268 Plucker on Spectrum Analysis. 



appearance. It becomes continuous ; the violet and blue lines 

 no longer arise from the ground, which has become lustrous, 

 and we notice at one extremity of the spectrum the red line be- 

 come broader, and surpassing in brilliancy the adjacent parts. 

 Lastly, if we direct the spectroscope towards the broad part of 

 the tube surrounding the electrode, where the light, before 

 entering the capillary tube is less concentrated, an intermediate 

 phenomenon is presented to our view. We still see the three 

 primitive lines, but while the red one remains pretty much as 

 before ; the two others appear in bloom, the violet more than 

 the blue. 



" Nitrogen gas behaves in a manner altogether different ; the 

 beautiful spectrum of this gas, as I at first obtained it by means 

 of the small induction apparatus, remains essentially the same 

 when the great apparatus is used without the bottle and its ten- 

 sion is augmented to about 100 millimetres. When, however, we 

 introduce the Leyden jar, all is changed ; the new spectrum con- 

 tains no trace of the old one : it is composed of a great number 

 of beautiful lines of refrangibility one (partially separated by fine 

 black lines), and not one of them is like the former spectrum. 

 Sulphur and selenium afford analogous results. The spectrum 

 of oxygen is weak under the old conditions, but if the tension of 

 the gas is about 100 millimetres it gives with the great induction 

 coil, and the Leyden jar, a spectrum of great beauty, composed 

 of lines of refrangibility one. The greater intensity of the 

 current brings out a great number of new lines. The same 

 thing happens with chlorine and iodine. 



"The former spectrum of the vapour of mercury was essen- 

 tially composed of three brilliant lines, of which one is double. 

 In the new one, other lines are added, especially red lines, and 

 a double orange line, which at first were not even indicated; 

 and at the same time the feeble lines on the ground of the first 

 spectrum are less developed, as is the case with hydrogen. In 

 this same spectrum of mercury, the green and orange rays, 

 sharply bounded when the temperature is weak, dilate them- 

 selves mere and more towards the red as the heat is increased." 

 Having thus illustrated the physical appearance of the spectra, 

 M. Plucker makes the following remarks on their employment 

 in chemical analysis : — " It seems that no compound body in a 

 gaseous state can escape decomposition if we augment its tem- 

 perature sufficiently. To effect this result, we introduce the 

 gaseous body into a Geissler tube, then we heat the minute 

 thread of gas in its capillary portion by means of an induction 

 current. We then examine the incandescent thread of gas 

 with a prism. In my former spectrum tubes a feeble current 

 sufficed to obtain the spectrum of highly rarefied gas, but in 

 this case the decomposition, if it occurs, is often partial. Two 



