BOCKS. 259 



Experiments on the changes which the texture and 

 chemical constitution of rocks experience from the action of 

 heat, have shown that volcanic masses,* (diorite, augitie 

 porphyry, basalt, and the lava of Etna) yield different pro- 

 ducts/ according to the difference of the pressure under which 

 they have been fused, and the length of time occupied during 

 their cooling ; thus, where the cooling was rapid, they form 

 a black glass, having a homogeneous fracture, and where the 

 cooling was slow, a stony mass of granular crystalline structure. 

 In the latter case, the crystals are formed partly in cavities 

 and partly enclosed in the matrix. The same materials yield 

 the most dissimilar products, a fact that is of the greatest 

 importance in reference to the study of the nature of erupted 

 rocks, and of the metamorphic action which they occasion. 

 Carbonate of lime when fused under great pressure, does not 

 lose its carbonic acid, but becomes when cooled, granular 

 limestone; when the crystallization has been effected by 

 the dry method, saccharoidal marble; whilst by the humid 

 method, calcareous spar and aragonite are produced, the 

 former under a lesser degree of temperature than the latter.'j' 

 Differences of temperature, likewise, modify the direction in 

 which the different particles arrange themselves in the act of 

 crystallization, and also affect the form of the crystal. J Even 

 when a body is not in a fluid condition, the smallest particles 

 may undergo certain relations in their various modes of 

 arrangement, which are manifested by the different action on 

 light. The phenomena presented by devitrification, and by the 

 formation of steel by cementation and casting, the transition 

 of the fibrous into the granular tissue of the iron, from the 

 action of heat,|| and probably, also, by regular and long con- 



* Sir James Hall, in the Edin. Trans., vol. v. p. 43, and vol. vi. p. 71 ; 

 Gregory Watt, in the Phil. Trans, of the Roy. Soc. of London for 1804, 

 Pt. ii. p. 279 ; Dartigues and Fleariau de Bellevue, in the Journal de 

 Physique, t. Ix. p. 456; Bischof, Wdrmelehre, s. 313 und 443. 



t Gustav Rose, in Poggend. Annalen, bd. xlii. s. 364. 



J On the dimorphism of sulphur, see Mitscherlich, Lehrbuch der 

 Chemie, 55-63. 



On gypsum as a uniaxal crystal, and on the sulphate of magnesia, 

 and the oxides of zinc and nickel, see Mitscherlich, in Poggend. A nnalen, 

 bd. xi. 8. 328 



^ II Coste. Versuche, am Creusot -uher das Iruchig iverden dt* Stab' 

 eisens. Elie de Beaumont, Mem. GeoL, t. ii. p. 411. 



s2 



