1891.] Influence of Pressure on the Spectra of Flames. 210 



derived from the residue of atmospheric air mixed with the condensed 

 gases. We took a photograph of them, and on comparing 1 this with 

 a photograph of the N0 2 bands, we found the two to be identical. 

 Except for the bands, and the bright lines of sodinm, the spectrum 

 appeared to be continuous, and to extend from about X 6200 to X 4150, 

 with the brightest part about X 5150. It increased in brilliance as 

 the pressure increased, as well as- in extent, being visible at 

 3 atmospheres pressure from about X 6720 to X 4040. The greater 

 distinctness of the N0 2 bands at the higher pressures was due both 

 to the greater brightness of the continuous spectrum and* to the 

 greater quantity of NO 2 formed. A large quantity of water accu- 

 mulated in the experimental tube, and when this was drawn off by 

 the stopcock s, it effervesced with escape of NO, and was found to be 

 strongly acid. A specimen titrated was found to contain very nearly 

 3 per cent, of nitric acid. The observations were continued up to a 

 pressure of 4*0 atmospheres-. There was no indication that the con- 

 tinuous spectrum had any connexion with the line spectrum of 

 hydrogen. There was no increase of brilliance in the neighbourhood 

 of the C, F, or G lines of hydrogen. The characters of the spectrum 

 were, however, better seen in the absence of NO 2 , and will be de- 

 scribed in the next section. 



Oxygen Burning in Hydrogen. 



In this case the colour of the flame was very different from that of 

 lydrogen burning in oxygen^ Instead of being yellow, it appeared, 

 the unaided eye, to have a lavender hue. In the spectroscope it 

 showed a perfectly continuous spectrum, brightest in the green, about 

 the region of the Frannhofer line 6, and very gradually fading away on 

 either side. On the red side it could be just traced up to about 

 X 6150, and on the violet side to about X 4285, at ordinary pressures. 

 The sodium lines were absent. With increase of pressure it in- 

 creased very much in brightness, and at 8 atmospheres pressure it 

 could be traced as low as X 6630 and as high as X 3990. 



The dispersion used was that of a direct- vision spectroscope (such 

 as was described by us r ' Boy. Soc. Proc.,' vol. 41, p. 449), equivalent 

 to three prismd of white flint glass, but the collimator and telescope 

 very short, so as to obtain plenty of light. With less dispersion, 

 3rhaps, the continuous spectrum might have been traced further. 

 > hotographs, however, showed that it scarcely extended into the 

 Itra- violet. There was no indication that this- spectrum was due to 

 expansion of the lines of either the first, or second, spectrum of 

 lydrogen. It is true that the maximum brightness (which could not 

 determined with any great accuracy) was not very far from F, but 

 indication of any second maximum in the neighbourhood of either 



