588 



NATURE 



[July 8, 1920 



Yet it interposes an impassable obstacle to the 

 ultra-violet rays, at least to those of shorter wave- 

 length than about 2900 ang-stroms. It cuts off 

 the iron spectrum at about the same point where 

 the solar spectrum ends. Speaking roughly and 

 generally, it may be said that glass is somewhat 

 more opaque than ozone — i.e. that with diminish- 

 ing wave-length the limit of transmission is 

 reached somewhat sooner. To make a statement 

 of this kind quite definite the thickness must of 

 course be specified. 



Sir William Huggins devoted a great deal of 

 attention to the spectra of the sun and stars in 

 the extreme ultra-violet region, using for the pur- 

 pose a reflecting telescope, and prisms and lenses 

 made of quartz or Iceland spar. In this way the 

 absorption of a glass objective was avoided. He 

 noticed in 1890 that the spectrum of Sirius showed 

 a number of bands near the extreme limit of atmo- 

 spheric transmission, the bands tailing off into 

 complete absorption. 



These bands were observed and discussed by 

 other authors, but no definite conclusion was 

 reached as to their origin until 191 7, when the 

 matter was taken up by my colleague. Prof. 

 Fowler, and myself. Our interest was stimulated 

 by an excellent photograph of the bands, taken 

 at Edinburgh Observatory under Prof. Sampson's 

 direction, which I show on the screen. We found 

 that the same bands were present in the solar 

 spectrum. It may seem strange that this had 

 not been observed long ago, considering how 

 closely the solar spectrum has been scrutinised 

 for more than a generation. As a matter of fact 

 this is one of the cases where a powerful instru- 

 rnent is a positive disadvantage. The bands are 

 diffuse, and under high dispersion they are un- 

 recognisable. In any case, they are less con- 

 spicuous than in the spectrum of Sirius, because 

 in the sun numerous metallic lines are superposed 

 upon them and distract the eye. 



Now the position and general aspect of these 

 bands suggested that they were connected with 

 the absorption which terminates the spectrum. 

 This led us to suspect that they were due to ozone, 

 and the suspicion was readily confirmed by experi- 

 ment. Burning magnesium ribbon gives a con- 

 venient source of continuous spectrum in the 

 ultra-violet region. Interposing a long tube con- 

 taining ozone between the burning magnesium and 

 the slit, a series of bands was photographed which 



exactly corresponded to those photographed in the 

 solar spectrum with the same instrument, as you 

 will see in the slide shown. 



Absence of Ozone near the Ground. 



We are then driven to the conclusion that the 

 absence of short waves from the spectra of the 

 sun and stars is due to absorption by terrestrial 

 ozone. But it was not thought desirable to let 

 the matter rest there. It is true that many 

 attempts had been made to determine the (no 

 doubt very small) quantity of ozone in air by 

 chemical means, but with very conflicting results, 

 because other constituents of air, such as oxides 

 of nitrogen, are liable to produce reactions not 

 unlike those of ozone. It seemed more satisfactory 

 to test the absorbing power of air near the ground 

 for ultra-violet rays, to which ozone is so opaque. 

 I used for this purpose a mercury vapour lamp in 

 a quartz vessel, which is a powerful source of 

 ultra-violet rays, and observed its spectrum four 

 miles away, so that the mass of air intervening 

 was as great as that between the midday summer 

 sun and the top of the Peak of Teneriffe, from 

 which observations of the extent of the solar spec- 

 trum have been made. The result was to show 

 that the mercury lamp spectrum was by no means 

 stopped when the solar spectrum stops, but that 

 it extended to the region where ozone is most 

 opaque. There is a strong mercury line (wave- 

 length 2536) at about this point which was dis- 

 tinctly photographed. Its intensity was of course 

 a good deal reduced relative to the visible spec- 

 trum by atmospheric scattering. But there was 

 no evidence whatever of ozone absorption. 



What conclusion can we draw? Evidently that 

 the absorbent layer of ozone in the air is high up, 

 and that there is little or none near the ground. 

 It may seem at first sight that this thin and in- 

 accessible layer of ozone, which we have learned 

 of by a chain of reasoning not less conclusive than 

 direct observation, is a matter of little importance 

 to man and his welfare. There could be no greater 

 mistake. It acts as a screen to protect us from 

 the ultra-violet rays of the sun, which without 

 such a protection would probably be fatal to our 

 eyesight : at least if one may judge from the 

 painful results of even a short exposure to such 

 rays, which those who have experienced it are not 

 likely to forget. 



The Future of the Iron and Steel Industry 



By Prof. H. C. H. Carpenter, F.R.S 



TOURING the spring of last year two Commis- 

 ■■-' sions were appointed by the Minister of 

 Munitions to visit and report upon certain steel- 

 producing areas in Western Europe. One of them 

 visited the steel works in Lorraine and certain 

 parts of the Saar Valley, the other journeying to 

 the occupied areas of Germany, Luxemburg, and 

 certain oarts of France and Belgium. The 

 NO. 2645, VOL. 105] 



in Lorraine. 



former was under the charge of Sir William 

 Jones, and included Messrs. Percy Cooper, Row- 

 land Harding, and Cosmo Johns, while the latter 

 was entrusted to Dr. F. H. Hatch, who had 

 with him Messrs. L. Ennis, James Henderson, 

 and Richard Mather. The Commissions were 

 absent about three weeks. The terms of reference 

 to them were the same and were to ascertain : — 



