380         ON  THE  CHANGE  OF BEFRANOIBILITY  OF  LIGHT.
probability the principle to which the blue colour was due was capable of entering into firm combination with acids, but did not combine with alkalies. It is, in fact, as we know, a vegetable base*.
213.    A solution of nitrate of uranium in ether is insensible, as if some of the elements of the ether entered into firm combination with the oxide of uranium.    In connexion with this circumstance, it is rather remarkable, that although the ether passes off by evaporation when the solution is left to itself in an open vessel, if heat be applied chemical action sets in, and the residue consists chiefly of a salt which has all the appearance of oxalate of uranium.    This salt, when washed and examined in the moist state, without very great concentration of light, was found to be insensible f.
214.    It is rare to meet with solutions so highly sensitive as those of quinine and esculine, but similar observations  may be made  on  a  great  number of solutions, by employing suitable methods.     The  most  searching  method   consists   in   forming  a bright and tolerably pure  spectrum, by  transmitting the sun's light  through a very broad slit, or  even  leaving  out  the   si it altogether.    It is desirable to use a lens of only moderate focal length in connexion with the prisms.    The solution having been placed in the spectrum, the acid, or other agent whose reactions it  is  desired   to   study, is to be added, and  the effect,  if any, observed.    It is usually advantageous to cover the slit with a blue glass, or similar absorbing medium;  but, sometimes effects take place in the bright part of the spectrum, which is intercepted by such a medium.    When false dispersion abounds, it is well to look down on the fluid through a Nicol's prism, so as to stop all light which is polarized in the plane of reflexion.
Negative results with reference to a mutual action of the m//,s-incident on sensitive .volutions.
215.    The antagonistic effects of the more and less refrangible rays, which have been observed in certain phenomena, induced
* [It is not a base but a glucoside.j t See note I.