SORBY STRUCTURE OF CRYSTALS. 



453 



On Me Microscopical Structure of Crystals, indicating the 

 Origin of Minerals and Rocks. By H. C. Sorby, Esq., 

 F.R.S., F.G.S., Corresponding Member of the Lyceum of Natural 

 History of New York, and of the Academy of Natural Sciences of 

 Philadelphia, &c. 



[Read December 2, 1857.] 

 [Plates XVI.-XIX.] 



CONTENTS. 



History of the Subject. 



I. Structure of Artificial Crystals. 



§ 1. Crystals formed from Solu- 

 tion in Water. 



a. Mode of Preparation and Ex- 



amination ; general and 

 special characters. 



b. Number, size, form, and ar- 



rangement of Cavities. 



c. Expansion of Fluids by Heat. 



d. Effects of Pressure. 



e. The Elastic Force of the Va- 



pour of Water. 



§ 2. Crystals formed by Sublima- 

 tion. 



§ 3. Crystals formed by Fusion. 



§ 4. General Conclusions. 



II. Structure of Natural Crystals. 



§ 1. Methods employed in exa- 

 mining Minerals and Rocks. 



§ 2. Water contained in Crystals. 

 § 3. Minerals contained in Se- 

 condary Rocks. 



a. Rock-salt, Calcite, &c. 



b. Quartz-veins. 



§ 4. Metamorphic Rocks. 



§ 5. Minerals and Rocks formed 

 by cooling from a state of 

 igneous fusion. 



§ 6. Minerals and Rocks formed 

 by the combined operation 

 of Water and Igneous Fu- 

 sion. 



a. Minerals in the blocks ejected 



from Vesuvius. 



b. Granitic Rocks. 



c. Temperature and Pressure 



under which Granitic Rocks 

 have been formed. 

 Description of the Plates. 



In this paper I shall attempt to prove that artificial and natural 

 crystalline substances possess sufficiently characteristic structures to 

 point out whether they were deposited from solution in water or 

 crystallized from a mass in the state of igneous fusion ; and also that 

 in some cases an approximation may be made to the rate at, and the 

 temperature and pressure under which they were formed. 



History of the Subject. — The existence of cavities in quartz, topaz, 

 and fluor-spar, containing fluid, has long been known. Mr. Sivright 

 found them also in calcite, barytes, and selenite. Sir David Brewster 

 discovered them in emerald, beryl, cymophane, peridot, felspar, and 

 rock-salt, as well as in a number of crystals formed artificially. In 

 his admirable paper in the 'Transactions of the Royal Society of 

 Edinburgh' (1824, vol. x. pt. 1. p. 1), he remarks that, being per- 

 suaded that water thus mechanically enclosed will be found in every 

 crystal deposited from solution, he was next desirous of finding it in 

 crystals formed by heat or sublimation ; but, in no case having been 

 able to discover the slightest trace of its existence, he considered, in 

 the absence of all other information on the subject, the result highly 

 favourable to the supposition of the aqueous origin of all minerals in 

 which cavities containing water had been discovered. No distinc- 



