40 Prof. Galbraith on the Composition of the Felspars of 



eight minutes, is about yj^o of the mechanical value of the 

 earth's motion. As the mean velocity of the vibrations might 

 be many times greater than has been supposed in this case, the 

 mass of the medium might be considerably less than this ; but 

 we may be sure it is not incomparably less, not 100,000 times 

 as small for instance. On the other hand, it is worth remarking 

 that the preceding estimate shows that what we know of the 

 mechanical value of light renders it in no way probable that the 

 masses of luminiferous medium in interplanetary spaces, or all 

 round the sun in volumes of which the linear dimensions are 

 comparable with the dimensions of the planets 5 orbits, are other- 

 wise than excessively small in comparison with the masses of the 

 planets. 



But it is also worth observing that the luminiferous medium 

 is enormously denser than the continuation of the terrestrial 

 atmosphere would be in interplanetary space, if rarified accord- 

 ing to Boyle's law always, and if the earth were at rest in a space 

 of constant temperature with an atmosphere of the actual den- 

 sity at its surface*. Thus the mass of air in a cubic foot of 

 distant space several times the earth's radius off, on this hypo- 

 thesis, would be jjj> — Tr&ib > wn ^ e there cannot, according to 



the preceding estimate, be in reality less than *-=? — - — Vtttt. which 



J 1560 x 10 17 



is 9 x 10 327 times as much, of matter in every cubic foot of space 



traversed by the earth. 



VI. On the Composition of the Felspars of the Granite of the 

 Dublin and Wicklow Mountains. By the Rev. Joseph A. 

 Galbraith, Erasmus Smith's Professor of Natural and Ex- 

 perimental Philosophy in the University of Dublin. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IN the Mourne Mountains of the county Down we have a 

 well-known case of the occurrence of albite as a constituent 



* "Newton has calculated (Princ. iii. p. 512) that a globe of ordinary 

 density at the earth's surface, of 1 inch in diameter, if reduced to the den- 

 sity due to the altitude above the surface of one radius of the earth, would 

 occupy a sphere exceeding iu radius the orbit of Saturn." — (Herschel's 

 Astronomy, Note on § 559.) It would (on the hypothesis stated in the 

 text) we may now say occupy a sphere exceeding in radius millions of mil- 

 lions of times the distances of any stars of which the parallaxes have 

 been determined. A pound of the medium, in the space traversed by the 

 earth, cannot occupy more than the bulk of a cube 1000 miles in side. The 

 earth itself, in moving through it, cannot displace less than 250 pounds of 

 matter. 



