544 Scientific Intelligence. 



case of the mercury lamp spectrum. It is thus evident that 

 the air near the surface of the earth is far more transparent 

 than the upper atmosphere to ultra-violet rays, when equal 

 masses are considered. Since the more refrangible limit of the 

 solar spectrum is known to be due to ozone, it follows that there 

 must be much more ozone in the upper air than in the lower. 



By timing the exposures, for long and short distances, so 

 as to give about the same photographic density in the green 

 and yellow regions of the spectrum, it was found that the 

 ultra-violet impressions fell off very rapidly in intensity as 

 the thickness of air increased. Strutt gives numerical data and 

 computations to show that this rapid decrease in intensity 

 cannot be due to the absorption of pure air, but that it may 

 be caused by the scattering of light by suspended particles 

 having diameters large compared to those of molecules but 

 small with respect to the wave-lengths concerned. An alternative 

 cause would be a small amount of ozone in the lower atmosphere. 

 Spectrograms were accordingly taken by passing the light 

 through a tube, 18 mm. long, containing calculable percentages 

 of pure ozone. "We may conclude then that even if the low 

 intensity of A 2536 in the long distance spectrum were wholly 

 due to ozone absorption, it would be accounted for by less than 

 0-27 mm. of ozone in 4 miles of air. We have already seen 

 that it is quite probable that an effect equivalent to 0*26 mm. 

 ozone is really due to atmospheric scattering. The close agree- 

 ment of the two figures is, no doubt, largely accidental, but 

 still, allowing for the somewhat uncertain deduction to be made 

 for scattering, it cannot be said that any undoubted effect 

 remains to be attributed to ozone absorption. In any case it 

 is certain that the ozone cannot exceed 0-27 mm." — Proc. Boy. 

 Soc, 94 A, 260, 1918. h. s. u. 



7. Molecular Frequency and Molecular 'Number. — The idea 

 of "atomic number" has been generalized by H. Stanley 

 Allen to apply to chemical compounds. He has introduced 

 the term "molecular number" to signify* the sum of the posi- 

 tive charges carried by the atomic nuclei contained in the 

 molecule. Thus when a molecule contains a atoms of an ele- 

 ment A, b atoms of B, c atoms of C, so that its chemical formula 

 is A a B b C c , the molecular number A r = aJV a -\-bJV b -\- cJS T c , where 

 JV a , N~ b , JV" C are the atomic numbers of the component elements. 

 For example, the molecular number of water (H 2 0) is 10, for 

 the nuclear charge of hydrogen is 1, and of oxygen is 8. The 

 molecular number generally, but not invariably, comes out even 

 due to the circumstance that when the valency is odd the atomic 

 number is usually odd also. 



The importance of the new definition lies in the fact that 

 certain simple empirical formulae which Allen has shown to 

 hold for the atomic numbers and other characteristic constants 



