532 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBIIJTY OF LIGHT. 
to me to be too complex to allow us to deduce any conclusion from the result respect- 
ing the direction of vibration. Besides, the result itself admits of easy explanation, 
by attributing it to the light which has entered the substance of the paper and come 
out again, which might be expeeted to be polarized by refraction. 
Effect of Heat on the Sensibility of Glass, 8fc. 
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 microcosmic 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 the blow- 
pipe, 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 refrangibility produced 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 refrangibility 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 be carried too far, so as to render imperceptible the 
peculiar properties of the substance dissolved. Yet it is wonderful what a degree 
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 pre- 
dicted with the utmost confidence. In such a case the light passes completely 
through the fluid long before it has produeed all the effect which it is capable of pro- 
ducing. But that concentration should be an obstacle to the exhibition of the phe- 
nomenon is not perhaps what we should have expected, and deserves an attentive 
consideration. 
186. Imagine a given sensitive substance to be held in solution, in a vessel of which 
the face towards the eye is plane, and the breadth in the direction of vision as great 
as we please ; and suppose the solvent, or at least the fluid used for diluting the solu- 
