102 On the Fluorescent Relations of certain solid Hydrocarbons. 



yielding a banded spectrum identical with that characterizing it 

 in its normal state. 



I should therefore conclude that no true combination had 

 occurred, and that the change from a yellow to a green colour 

 resulted from the presence of minute black particles produced 

 by the action of the sulphuric acid upon some small portion of 

 an adherent impurity. This view is confirmed by the fact that 

 with petrollucene, which is almost colourless, sulphuric acid pro- 

 duces a dark green colour, approaching to black. Continued 

 application of heat causes thallene, treated as above, to dissolve, 

 forming a compound perfectly soluble in water. 



The picric-acid compounds, both of anthracene and thallene, 

 show no fluorescence ; so that this gives us a convenient method 

 of determining the point of saturation. 



Examined with BecquereFs phosphoroscope, the duration of 

 fluorescence in solid thallene was found to be about - 5 Jq of a 

 second ; but that of its solution was too brief to be appreciable 

 even at the highest velocity which could be given to the in- 

 strument. 



To exhibit the fluorescence of thallene, I have found the fol- 

 lowing methods most effective : — 



1st. The electric light, without any lenses, is allowed to fall 

 directly on it through a plate of blue or violet glass. In this 

 way one can show various devices, filling an entire stage without 

 difficulty. Allowing the light to pass through the condensing- 

 lenses of a lantern greatly reduces the effect. 



2nd. Burning magnesium wire behind a blue glass. The 

 effect here also is very powerful. 



3rd. Connecting in series a number of nitrogen-spectrum 

 tubes and exciting them with a powerful coil. The great lack 

 of general illuminating-power in the light so produced makes 

 the effect in this case very striking. 



A lime-light behind a blue glass will exhibit the fluorescence 

 of thallene fairly well, but not that of petrollucene. 



The violet end of a projected spectrum might be regarded as 

 a good source of exciting light, but is, as a matter of fact, infe- 

 rior to any of those before mentioned. Nevertheless the other- 

 wise invisible bands of metallic spectra, up to the position of the 

 group O, may be exhibited to the largest audiences by the use of 

 a screen covered with thallene. In painting designs and screens 

 with thallene, it is best to grind it with damar varnish and ben- 

 zole. A glass goblet coated with thallene is well suited for 

 Gassiot's electric cascade. 



