364          ON  THE  CHANGE  OF  REFRANGIBILITY  OF   LIGHT.
Effect of Heat on the Sensibility of Glass, etc.
184.    The sensibility of glass is temporarily destroyed by heat. The glass may be heated by holding it in the flame of a spirit-lamp, as a heat much short of redness is sufficient.    This takes place even with glass coloured by oxide of uranium, which is in general so highly sensitive.    The sensibility returns again  as the glass cools.    A bead of rnicrocosrnie salt, containing uranium  in  its highest  state  of   oxidation,  is  very  sensitive   when   cold,   but insensible when hot.    The sensibility gradually comes on as the bead cools.    A solution of nitrate of uranium in water on being heated has its sensibility impaired, very much so by the time the temperature reaches the boiling-point.    The sensitive compounds, whatever may have been their precise nature, obtained by fusing the sulphates of soda and potassa on charcoal before.^4 the blowpipe, were insensible while hot.    The few vegetable solutions which I have  examined   with  this  object  did   not  seem   to  have  their sensibility affected by being heated.
Effect of Concentration* and Dilution.
185.    In investigating the change of rcfrangibility produced }jl                              by a sensitive substance in solution, it is almost always convenient
to have the solution weak. This however is by no means merely a matter of convenience, for the quantity of light which the medium is capable of giving back with a changed rcfmngibility is often materially diminished by increasing the concentration of the solution. Thus a solution which, when in a concentrated state, exhibits no sensible dispersive reflexion, will often exhibit when much diluted a very copious appearance of that nature. On the other hand, the dilution may of course he carried too far_3 so as to render imperceptible the peculiar properties of the substance dissolved. Yet it is wonderful what a decree of dilution a highly sensitive solution will bear before its sensibility ceases to be perceptible.
That the sensibility will be diminished, and will at last become imperceptible, if only the dilution be carried far enough, is nothing more than might have been predicted with the utmost confidence. In such a case the light passes completely through