210 PRESENT PROBLEMS OF INORGANIC CHEMISTRY. 



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

 and Richards has recently confirmed his resuks, while Rayleigh and 

 Lednc consistently obtained densities which, even w^hen corrected so 

 as to equalize the numbei's of molecules in equal volumes, give the 

 lower figure 14.002. The diii'erence 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 

 preparation of pure compounds of nitrogen, such as salts of hydra- 

 zine, 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 jjeroxide. I have just 

 heard from my former student, W. R. W. Gray, that he has recovered 

 Stas's number l^y combining 2N0 Avith O., AA'hile the density of NO 

 leads to the lower value for the atomic weight of nitrogen. 



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 iodine is concerned. Recent determina- 

 tions give the figures 127.5 (Gutbier), 127.6 (Pellini), and 127.9 

 (Kothner). But is that of iodine as accurately known? It would 

 appear advisable to revise the determination of Stas, preparing the 

 iodine preferably from an organic compound, such as iodoform, 

 Avhich can be produced in a high state of purity. The heteromor- 

 phism of selenates and telhirates, 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. A^Hiat 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 the 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 Ave are 

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

 change of thorium into thorium X is perhaps hardly an explanation 

 of the divergencies; yet it must be considered; but of this, more 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 haA^e been made are specula- 

 tive rather than based on experiment. The existence of many peroxi- 

 dized substances, such as percarbonates, perborates, persulphates and 

 of crystalline compounds of salts Avith hydrogen peroxide, makes it 

 difficult to draw any indisputable conclusions as regards valency from 



