TROWBRIDGE. — SPECTRA AT HIGH TEMPERATURES. 685 



even if the walls do not show a characteristic spectrum they may influ- 

 ence by a species of katalytic action the reaction between the rarefied 

 gases and the vapors of the metallic terminals. 



It is interesting to observe the appearance of successive orders of 

 spectra photographed by the camera I have described with progressively 

 increasing strengths of electrical discharges as well as repeated discharges 

 of the same strength. When, for instance, the capillaries are filled with 

 rarefied water vapor one generally obtains with comparatively weak 

 electrical discharges hydrocarbon spectra which possibly arise from 

 traces of carbonic acid in the capillaries or from traces of grease in the 

 stopcocks of the pump. These banded spectra decrease in strength as 

 the strength of the electrical discharges increases, and finally disappear 

 with powerful discharges. The hydrocarbon combinations are evidently 

 broken up and new reactions and combinations enter. We therefore 

 may not be able to recognize in these new reactions the presence of a 

 simple component. 



In the study of the spectra of heavenly bodies which may be con- 

 sidered from the point of view of furnaces, this fact I think should be 

 borne closely in mind. Substances may be present which do not appear 

 in the spectra observed at great distances ; ' for the relative brightness of 

 metallic lines and gaseous lines is much modified by the combinations 

 which enter in an environment of high temperature. 



Figure 3 (the photographs are reproductions of the negatives, and are 

 not positives) is a reproduction of the spark spectrum of calcium in the 

 neighborhood of the H.H. lines of the solar spectrum taken by the 

 method of successive discharges of known amounts of electrical energy. 

 The discharges ran from one to five. It is interesting to notice on the 

 negatives that the photometric intensity of the lines estimated by the 

 blackness is not directly proportional to the amount of energy. Thus 

 the spectrum produced by four discharges is not twice as intense as that 

 produced by two discharges. It is noticeable, also, that the calcium 

 lines, wave lengths 3737 and 3706, are stronger on the negative than 

 those which coincide with the H.H. lines, and always appear with these 

 lines. 



Figure 4 is a negative of discharges running from one to four in a 

 Geissler tube of glass which did not contain an appreciable amount of 

 lead in its composition. The capillary was two millimeters, internal 

 diameter, and four inches in length. The pressure of hydrogen was one 

 millimeter. It is noticeable that the lines which coincide with the H.H. 

 lines are not accompanied by the lines, wave lengths 3737 and 370G; 



