HISTORICAL. 291 



The dispersed radiation from gases in vacuum-tubes has been less 

 extensively investigated. Runge and Paschen' investigated the infra- 

 red emission spectrum of hehum (cleveite gas) to 7 fi, using for the pur- 

 pose a bolometer and a fluorite prism. They found strong emission 

 bands at 0.729 /x, 1.134 yu,, and 2.057 /x. The bands shift slightly for the 

 different vacuum-tubes used, and also vary in intensity in a manner 

 that could not be explained. No lines were found beyond the region of 

 2 fji. The three strong lines belong to those of the visible spectrum as 

 predicted by the spectral series formulae. 



Angstrom^ measured the total radiation of gases in vacuum-tubes and 

 remarked that to do so is a difficult matter, so that after dispersing the 

 radiation the task is well-nigh impossible. In fact, the bolometer used 

 by him was not sensitive enough to measure the intensity of the indi- 

 vidual lines, hence only the total radiation was measured. For the 

 positive column, at constant pressure. Angstrom found that the total 

 radiation as well as the luminous radiation is proportional to the cur- 

 rent. For hydrogen the total radiation is a maximum at a pressure of 

 1.02 mm., while the luminous radiation decreases with increase in pres- 

 sure. For nitrogen the total radiation is a minimum at 3.6 mm. pres- 

 sure. In the work of Drew (to be mentioned presently) the curves of 

 total radiation of air also show a minimum, which, however, shifts with 

 increase in size of the tube, the minimum being at a pressure of about 

 1.7 mm. for tube 0.9 cm. in diameter, and at a pressure of 0.5 mm. for a 

 tube 1.8 cm. in diameter. 



From the fact that the total radiation increases, while the luminous 

 radiation decreases, with increase in pressure of the gas. Angstrom 

 concluded that there are two kinds of radiation present during the 

 electrical discharge, viz, "regular" and "irregular" {i. e., lumines- 

 cence). With decrease in pressure the former decreases while the 

 irregular radiation increases in proportion as the motions are less ob- 

 structed by the mass of the gas. At constant pressure a certain pro- 

 portion of the energy in each molecule is converted into radiation; as 

 the strength of the current increases the number of active molecules, 

 and hence the radiation, increases in the same proportion. 



The number of the active molecules being relatively small, the damp- 

 ening effect of the rest may be taken as constant, and the composition of 

 the radiation remains practically unaltered as the current increases. 

 On increasing the pressure the dampening effect changes, and the radia- 

 tion becomes richer in infra-red rays. A greater proportion of the 



' Runge & Paschen : Astrophys. Jour., 3, p. 4, 1896. 

 ^ Angstrom ; Ann. der Physik (3), 48, p. 493, 1893. 



