394 Royal Institution : — 



fluorescent substance (as distinguished from a mixture) is examined 

 in a pure spectrum, it is found that on passing from the extreme 

 red to the violet and beyond, the fluorescence commences at a cer- 

 tain point of the spectrum, varying from one substance to another, 

 and continues thence onwards, more or less strongly in one part 

 or another according to the particular substance. The colour of 

 the fluorescent light is found to be nearly constant throughout 

 the spectrum. Hence, when in a solution presented to us, and ex- 

 amined in a pure spectrum, we notice the fluorescence taking, as it 

 were, a fresh start, with a different colour, we may be pretty sure 

 that we have to deal with a mixture of two fluorescent substances. 



It might be inferred a priori, that fluorescence at any particular 

 part of the spectrum would necessarily be accompanied by absorp- 

 tion, since otherwise there would be a creation of vis viva ; and 

 experience shows that rapid absorption (such as corresponds to 

 a well-marked minimum of transparency indicated by a determinate 

 band of absorption in the transmitted light) is accompanied by 

 copious fluorescence. But experience has hitherto also shown, 

 what could not have been predicted, and may not be universally 

 true*, that conversely, absorption is accompanied, in the case of a 

 fluorescent substance, by fluorescence. 



From what precedes it follows that the colour of the fluorescent 

 light of a solution, even when the incident light is white, or merely 

 sifted by absorption, may be a useful character. To illustrate this, 

 the electric light, after transmission through a deep-blue glass, was 

 thrown on solutions in weak ammonia of two crystallized substances, 

 sesculine and fraxine, obtained from the bark of the horse-chestnut ; 

 the latter of the two occurs also in the bark of the ash, in which, 

 indeed, it was first discovered. Both solutions exhibited a lively 

 fluorescence ; but the colour was different, being blue in the case of 

 sesculine, and bluish green in the case of fraxine. A purified solu- 

 tion obtained from the bark exhibits a fluorescence of an interme- 

 diate colour, which would suffice to show that sesculine would not 

 alone account for the fluorescence of the solution of the bark. 



When a substance possesses well-marked optical properties, it is 

 in general nearly as easy to follow it in a mixture as in a pure solu- 

 tion. But if the problem which the observer proposes to himself 

 be, Given a solution of unknown substances which presents well- 



* Fluorescent substances, like others, doubtless absorb the invisible 

 heat-rays lying beyond the extreme red, in a manner varying from one sub- 

 stance to another. Hence, if we include such rays in the incident spectrum, 

 we have an example of absorption not accompanied by fluorescence. But 

 the invisible heat-rays differ from those of the visible spectrum (as there is 

 every reason to believe) only in the way that the visible rays of one part of 

 the spectrum differ from those of another, that is, by wave-length, and 

 consequently by refrangibility, which depends on wave-length. Hence it 

 is not improbable that substances may be discovered which absorb the 

 visible rays in some parts of the spectrum less refrangible than that at 

 which the fluorescence commences ; and mixtures possessing this property 

 may be made at pleasure. Nevertheless the speaker has not yet met with 

 a pure fluorescent substance which exhibits this phenomenon. 



