ARTICLES 



THE SPECTRUM OF HYDROGEN 



By S. BARRATT. B.A. 



Balliol College, Oxford 



Hydrogen is known to emit several totally distinct spectra. 

 Of these the most frequently observed is a series spectrum in 

 the visible and ultra-violet regions, which is accompanied by 

 other related series in the infra-red and far ultra-violet. An 

 entirely distinct line or band spectrum is observed under suitable 

 conditions, which is called the secondary spectrum. It is not 

 so intense as the series spectrum, but contains a large number 

 of lines. There is also a continuous spectrum, observed in 

 hydrogen-filled vacuum tubes, but of which little is known, 

 except that its intensity distribution precludes it from being a 

 temperature radiation.-^ 



The chief object of this article is to describe some recent 

 work on the secondary spectrum, but an account of the series 

 spectrum will not be out of place. 



The main series of hydrogen, usually known as the " Balmer" 

 series, is of exceptionally wide occurrence. It is found in 

 nebulae and remains a prominent feature of several later types 

 of stars, such as Sirius and S-Aquilse. Its prominence in the 

 sun is evident from the fact that Fraunhofer chose the first three 

 lines of the series. Ha, H/3, and H7, to mark the C, F, and G 

 regions, in his classification of the solar spectrum. The red C 

 line is responsible for the red colour of solar prominences, and 

 has been observed at a height of 8,000 km. above the normal 

 photospheric surface. 



In the laboratory the Balmer series is obtained from vacuum 

 tubes — indeed it is rarely absent from them — and also from the 

 arc and spark in hydrogen. Curiously enough, it has rarely, 

 if ever, been observed in flames. The spectruni of burning 

 hydrogen gives only a series of bands in the ultra-violet, which 

 are commonly attributed to water-vapour. The introduction 

 of hydrogen salts, such as hydrogen chloride, into flames does 



^ I.e. a continuous spectrum emitted by an opaque body in virtue of its 

 temperature alone, in which the energy distribution obeys Planck's Law. 



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