Polytechnic Association. 915 



the fluorescence-exciting rays ; so is the light from burning magnesium 

 from the ignited lime of the lime-light, only in these cases we 

 must sift out the excess of the longer waves, which are also present, 

 by the use of a plate of blue glass. Sunlight I have strangely omitted, 

 as it comes next to the electric arc, but requires the blue glass also. 

 I may mention here that a very neat arrangement has been made by 

 Messrs. Hawkins and Wale, of Hoboken, to exhibit this fluorescence. 

 It consists of a card with a flower painted in fluorescent material on 

 it. This is contained in a little folding pocket of glass, which can be 

 carried in one's pocket, and exhibited at a moment's notice, in day- 

 light brilliantly, and even to some extent by gas-light. 



Such, then, being the limitations as to the exciting light and the 

 sources from which we can derive it, we inquire as to what bodies are 

 capable of exhibiting this action. 



If we apply the most accurate tests and use the most powerful 

 means of excitement at our command, we find that it is much easier 

 to say what does not than what does fluoresce, for almost all organic 

 matters and many minerals possess this property in some degree. 

 Thus, paper, muslin, leaves of plants, alumina or clay, in many of its 

 combinations, the sulphides of lime, etc., show this property. 



{Note. — I hold, with Hagenbach, the identity in nature of fluores- 

 cense and phosphorescence.) 



But if we confine ourselves to bodies whose fluorescent properties 

 are easily perceived, our list is by no means so long. 



One of the first substances in which this property was perceived is 

 a solution of the acid sulphate of quinine. 



This I will exhibit to you in the following manner : 



I have here some large vessels of water, so placed that they will be 

 lit by the electric light, filtered of all its blue and violet rays by means 

 of a plate of glass. 



Into one of them I pour a little of this solution of quinine sulphate, 

 and as it mixes with the water you see that it all lights up with a 

 blue luminosity which makes it seem opaque, but yet this candle 

 shows clearly through it, and, by the gas-light, the water looks as clear 

 as before. 



Into this other jar I pour a solution of sesculin, the principal of 

 the horse-chestnut bark, and you see that it lights up in a similar 

 manner. The quantity required in this case is so small that a 

 fluorescence could be detected in water which contained but 

 1-20,000,000 part of assculin. 



