86 Professor Dewar [April 1, 



a very slight displacement, but it was not so definite that one could 

 be sure of it. 



These observations, however, led us to some other interesting 

 results. In the first place, one of the two images of the lithium line 

 almost always was reversed — that is, showed a dark line down the 

 middle. This was the line given by the flash approaching the slit. 

 The receding flash in the other limb of the tube gave as broad a 

 bright line, but it had no dark line in its middle. This observation 

 was made a great many times ; and the fact of the reversal established 

 independently in the case of some other metallic lines by means of 

 l^hotographs. These reversals show that in the wave of explosion the 

 temperature of the gases does not reach its maximum all at once; 

 but the front of the wave is cooler than the part which follows and 

 absorbs some of its radiation, while the rear of the wave does not 

 produce the same effect. One would suppose that there must be 

 cooler lithium-vapour in the rear of the wave as well as in its front ; 

 but it is possible that the absorption produced by it extends over the 

 whole width of the line, and not only over a narrow strip in the middle. 

 For we observed that when a little lithium carbonate was freshly put 

 into the tube, the red line was so much expanded as to fill the whole 

 field of view — that is to say, it was some ten or twelve times as wide 

 as the distance between the two yellow lines of sodium ; but by 

 washing out the tube with water (that is, by reducing the quantity 

 of lithium present in the tube), the line could be reduced in width 

 until it was no wider than one-tenth of the distance between the two 

 sodium lines. This seeins to prove that the breadth of the line is 

 directly dependent on the amount of lithium present. 



M. Fievez has, in a recent publication (' Bulletins de I'Academie 

 roynle de Belgique'), concluded, from observations on sodium, that the 

 widening of the lines is solely due to elevation of temperature. The 

 flash of the exploding gases cannot be raised in temperature by the 

 presence of a minute quantity more of a lithium compound ; so that 

 in our case the widening cannot be ascribed to anything but the 

 increase in the quantity of lithium present, or to some consequence 

 of that increase. It is not improbable that the amount of lithium 

 vaporised in the front of the wave of explosion is less than in the 

 following part, and hence the absorption-line is not so wide as tho 

 bright line behind it, while in the rear of the wave the absorption 

 extends over the whole width of the bright band, and so is not so 

 easily noticed. Only twice amongst many observations was any 

 reversal of the lithium line seen in the receding wave of explosion. 



On observing the flash with a spectroscope of small dispersion 

 instead of that with the grating, the continuous spectrum was very 

 bright, but the metallic lines stood out still brighter ; not only the 

 red line of lithium, but the orange, the green, and the blue lines were 

 very bright, and continued so when the pressure of the gases before 

 explosion was reduced from one atmosphere to one-third of an at^ 

 mosphere, The violet line was not seen, but it may have been so 



