﻿certain Gases in Geissler's Tubes. 411 



visible, besides the characteristic hydrogen-lines ; at two places 

 in the green a bright glow was to be seen ; and also between H /3 

 and H <y the two previously mentioned bright bands were vi- 

 sible. 



On further rarefaction to 3 or 2 millims. the characteristic 

 lines retained the same brightness, and everything else disap- 

 peared almost entirely from the spectrum ; yet, with a simulta- 

 neous enfeeblement of the bright lines, part of the continuous 

 spectrum reappeared in the green, in the form of about five bright 

 fields, when the gas was rarefied to fractions of a millimetre pres- 

 sure. These observations show that the hydrogen-spectrum de- 

 scribed belongs, in fact, to a lower temperature than that consist- 

 ing of the three lines ; for with increasing density of the gas in the 

 spectrum-tube the temperature must become lower, since the 

 induction-current experiences a greater resistance in the denser 

 gas, and a larger quantity of the gas has to be heated. But, 

 just as a great density of the gas does not permit the full inten- 

 sity of the current to be developed, so the current is also enfeebled 

 by great rarefaction ; for by adequate rarefaction the current in a 

 spectrum-tube may be completely stopped. The occurrence, 

 therefore, of the continuous spectrum both with greater density 

 and with greater tenuity of the gas proves that it belongs to a 

 lower temperature. 



4. By the experiments communicated in the preceding, the proof 

 has been furnished that the continuous hydrogen-spectrum be- 

 longs to a lower temperature ; and the question arises how it hap- 

 pens that Geissler's spectrum-tubes, which contain hydrogen 

 under a pressure of from 5 to 10 millims. (a pressure favourable, 

 therefore, for producing the line-spectrum), yet after some time, 

 after lengthened use, may yield the continuous spectrum. The 

 observation that this spectrum is especially seen when, after 

 lengthened use, the tube is exposed to the action of a feebler 

 current, led to the supposition that (possibly owing to a super- 

 ficial melting of the electrodes) greater resistance was offered to 

 the passage of the induction-current. That such a fusion of the 

 electrodes has an influence of that kind was established by a 

 series of experiments. In the extreme degrees of exhaustion 

 mentioned above, and which are to be subsequently discussed, 

 the resistance in the spectrum-tubes was so great that the whole 

 of the positive electrode became incandescent. It thereby be- 

 came quite bent, and after some time was partially fused, so that 

 it looked like a series of small pearls on a thread. After this de- 

 formation of the electrodes had occurred, the tube, on gradual ex- 

 haustion of the hydrogen, always exhibited the continuous spec- 

 trum, even under pressures at which the green light was other- 

 wise scarcely visible. The continuous spectrum was extremely bril- 



