6 On Double Spectra. [January, 



of barium contains no line of the metal, since barium oxide 

 is less easily decomposed than either calcium oxide or 

 strontium oxide. 



But amongst all the additions made to our knowledge of 

 spectrum analysis within these ten years, none is so startling 

 as the discovery, which we owe to Pliicker, that a substance 

 may give two totally different spectra which have no line 

 or band in common. In a paper published in the " Philo- 

 sophical Transactions " for 1865, Pliicker and Hittorf 

 describe double spectra of nitrogen, sulphur, selenium, hydro- 

 gen, and iodine. Nitrogen exhibits this peculiarity in a 

 marked manner. In order to obtain its spectrum it is neces- 

 sary to employ electricity, as no flame is hot enough. If 

 an ordinary vacuum-tube containing nitrogen have the 

 current from an induction coil sent through it, the narrow 

 part of the tube gives out a purple light, which is resolved 

 by the prism into the spectrum represented in the chromo- 

 lithograph — a spectrum consisting of an immense number of 

 shaded bands. 



If, instead of using nitrogen at low pressures, we let the 

 spark pass in the gas at the ordinary pressure, and intensify 

 it by connecting the two wires with the outer and inner 

 coatings of a moderate sized Leyden jar, we obtain an 

 intensely bright light, which gives a spectrum also repre- 

 sented in the chromo-lithograph. This second spectrum is 

 entirely different from the other, consisting only of sharply 

 defined bright lines. Pliicker terms these spectra, spectra 

 of the first order, and of the second order, respectively. It 

 will be observed that these spectra possess, respectively, the 



The vibrations which produce light depend, so far as we can see, on the 

 manner in which the atoms of a molecule are in equilibrium. We see from 

 the occurrence of lines in all parts of the spectrum, that there are in the 

 molecule several different vibrations executed simultaneously, and these corres- 

 pond to the greater or less intensity of the force by which the atoms are main- 

 tained in their position of equilibrium. The more intense the force by which 

 an atom is held in equilibrium, the faster it will vibrate when set in motion. 



When cyanogen is moderately heated its molecules vibrate in the regular 

 way indicated by the cyanogen spectrum ; but when the temperature is raised 

 the compound is dissociated, and the carbon atoms vibrate uninfluenced by 

 those of nitrogen. Now, when this takes place, the vibrations of the cyanogen 

 which first disappear must be those due to the closest intimacy — that is, the 

 most rapid. Hence the carbon spectrum comes in at the blue end. 



Further, the vibrations of an atom about its position of equilibrium will not 

 all be of equal length, and so will produce light of varying intensity. If the 

 vibrations are quite cycloidal, all will be executed in the same time, and we 

 shall have a sharp bright line, but if the vibrations are like those of an ordinary 

 pendulum, the smaller vibrations will be performed inless time than the larger, 

 and the result will be a band fading off into the blue. If, on the other hand, 

 the vibrations of small amplitude are executed more slowly than those of larger 

 amplitude, the result will be a band fading off into the red. 



