MOUNT WILSON OBSERVATORY. 241 



ments whose ionization potentials can be investigated under laboratory 

 conditions. For any element the changes depend upon the degree 

 of ionization over the photosphere and the susceptibility to excitation, 

 and these, moreover, vary for the different lines. 



The identification of the enhanced line of magnesium, X4481, 

 in the solar spectrum seems to be definitely settled. King discovered 

 its duplicity in the tube-arc spectrum, and Fowler has accurately de- 

 termined the wave-lengths of the components. The two weak solar 

 lines are much strengthened over faculse and weakened in spots. They 

 are therefore enhanced lines. Solar spectrograms of high dispersion 

 yield the same separation and relative intensity as the best laboratory 

 plates. The solar wave-lengths are 0.007 a longer than the terrestrial 

 wave-lengths. The identification is as certain as for any other solar 

 line. 



OXYGEN IN PLANETARY ATMOSPHERES. 



The absence of any evidence that oxygen is present in the atmosphere 

 of Venus seems surprising, in view of the abundance of oxygen in the 

 earth's atmosphere, but it falls into line with several other facts pointed 

 out b}^ Russell. 



(1) The oxygen triplet at X7772-75 is practically extinguished in 

 the spot-spectrum, though the lines of titanium and the bands of tita- 

 nium oxide are strong in spots. 



(2) The igneous rocks of the earth's crust contain so much ferrous 

 oxide that the whole of the free oxygen of the atmosphere would not 

 nearly suffice to convert it into ferric oxide. 



(3) Volcanic gases appear, on the whole, to contain a good deal of 

 unoxidized material (carbon monoxide, free sulphur, etc.). 



All these facts can be explained on the assumption that the outer 

 layers of the sun (from which it now appears probable that the planets 

 were formed by eruptions at some remote period) do not contain enough 

 ox3^gen to combine with all the metallic elements present. A planet 

 in its initial state would then have an incompletely oxidized surface 

 and an atmosphere devoid of free oxygen. 



The oxygen of the earth's atmosphere may be an organic product, 

 given off by vegetation during geological time, and balanced by an 

 accumulation of carbonaceous organic residues in the sedunentary 

 rocks. The latter would correspond to a layer of coal covering the 

 earth's surface about 2 feet thick, which does not appear to be an im- 

 possible amount. 



SOLAR ROTATION. 



The observations by Mr. St. John now cover 7 years. The mean 

 value of the linear velocity at the equator is 1.93 km. per second, with 

 small and irregular variations from year to year. For the year of spot 

 maximum, 1917, the value is about 1.5 per cent above the mean. Lit- 

 tle significance, however, can be attributed to this small deviation for 



