PHYSICS NINETEENTH CENT.-SPECTROSCOPY. 493 



In 1865 Pliicker and Hittorff showed that many of the elementary 

 gaseous bodies will yield different spectra according to the conditions 

 of pressure and temperature. For example, these observers found 

 that nitrogen, when rarefied beyond a certain point, will not permit 

 the passage of the induction-sparks ; but under a tension of less than 

 one millimetre of mercury, the current passes and the tube becomes 

 luminous, giving off a yellow-coloured light, the spectrum of which 

 consists of a large number of coloured bands, or narrow luminous 

 spaces less sharply denned than the "line" spectra. A slight change 

 either in the intensity of the spark or density of the gas causes the 

 nitrogen to glow with a bluish instead of a yellowish light, which gives 

 another band spectrum different from the former. If the temperature 

 due to the electric discharge is still further increased by including a 

 Leyden jar in the circuit, the light changes to white, and the spectrum 

 which this yields is now one of bright lines on a dark ground. These 

 changes were explained by attributing them to the existence of different 

 allotropic states of nitrogen. Chemists are indeed not acquainted with 

 nitrogen except in one state ; but they are familiar with certain other 

 elements sulphur, for example in states in which different physical 

 properties are exhibited by the same (i.e., chemically identical) sub- 

 stance states. These are also known to depend upon temperature, 

 so that Pliicker and HittorfFs explanation has much probability in its 

 favour. The same observers have found that by increasing the tem- 

 peratures sufficiently, oxygen and certain other elementary gases yield 

 continuous (page 484) spectra. 



It would be impossible within our limits to mention a tithe of the 

 many interesting discoveries which have rewarded the labours of spec- 

 troscopists. A multitude of skilful experimentalists in every country of 

 Europe and in North America have for the last twenty years been 

 engaged in enlarging our knowledge of the spectra of the chemical 

 elements 'and compounds. But as we have probably dwelt upon 

 spectrum analysis in its application to terrestrial substances sufficiently 

 to give the reader some notion of the kind, though not of the number, 

 of facts upon which this new method of investigation is based, we 

 now turn to another development of the subject, which is of the 

 highest general and scientific interest, inasmuch as the discoveries 

 which it includes have greatly influenced our views of the constitution 

 of the universe. 



The stars of heaven, ever as it seemed carried round the earth in 

 the unhasting and unresting revolution of their sphere, must, even in 

 the most remote period of human history, have attracted men ; s eyes 

 and thoughts. How many minds have pondered over the mysteries 

 of the midnight sky ! How many eyes have anxiously scanned the 

 constellations, to read, if might be, in their bright configurations the 

 preordained destinies of men ! Gliding on silently and unchangeably, 

 far above the mutations and turmoil of the generations of mankind, 



