162 Profs. Liveing and Dewar on the Spectral Lines 



sharply defined lines to make such a displacement appreciable. 

 With lines of longer wave length than the yellow sodium 

 lines, the displacement would be proportionately greater. 

 Further, if a receding explosion could be observed simul- 

 taneously with an advancing explosion, the relative shift of 

 the line would be doubled, one image of the line observed 

 being thrown as much towards the less-refrangible side of its 

 proper position as the other was thrown towards the more- 

 refrangible side. The two images of the red line of lithium 

 would in this way be separated by a distance of about ^ of a 

 unit of Angstrom's scale; a quantity quite appreciable, though 

 much less than the distance between the components of & 3 , and 

 about equal to the distance of the components of the less 

 refrangible of the pair of lines E. "We thought therefore that 

 we might test theory by experiment. 



A preliminary question had, however, to be answered. 

 What lines could be seen in the flash of the exploding gases ? 

 We were pretty certain that the hydrogen lines could not be 

 seen, but that probably we might get sufficient dust of sodium 

 compounds floating in the gas to make the sodium lines 

 visible. A preliminary observation was made on the flash of 

 mixed hydrogen and oxygen in a Cavendish's eudiometer, 

 which showed not only the yellow sodium lines, but the orange 

 and green bands of lime and the indigo line of calcium all 

 very brightly, as well as other lines not identified. The flash 

 is very instantaneous, but nevertheless produces a strong im- 

 pression on the eye ; and by admitting the light of a flame 

 into the spectroscope at the same time as that of the flash, the 

 identity of the lines was established. That sodium should 

 make itself seen was not surprising, but that the spectrum of 

 lime should also be so bright had not been anticipated. At 

 first we thought that some spray of the water over which the 

 gases were confined must have found its way into the eudio- 

 meter ; but subsequent observations led us rather to suppose 

 that the lime was derived from the glass of the eudiometer. 

 The lime-spectrum made its appearance when the eudiometer 

 was quite clean and dry, and when the gases had been stand- 

 ing over water for a long time. 



To obtain the high dispersion requisite, as already explained, 

 we made use of one of Rowland's magnificent gratings, with 

 a ruled surface of 3£ by 2^ inches, and the lines 14,438 to the 

 inch. One telescope fitted with a collimating eyepiece served 

 both as collimator and observing-telescope; and by this means 

 we were able to use the spectra of the third and fourth orders 

 with good effect. 



Observations were made with this instrument on explosions 



