Fluor spar, and some other substances, when raised to a temperature still under redness, emit light.  During the ages which have elapsed since their formation, this capacity of shaking the ether into visual tremors appears to have been enjoyed by these substances.  Light has been potential within them all this time; and, as well explained by Draper, the heat, though not itself of visual intensity, can unlock the molecules so as to enable them to exert their long-latent power of vibration.  This deportment of fluor spar determined Stokes in his choice of a name for his great discovery:  he called this rendering visible of the ultra-violet rays Fluorescence.

By means of a deeply coloured violet glass, we cut off almost the whole of the light of our electric beam; but this glass is peculiarly transparent to the violet and ultra-violet rays.  The violet beam now crosses a large jar filled with water, into which I pour a solution of sulphate of quinine.  Clouds, to all appearance opaque, instantly tumble downwards.  Fragments of horse-chestnut bark thrown upon the water also send down beautiful cloud-like strife.  But these are not clouds:  there is nothing precipitated here:  the observed action is an action of molecules, not of particles.  The medium before you is not a turbid medium, for when you look through it at a luminous surface it is perfectly clear.

If we paint upon a piece of paper a flower or a bouquet with the sulphate of quinine, and expose it to the full beam, scarcely anything is seen.  But on interposing the violet glass, the design instantly flashes forth in strong contrast with the deep surrounding violet.  President Morton has prepared for me a most beautiful example of such a design which, when placed in the violet light, exhibits a peculiarly brilliant fluorescence.  From the experiments of Drs. Bence Jones and Dupre, it would seem that there is some substance in the human body resembling the sulphate of quinine, which causes all the tissues of the body to be more or less fluorescent.  All animal infusions show this fluorescence.  The crystalline lens of the eye exhibits the effect in a very striking manner.  When, for example, I plunge my eye into this violet beam, I am conscious of a whitish-blue shimmer filling the space before me.  This is caused by fluorescent light generated in the eye itself.  Looked at from without, the crystalline lens at the same time is seen to gleam vividly.

Long before its physical origin was understood this fluorescent light attracted attention.  Boyle describes it with great fulness and exactness.  ‘We have sometimes,’ he says, ’found in the shops of our druggists certain wood which is there called Lignum Nephriticum, because the inhabitants of the country where it grows are wont to use the infusion of it, made in fair water, against the stone in the kidneys.  This wood may afford us an experiment which, besides the singularity of it, may give no small assistance to an attentive considerer