But the phenomenon of phosphorescence finally {61} disposes of any such description, for the two phenomena have no sharp distinction between them. Some substances are known in which the phosphorescence lasts for such an extremely small fraction of a second after the stimulating waves are withdrawn that it is difficult to know whether to call the effect fluorescence or phosphorescence. It is probable, therefore, that both are due to the same action. Now a wave of orange light completes about five hundred million million vibrations in one second, and therefore if an orange-coloured phosphoresence were to last for only one five-hundredth of a second it would mean that the electrons responsible for it vibrate one million million times after the stimulus is removed. This is hardly credible, and becomes more credible when we remember that in some phosphorescent substances the effect lasts for many hours. +Chemical Theory of Phosphoresence.+--It is more probable that the stimulating rays produce an actual chemical change in the phosphorescent substance. For instance, it is possible that the vibrations of a certain type of electron in one kind of atom become so violent as to detach it from the atom and the temporarily free electron attaches itself immediately to another kind of atom. The new arrangement may be quite stable; it is so in the action of light on a photographic plate, but it may only be stable when the electrons are being driven out of their original atoms, and in this case the electrons will begin to return to their old allegiance as soon as the stimulus is withdrawn. In the return {62} process the electrons will naturally be agitated, and will therefore emit waves having their characteristic period. The rate at which the return process takes place will evidently depend upon the stability of the new arrangement. If it is extremely unstable, the whole return may only occupy a fraction of a second, but if it is nearly as stable as the original arrangement the return may be extremely slow. On this view, then, those substances will phosphoresce which have an electron which is fairly easily detached from its atom and which will attach itself to another atom, forming an arrangement which is less stable than the original. +Temperature and Phosphorescence.+--A confirmation of this chemical view is provided by the effect of temperature on phosphorescence. The rate of a chemical change is usually very largely increased by rise of temperature, and further, at very low temperatures a large number of chemical changes which take place quite readily at ordinary temperatures do not take place at all.