140 GEOLOGY OF THE LOTHIANS. 



exhibited a fracture like that of porcelain, with little lustre ; 

 and its colour was changed from black to dark brown. Being 

 exposed to heat, it was found to be fusible only at 31° ; that 

 is, it was less fusible than the glass by 13° or 14°. 



" Numerous and varied experiments have since proved, in 

 the clearest manner, that, in any temperature, from 21° to 

 28° inclusive, the glass of this whin passes from a soft, or 

 liquid state, to a solid one, in consequence of crystallization ; 

 which is differently performed at different points of this 

 range. In the lower points, as at 23, it is rapid and imper- 

 fect ; in higher points, slower and more complete, every in- 

 termediate temperature affording an intermediate result. 

 I likewise found, that crystallization takes place, not only 

 when the heat is stationary, but likewise when rising or 

 sinking, provided its progress through the range just men- 

 tioned is not too rapid. Thus, if the heat of the substance, 

 after fusion, exceeds one minute in passing from 21° to 23°, 

 or from 23° to 21°, the mass will infallibly crystallize, and 

 lose its vitreous character. These facts enabled me to account 

 for the production of the substance resembling the liver of 

 an animal, which I obtained in my first attempts to crystal- 

 lize the melted stone. Not being then aware of the tem- 

 perature proper for complete crystallization, I had allowed 

 it to be passed over rapidly by the descending heat, and I 

 had begun the slow cooling in those lower points, at which 

 the formation of this intermediate substance takes place. 



" By the same means I was enabled to explain the unex- 

 pected result, which I obtained in endeavouring to convert 

 the glass of this stone into crystallite,* The fire applied 

 to the crucible, containing fragments of the glass, had been 



* The crystallized substance, obtained from the slow cooling of trap- 

 glass, was denominated crystallite by Sir James Hall. 

 5 



