, and can only be 

 attributed to a variation in the character of the gas/- 



"Is it possible," he concludes, " that the difference is 

 independent of impurity, the nitrogen being in a different 

 (dissociated) state ? " 



Next year his paper on the densities of the principal gases 

 was communicated to the Royal Society, in which it was con- 

 clusively shown that chemically prepared nitrogen was lighter 

 than that obtained from the atmosphere. Atmospheric nitro- 

 gen contained some unknown impurity, and Lord Rayleigh 

 undertook to separate out the nitrogen from the air, and 

 determine the nature of this unknown substance. This he did 

 by effecting the combination of the nitrogen by the electric 

 discharge, while Sir Wm. Ramsay, who at this stage had become 

 associated with him in the work, accomplished the same result 

 by passing air over heated magnesium, with the result that at 

 the Oxford Meeting of the British Association in 1 896 they were 

 able to announce that the atmosphere contains about one- 

 half per cent, of a new gas — Argon. Rayleigh had started 



