420 The Rev. J. A. Galbraith on the Felspars occurring 



Thomson (Phil. Trans. 1854), have shown that the quantity de- 

 noted by K is practically inappreciable; so that both this quan- 

 tity, and the terms containing h as a factor, may be neglected 

 in equation (25) and those which follow it, until there is experi- 

 mental evidence of their having sensible values. 



In the preceding paper, the laws of the expansive action of 

 heat are deduced from a mechanical hypothesis, called that of 

 Molecular Vortices. Those laws are capable of being expressed 

 and proved independently of any hypothesis (see Phil. Trans. 

 1854, parti, and a paper "On the Science of Energetics," Edinb. 

 Phil- Journ. July 1855) ; but it is nevertheless considered, that 

 a molecular hypothcfis which has already led to the anticipation 

 of some laws subsequently coufirmed by experiment, may possibly 

 lead hereafter to the anticipation of more such laws, and may at 

 all events be regarded as interesting in a mathematical point of 

 view ; although its objective reality, like that of other molecular 

 hypotheses, be incapable of absolute proof. 



LVIII. On the Felspars occurring in the Granites of the Dublin 

 and TVicklow Mountains. By tlie Rev. Joseph A. Galbraith, 

 Erasmus Smith's Professor of Natural and Experimental P/d- 

 losophy in the University of Dublin. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



IN the last Number of your Journal, Mr. Jennings observes 

 that I am incorrect in stating that it is necessary to deter- 

 mine the quantities of silex, alumina, &c. in a rock or mineral 

 in order to obtain the alkalies which it contains. I beg leave to 

 say that I never made such a statement ; I certainly did observe, 

 in reference to a single and incomplete analysis which he fur- 

 nished in order to settle an important question, that where accu- 

 racy was an object, it was most desirable to have a complete 

 analysis, in which each constituent was weighed, and this for the 

 obnous reason, that if all the parts, reduced to per-centages, 

 on being added together yielded one hundred, the utmost confi- 

 dence might be placed in a result which thus checked itself: in 

 the justice of this statement I am sure that every chemist who 

 reads your Journal will acquiesce. 



With regard to the mineral constitution of the granite rocks 

 of the Dublin and Wicklow chain, the question at issue is that 

 which was raised about three years ago by Sir Robert Kane, 

 when he stated to the Royal Irish Academy that the soda in 

 these granites was so preponderant over the potash, as to lead 

 to the conclusion that the potash should be refeiTed to the mica 



