1869.] On the Structure of Rubies, Sapphires, Diamonds, Src. 291 



limb is observed, these bright lines indicating probably an outer shell of the 

 photosphere of a gaseous nature. 



iii. That a sun-spot is a region of greater absorption. 



iv. That occasionally photospheric matter appears to be injected into the 

 chromosphere. 



May not these facts indicate that the absorption to which the reversal 

 of the spectrum and the Fraunhofer lines are due takes place in the 

 photosphere itself or extremely near to it instead of in an extensive outer 

 absorbing atmosphere ? And is not this conclusion strengthened by the con- 

 sideration that otherwise the newly discovered bright lines in the solar 

 spectrum itself should be themselves reversed on Kirchhoff's theory? this, 

 however, is not the case. We do not forget that the selective radiation of 

 the chromosphere does not necessarily indicate the whole of its possible 

 selective absorption ; but our experiments lead us to believe that, were any 

 considerable quantity of metallic vapours present, their bright spectra would 

 not be entirely invisible in all strata of the chromosphere. 



February 18, 1869. 



Lieut. -General SABIXE, President, in the Chair. 



The Most Noble the Marquis of Salisbury and the Right Hon. Lord 

 Houghton were admitted into the Society. 



The following communications were read : — 

 I. " On the Structure of Rubies, Sapphires, Diamonds, and some 

 other Minerals." By H. C. Sorby, F.R.S., and P. J. Butler. 

 Received December 8, 1868. 



[Plate VII.] 



For many years Mr. Butler has had the opportunity of examining very 

 many rubies, sapphires, and diamonds, and has taken advantage of it in 

 forming a most interesting collection, cut and mounted as microscopical 

 objects. He had very carefully studied the included fluid- cavities, and 

 ascertained many curious facts. Mr. Sorby had for some time paid much 

 attention to the microscopical structure of crystals, and published a paper * 

 in which he showed that their microscopical characters often serve to 

 throw inuch light on the origin of rocks. Mr. Butler therefore placed the 

 whole of his collection in Mr. Sorby's hands for careful examination, and it 

 was decided that a paper should be written by the two conjointly ; and 

 since Mr. Sorby had previously made many experiments in connexion with 

 the expansion of liquids, as already described in a paper published in the 

 Philosophical Magazinef, he took advantage of the opportunity to investi- 



* Quarterly Journal of Geo!. Soc, 1S58, vol. xiv. p. 453. 



f " On the Expansion of Water and Saline Solutions at High Temperatures," August 

 1859, vol. xviii. p. 81. 



