Some general remarks of Mr. Heinrich, on the phos phorescence excited by exposure to light, belong also to this division of our article ; and they serve to explain a point of the subject which Dessaignes and others have left somewhat imperfect. The first relates to the duration of the light, which varies exceedingly in the different sub stances susceptible of this property, to whatever class of bodies they may belong. Thus, for example, while the diamond and fluor spar will continue to display their light for an hour, there is scarcely another mineral in which it lasts for a minute. No relation has been observed be tween the brightness and the duration of the light.
In all cases, except that of the diamond, where light is emitted by minerals, it is white; and this result is not al tered, whether they are caused. to shine in consequence of the white light of the sun or the day, or by exposure to the coloured rays of the prism.' Although the direct rays of the sun have greater power in exciting the luminous action than the light of day only, long exposure to these is injurious, as, by exciting heat in the substance, it di minishes the phosphorescent power. The white substances
are more active in their powers of yielding light than the coloured ones, and this property also diminishes as we ap proach nearer to the dal kest or black substances. Immer sion in water does not extinguish the light of bodies that are in the act of emitting it ; nor is any very sensible dif ference produced by change of temperature.
In cases of exposure to heat, we formerly remarked, that the whole substance throughout was luminous ; and the same was observed in those experiments where the phosphorescence was excited by light. Grooves being cut deeply into the substances when in a luminous state, it was observed to be emitted by the deepest of these as strongly as by the surface. Mr. Heinrich has also remark ed, in confirmation of Dessaigne's observations, that, in some cases, polishing destroyed the phosphorescent qua lity in those substances which were luminous when rough, while in others the light was only diminished.