PRESENT PROBLEMS 261 



sibly likely to throw light on the question. First, there is a striking 

 anomaly in the atomic weight of nitrogen, determined by analysis 

 and determined by density. Stas obtained the number 14.04 (0 = 16), 

 and Richards has recently confirmed his results; while Rayleigh 

 and Leduc consistently obtained densities which, even when corrected 

 so' as to equalize the numbers of molecules in equal volumes, give 

 the lower figure 14.002. The difference is 1 in 350; far beyond any pos- 

 sible experimental error. Recently, an attempt to combine the two 

 methods has led to a mean number; but that result can hardly be 

 taken as final. What is the reason of the discrepancy? Its discovery 

 will surely advance knowledge materially. I would suggest the pre- 

 paration of pure compounds of nitrogen, such as salts of hydrazine, 

 methylanine, etc., and their careful analysis; and also the accurate 

 determination of the density and analysis of such gaseous compounds 

 of nitrogen as nitric oxide and peroxide. I have just heard from my 

 former student, R. W. Gray, that he has recovered Stas's number 

 by combining 2NO with 2 ; while the density of NO leads to the 

 lower value for the atomic weight of nitrogen. 1 



The question of the atomic weight of tellurium appears to be set- 

 tled, at least so far as its position with regard to the generally accepted 

 atomic weight of iodin is concerned; recent determinations give the 

 figures 127.5 (Gutbier), 127.6 (Pellini), and 127.9 (Kothner). But 

 is that of iodin as accurately known? It would appear advisable to 

 revise the determination of Stas, preparing the iodin preferably from 

 an organic compound, such as iodoform, which can be produced in a 

 high state of purity. The heteromorphism of selenates and tellurates, 

 too, has recently been demonstrated; and it may be questioned 

 whether these elements should both belong to the same group. 



The rare earths still remain a puzzle. Their number is increas- 

 ing yearly, and their claim to individuality admits of less and less 

 dispute. What is to be done with them? Are they to be grouped by 

 themselves as Brauner and Steele propose? If so, how is their con- 

 nection with other elements to be explained? Recent experiments 

 in my laboratory have convinced me that in the case of thorium, at 

 least, ordinary tests of purity such as fine crystals, constant subliming 

 point, etc., do not always indicate homogeneity; or else that we are 

 sadly in want of some analytical method of sufficient accuracy. The 

 change of thorium into thorium X is perhaps hardly an explanation of 

 the divergences; yet it must be considered; but of this, anon. 



To turn next to another problem closely related to the orderly 

 arrangement of the elements, that of valency, but little progress 

 can be chronicled. The suggestions which have been made are specu- 



1 Note added February, 1906: Researches by Gray and by Guye have since 

 shown that Stas's results are in error; and determinations by Richards allow 

 the same conclusion to be drawn. The actual atomic weight cannot differ much 

 from 14.007. 



