116 THE GREAT WORK OF SORBY : 



ground to a thinness of about one-thousandth of an inch allowed 

 light to pass, and with the Microscope it became possible to see their 

 structure more clearly than the texture of the coarsest granite had 

 hitherto appeared. Rock-slices, having been ground down flat, 

 were admirably adapted to the application of polarized light, and to 

 one who had already a working knowledge of optics, the vagaries of 

 the vector variations of the optical properties of minerals proved to 

 be no deterrent. Finding no treatise on this subject ready-made, 

 Sorby designed, and, with his own hands constructed, a polariscope 

 to work either with parallel or with convergent light, and the very 

 instrument which he then made is still in use in the Sheffield 

 University Physical Laboratory. 



Researches on Me«eon7e5.— Subsequent to his early Petrological 

 Researches, Sorby turned his attention to the Microscopical Study 

 of Meteorites. 



In his Paper " On the Microscopical Structure of Meteorites " 

 (Royal Society Proceedings, 1864, p. 333) he pointed out that he had 

 applied to the Study of Meteorites the principles he had made use of 

 in the investigation of terrestrial rocks described in his various Papers 

 and specially in that on the Microscopical Structure of Crystals (Quar- 

 terly Jnl. Geol. Soc. 1858, Vol. XIV, p. 453). He there showed that 

 the presence in Crystals of " fluid, glass, stone, or gas cavities " enabled 

 the conditions under which the crystals were formed to be 

 satisfactorily determined. There were also other methods of 

 enquiry still requiring much investigation and a number of experi- 

 ments to be made, but not wishing to postpone the publication of 

 certain facts he gave a short account of them in this Paper. 



This Paper was followed by another " On the conclusion to be 

 drawn from the Physical Structure of some Meteorites" (B.A. Report,. 

 1864, p. 70), in which Sorby pointed out that he had previously ^ ho wn. 

 that the earliest condition of meteorites of which their microscopical 

 structure furnishes evidence was that of igneous fusion. There 

 were, however, some, like the Pallas Iron, consisting of a mixture of 

 Iron and Olivine which apparently strongly opposed this view if 

 judged from what occurred when melted artificially ; for then the 

 Iron being so much more dense would sink to the bottom and the 

 Olivine rise to the top like slag in a furnace. The object of thi& 

 Paper was however to show that this difference in density 

 depended on the force of gravitation and that, on the surface 

 of a small planetary body, or towards the interior of a larger 

 planetary body, Iron and Olivine might remain mixed in a. 

 state of fusion long enough to allow of gradual crystallisation. 

 Such meteorites should therefore be considered evidence of fusion 

 where the force of gravitation was very small ; and this conclusion 

 might be valuable in deciding between rival theories of their origin. 



Application of Sorby's Work to Metallurgy.— At the time these 

 researches were carried out, although the Science of Metallurgy had 

 advanced at a great rate, Chemical analysis remained the ultimate 

 arbiter of the quality of any metal. The work, however, of Gore, 



