The cube of fluor spar which showed these effects was externally colourless to the depth of 1/20 of an inch; then came one or two strata parallel to the faces of the cube showing the ruddy fluorescence, while the blue fluorescence extended to a much greater depth and had a stratified appearance. This crystal was eminently phosphorescent, its blue phosphorescence being arranged in strata parallel to the face of the cube like the blue fluorescence, but it was not perceptible beyond a very moderate distance below the surface at which the exciting cause entered, that cause being the photographic rays of extremely high refrangibility of the electric spark—taken, as in all these experiments, between the secondary terminals of an induction coil in connection with the coatings of a Leyden jar, and refracted by quartz prisms. Mr. Stokes has employed fluorescence as a means of tracing substances in impure chemical solutions. When a pure fluorescent substance 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 certain point of the spectrum, varying from one substance to another, and continues from 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 examined 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.’ Experience as well as theory shows that rapid absorption 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. The phosphorescent light of insects, fish, and plants is owing to chemical action, which produces many luminous phenomena; but a great number of inorganic and organic substances shine in the dark with a phosphorescence which is nearly allied to fluorescence. It is produced by exposure to the sun, by heat, electricity, insulation, cleavage, friction, and motion. For if a bottle containing nitrate of uranium be shaken, it shines spontaneously with a vivid light; even the hand shows phosphorescence in the dark after being exposed to the sun.