166 Scientific Proceedings, Royal Dublin Society. 
minerals are those containing iron or manganese as constituents, 
and it sometimes happens that a silicate which, when containing 
a small quantity of iron (as what is commonly called an impurity), 
is pleochroic, loses the pleochroism when the ferruginous impurity 
is absent. If then we have a ferrous constituent acting in one 
direction, and no ferruginous constituent in another, the mineral 
may be colourless in the latter, and green in the former direction. 
If there exist a ferrous in one direction, and a ferric constituent 
in another direction, the salt may be expected to be green in the 
former, and yellow or reddish in the latter direction. Other cases 
will naturally suggest themselves. 
Although this paper is only an abstract of the demiled Memoir, 
I cannot let the opportunity pass of thanking my friends who 
have helped me with their criticisms and advice in this inquiry. 
Dublin is fortunate in possessing a number of distinguished phy- 
sicists—I need only mention Professors Fitzgerald and Preston, 
and Doctors Johnstone Stoney, Joly, and Trouton, from all of 
whom I have received great help in discussion, but chiefly from 
Professor Fitzgerald, without whose encouragement the investiga- 
tion, which was commenced some four or five years ago, would 
never have been completed. 
