1 86 The Founders of Geology LECT. 



master, could not agree with him in this verdict. He was 

 confirmed in his opinion by an accident which had 

 occurred at the Leith glass- works, where a large mass of 

 common green glass, that had been allowed to cool 

 slowly, was found to have lost all the properties of glass, 

 becoming opaque, white, hard and crystalline. Yet a 

 piece of this substance, when once more melted and rapidly 

 cooled, recovered its true vitreous characters. Hall's 

 shrewd instinct at once applied this observation to the 

 Huttonian doctrine of the igneous origin of granite and 

 other rocks. It had been objected to Button's views 

 that the effect of great heat on rocks was to reduce 

 them to the condition of glass, but that granite and whin- 

 stone, being crystalline substances, could never possibly 

 have been melted. Yet here, in this glass-house material, 

 it could be demonstrated that a thoroughly molten glass 

 could, by slow cooling, be converted into a crystalline 

 condition, and could be changed once more by fusion into 

 glass. Hutton had overlooked the possibility that the 

 results of fusion might be modified by the rate of cooling, 

 and Hall at once began to test the matter by experiment. 

 He repeated the process by which the devitrified glass 

 had been accidentally obtained at the glass-house, and 

 found that he could at will produce, from the same mass of 

 bottle glass, either a glass or a stony substance, according 

 to the rate at which he allowed it to cool. 



Sir James was too loyal a friend and too devoted an 

 admirer of the author of the TJieory of the Earth to pursue 

 these researches far during the philosopher's lifetime. " I 

 considered myself as bound," he tells us, " in practice to 

 pay deference to his opinion, in a field which he had already 



