On the Felspars of the Dublin and WicUow Mountains. 115 

 article on polarization produced by compression thus expressed 

 hhuself — "The phenomena of cleavage and of hemitrope forms 

 would clearly indicate that the integrant molecules are spheres 

 lach hanug sl. poles on its surface." This article was quoted 

 by Dana hAhe American Journal of Science, and formed the 

 W of his investigations, without however being acknowdedged 

 Sii David Brewste? goes on to say that ^t --vin^ to the ^^^^^^ 

 ferent de-rees of force possessed by the different pole, that 

 he different cleavages are due; and m this he is dearly in 

 error For if a cube be divided either by the cubical or octahe- 

 dral cleavage, every molecule will eventually become separa ed 

 from ery'other/ln fact, to divide a crystal m which the 

 amn^ement is cubical by the octahedral cleavage, we must at 

 Iridic, separate ever^iolecule frem t^ree adjacex^t mo^^^^^ 

 cules or in other words, we must overcome the cohesion ot 

 h ee of its adjacent poles; but it was to the unequal cohe 

 of these very three poles that he ascribed the liabihty to 



sion 

 cleavage. 



Dana"also, as I have already stated, pointed out that in the 

 seconTand'remaining systems the -o^-ules are no long 

 spheres, but spheroids or ellipsoids ; and with this niocli- 

 ficatS all I have said applies word for word to the other 

 systems. 



XV. On the Felspars occurring in the Gramtes oftheDuUmand 

 Wickloiv Mountains. By the Rev. Joseph A Galbraith 

 mT Erasmus Smith's Professor of Natural and Experimental 

 Philosophy in the University of Dublin. 

 To the Editors of the Philosophical Magazine and Journal 



Gentlemen, 

 A COMMUNICATION from Mr. Jennings which appeared 

 A in your last Number, on the Felspars of the Dublin 

 Mountains! makes it necessary for me to request that you will 

 rnse'f r your Journal a short account o a discussion which 

 arose some time siace m the Royal Irish Academy on this 



'"*f'n'l*he month of January 1853, Sir Robert Kane brought 

 under the nSe of the Academy an analysis of the waters which 

 escend frmn the sides of the Dublin Mountains. The following 

 tSs analysis of the residue after evaporation, of what he con- 

 sidered to be a typical specimen :— 



