138 THE HISTORY OF SCIENCE 



was encouraged in this view by Black, who, as al- 

 ready implied, had made a special study of limestone 

 and had demonstrated that lime acquires its caus- 

 ticity through the expulsion of carbonic acid. 



Hall conjectured in addition that the rate at which 

 the fused mass cooled might have some bearing on 

 the structure of igneous rocks. An accident in the 

 Leith glass works strengthened the probability of 

 his conjecture and encouraged him to experiment. 

 A pot of green bottle-glass had been allowed to cool 

 slowly with the result that it had a stony, rather a 

 vitreous structure. Hall experimenting with glass 

 could secure either structure at will by cooling rap- 

 idly or slowly, and that with the same specimen. 



He later enclosed some fragments of whin stone in 

 a black-lead crucible and subjected it to intense heat 

 in the reverberating furnace of an iron foundry. 

 (He was in consultation with Mr. Wedgwood on the 

 scale of heat, and with Dr. Hope and Dr. Kennedy, 

 chemists.) After boiling, and then cooling rapidly, 

 the contents of the crucible proved a black glass. 

 Hall repeated the experiment, and cooled more slowly. 

 The result was an intermediate substance, neither 

 glass nor whinstone a sort of slag. Again he heated 

 the crucible in the furnace, and removed quickly to 

 an open fire, which was maintained some hours and 

 then permitted to die out. The result in this case 

 was a perfect whinstone. Similar results were ob- 

 tained with regular basalts and different specimens 

 of igneous rock. 



Hall next experimented with lava from Vesuvius, 

 Etna, Iceland, and elsewhere, and found that it be- 

 haved like whinstone. Dr. Kennedy by careful chem- 



