NOTES ON ATOMIC WEIGHTS. 209 



3,000,000. These simple experiments give us some idea 

 as to how hard it is to obtain even very simple compounds 

 in a state of absolute purity. Having thus laid the founda- 

 tions for his further work, and shown that the combining 

 proportions of elements are mathematically exact, Stas con- 

 sidered no labour too great if thereby he could obtain more 

 accurate values for these proportions. Any work done 

 since his determinations has only tended to uphold his 

 values and to increase our admiration for his work. 



The great value of very accurate experimental work 

 has been most strikingly exemplified by Lord Rayleigh's 

 determinations of the density of nitrogen (6). He 

 found that the nitrogen which he could obtain from air 

 alone by removing the oxygen was very little denser, but 

 was always denser than that prepared from the air with the 

 aid of ammonia by Harcourt's method, and that the nitro- 

 gen prepared from ammonia or from any compound had 

 always the same density, and that this was still lighter than 

 that partly from air and partly from ammonia. From this 

 he concluded that besides nitrogen the atmosphere must 

 contain another constituent still denser, which like nitrogen 

 resisted the action of iron and copper as well as their oxides, 

 even when very strongly heated. By combining the 

 nitrogen with oxygen after the method of Cavendish, or by 

 causing the nitrogen to unite with metallic magnesium, a 

 new gas to which the name of argon has been given was 

 finally separated by Rayleigh and Ramsay after much 

 laborious work. The detection in the atmosphere of a 

 constituent hitherto unsuspected as well as its isolation are 

 apparently only the first fruits of a number of more or less 

 startling discoveries Mowing directly from Lord Rayleigh's 

 very accurate work. The molecular weights of argon 

 (7) and helium (8) are respectively 40 and 4, and if their 

 molecules are monatomic this would give us the same 

 numbers for their atomic weights, but if the molecules are 

 diatomic, as is probable, these numbers would be halved for 

 the atomic weights. It is far from certain that either what 

 we call argon or what we call helium is not a mixture of 

 several similar substances. 



