solid Hydrocarbons found in Petroleum Distillates. 95 



thallene, however, in place of becoming less brilliant towards 

 the upper end, they glow with remarkable intensity; and all 

 about 14 we find a brilliant green ground on which the lines 

 H, H' and others appear with beautiful distinctness. 



Below this the colour is a bluish white as far as a little below 

 F, beyond which there is no fluorescent action, and the spec- 

 trum looks alike whether it fall on the thallene or on the card. 



The brightness of the spectrum on the thallene, however, is 

 not uniform between 8 and 14, but presents a series of gradu- 

 ated maxima and minima, which are represented in the woodcut 



To measure the positions of these maxima, the simple device 

 already alluded to was employed. A pinhole was pierced in the 

 fluorescent paper at some convenient point, as at a (fig. 7) ; and 

 by sliding the entire card or screen in the grooves which sup- 

 ported it, this pinhole was brought to the part of the spec- 

 trum whose location it was desired to fix (for example, as in 

 the figure, at the middle of the broad maximum a) ; and then, 

 the spectroscope being placed behind E, the refrangibility of the 

 light passing through the pinhole is directly measured. 



A comparison of fig. 7 with fig. 6, bearing ill mind that the 

 scale of the latter is double that of the former in actual dimen- 

 sions, though the numbers correspond, will show that the 

 maxima in fig. 7 exactly correspond with the absorption-bands 

 of fig. 6. 



This indicates that the absorption in this case is in very close 

 relations with the fluorescence of the substance ; and to this con- 

 clusion many phenomena to be presently noticed give confirma- 

 tion. That such a connexion should exist is eminently natural, 

 since those rays which expend their force in fluorescence, i. e. in 

 producing vibrations of a less rate, must evidently disappear ; but 

 it does not follow that all rays which are absorbed develop fluor- 

 escence; and so, as in the uranium salts, we may expect to 

 find absorption-bands which have no immediate relation to fluor- 

 escence, although here also there will always be a special absorp- 

 tion corresponding to the maximum of fluorescence-exciting 

 power. This was observed by Stokes at the outset. What we 

 have to notice here is that this body thallene has no appreciable 

 absorption other than that due to its fluorescence — or, in other 

 words, that nearly all the rays absorbed by it are converted into 

 fluorescent motions and not into those of heat. 



1 have spoken only of the rays as high as H 7 when referring 

 to the fluorescence excited by a pure spectrum on a screen of 

 thallene ; but it must not be supposed that this is the upper 

 limit of the effect. With lenses and prisms of quartz^ I have 



