Plucker on Spectrum Analysis. 269 



of these tubes — one containing carbonic acid, and the other 

 carbonic oxide — give the same spectrum, namely, that of the 

 last gas, which is not essentially changed by augmenting the 

 force of the induction coil : as beyond a certain limit the tem- 

 perature of the gas is not increased. In the new tubes, con- 

 taining gas of a greater density, a stronger coil is required than 

 in the old tubes, to bring the gas to a given heat ; but the gas 

 assumes a much higher temperature as we increase the power 

 of the coil. Thus, in two recent experiments we gave to the 

 two gases a pressure of 100 millimetres, and illuminated them 

 with discharges of a great Ruhmkorff coil. In discharging the 

 apparatus in an ordinary way we obtained the ordinary spec- 

 trum, that of the carbonic oxide ; but upon interposing a Leyden 

 jar of convenient dimensions, we instantly descried the beautiful 

 spectrum of oxygen, identical with that obtained when the tubes 

 were filled with pure oxygen at the same pressure, and trans- 

 mitted a current of electricity in the same way. . . . I therefore 

 conclude, that at a lower temperature the carbonic acid is re- 

 solved into carbon and carbonic oxide, and at a higher one this 

 latter gas is itself decomposed, whether we introduce it in the first 

 instance into the tube, or obtain it by the action of the current 

 upon carbonic acid. This is not all, for immediately after the 

 decomposition the temperature falls, and the recomposition of 

 oxygen and carbon ensues. 



" In citing these examples of the decomposition of bodies, 

 as evidenced by spectral analysis, the decomposition of the 

 vapour of water must not be passed over. We introduce water 

 into the interior of the new tubes, and, before sealing them in a 

 lamp, we boil the water to expel the air. If we then make the 

 electric current pass, without the Leyden jar, we obtain only 

 the three rays of hydrogen on a dark ground. With the addi- 

 tion of the Leyden jar we get the oxygen spectrum also, clearly 

 defined. This experiment illustrates the facility with which the 

 current traverses hydrogen gas. If a rarefied gas contains the 

 least trace of water, the water is decomposed, and the hydrogen 

 rays, especially the red and blue, are exhibited in the clearest 

 manner. If, for example, we cause the electric discharge to pass 

 through a tube containing nitrogen which has been dried, but 

 without extreme care, and we establish a communication with a 

 mercurial evacuator, we see first, as we produce a vacuum, the 

 beautiful spectrum of nitrogen, which is replaced by that of 

 hydrogen as the limit of rarefaction is approached. As a third 

 illustration, I shall take chloride of zinc. After having introduced 

 a small quantity into the spectrum tube, the vacuum is made as 

 complete as possible. We then obtain, on heating the tube, first 

 the chlorine spectrum, slightly developed, but easily recognizable; 

 afterwards continuing to heat the gas, this spectrum, which at 



