36 POPULAR SCIENCE MONTHLY. 



vance knowledge materially. I would suggest the preparation 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, W. 

 E. W. Gray, that he has recovered Stas's number by combining 2N"0 

 with 0,; while 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 determinations 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, 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 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 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 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 have 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 with hydrogen peroxide, makes it dif- 

 ficult to draw any indisputable conclusions as regards valency from a 

 consideration of oxygen compounds. Moissan's brilliant work on 

 fluorides, however, has shown that SF 6 is capable of stable existence, 

 and this forms a strong argument in support of the hexad character of 

 sulphur. The tetravalency of oxygen, under befitting conditions, too, 

 is being acknowledged, and this may be reconciled with the existence 

 of water of crystallization, as well as of the per-salts already men- 

 tioned. The adherence of ammonia to many chlorides, nitrates, etc., 

 points to the connecting link being ascribable to the pentavalency of 

 nitrogen ; and it might be worth while investigating similar compounds 



