APPENDIX II. 489 



the peroxide of uranium, U0 4 , prepared by Fairley ; 14 while Os0 4 is the 

 highest oxide giving salts. The line of argument which is inspired by the 

 periodic law, so far from being weakened by the discovery of peroxides, is 

 thus actually strengthened, and we must hope that a further exploration of 

 the region under consideration will confirm the applicability to chemistry 

 generally of the principles deduced from the periodic law. 



Permit me now to conclude my rapid sketch of the oxygen compounds by 

 the observation that the periodic law is especially brought into evidence hi 

 the case of the oxides which constitute the immense majority of bodies at our 

 disposal on the surface of the earth. 



The oxides are evidently subject to the law, both as regards their chemical 

 and their physical properties, especially if we take into account the cases of 

 polymerism which are so obvious when comparing C0 2 with Si0 2n . In order 

 to prove this I give the densities s and the specific volumes v of the higher 

 oxides of two short periods. To render comparison easier, the oxides are all 

 represented as of the form E. 2 n . In the column headed A the differences 

 are given between the volume of the oxygen compound and that of the parent 

 element, divided by n that is, by the number of atoms of oxygen in the 

 Compound : 15 



I have nothing to add to these figures, except that like relations appear in 

 other periods as well. The above relations were precisely those which made 

 it possible for me to be certain that the relative density of ekasilicon oxide 

 would be about 4-7 ; germanium oxide, actually obtained by Winkler, proved, 

 m fact, to have the relative density 4-703. 



The foregoing account is far from being an exhaustive one of all that has 

 already been discovered by means of the periodic law telescope in the bound- 

 less realms of chemical evolution. Still less is it an exhaustive account of all 

 that may yet be seen, but I trust that the little which I have said will account 



14 The compounds of uranium prepared by Fairley seem to me especially instructive 

 in understanding the peroxides. By the action jof hydrogen peroxide on uranium oxide, 

 TJO 3 , a peroxide of uranium, UO 4 ,4H 2 O, is obtained (U = 240) if the solution be .acid; but 

 if hydrogen peroxide act on uranium oxide in the presence of caustic soda, a crystalline 

 deposit is obtained which has the composition Na4UO8,4H 2 O, and evidently is a combina- 

 tion of sodium peroxide, Na^O^ with uranium peroxide, UO^ It is possible that the 

 former peroxide, UO 4 ,4H 2 O, contains the elements of hydrogen peroxide and uranium 

 peroxide, XJ 2 O 7 , or even U(OH) 6 ,H 2 O 2 , like the peroxide of tin recently discovered by 

 Spring, which has the constitution Sn 2 6 ,H2O 2 . 



15 A thus represents the average increase of volume for each atom of oxygen con- 

 tained in the higher salt-forming oxide. The acid oxides give, as a rule, a higher value 

 of A, while in the case of the strongly alkaline oxides its value is usually negative. 



