[22] The volume of oxygen (judging by the table on p. 36) is evidently a variable quantity, forming a distinctly periodic function of the atomic weight and type of the oxide, and therefore the efforts which were formerly made to find the volume of the atom of oxygen in the volumes of its compounds may be considered to be futile. But since a distinct contraction takes place in the formation of oxides, and the volume of an oxide is frequently less than the volume in the free state of the element contained in it, it might be surmised that the volume of oxygen in a free state is about 15, and therefore the specific gravity of solid oxygen in a free state would be about O·9. S M/_s_ Volume of Oxygen H_{2}O 1·0 18 ?- 22 Li_{2}O 2·0 15 - 9 Be_{2}O_{2} 3·06 16 + 2·6 B_{2}O_{3} 1·8 39 + 10·0 C_{2}O_{4} 1·6 55 + 10·6 N_{2}O_{5} 1·64 66 ?+ 4 Na_{2}O 2·6 24 - 22 Mg_{2}O_{2} 3·5 23 - 4·5 Al_{2}O_{3} 4·0 26 + 1·3 Si_{2}O_{4} 2·65 45 + 5·2 P_{2}O_{5} 2·39 59 + 6·2 S_{2}O_{6} 1·96 82 + 8·7 Cl_{2}O_{7} ?1·92 95 + 6 K_{2}O 2·7 35 - 35 Ca_{2}O_{2} 3·25 34 - 8 Sc_{2}O_{3} 3·86 35 ? 0 Ti_{2}O_{4} 4·2 38 + 3 V_{2}O_{5} 3·49 52 + 6·7 Cr_{2}O_{6} 2·74 73 + 9·5 Cu_{2}O 5·9 24 + 9·6 Zn_{2}O_{2} 5·7 23 + 4·8 Ga_{2}O_{3} ?5·1 36 + 4 Ge_{2}O_{4} 4·7 44 + 4·5 As_{2}O_{5} 4·1 56 + 6·0 Sr_{2}O_{2} 4·7 44 - 13 Y_{2}O_{3} 5·0 45 ?- 2 Zr_{2}O_{4} 5·5 44 0 Nb_{2}O_{5} 4·7 57 + 6 MoO_{6} 4·4 65 + 6·8 Ag_{2}O 7·5 31 + 11 Cd_{2}O_{3} 8·0 32 + 3 In_{2}O_{3} 7·18 38 + 2·7 Sn_{2}O_{4} 7·O 43 + 2·7 Sb_{2}O_{5} 6·5 49 + 2·6 TeO_{6} 5·1 68 + 4·7