INTRODUCTION. 73 



thus paved the way for Darwin's still broader, biological con- 

 ceptions upon the same basis. 



Hutton's scientific spirit and genial personality won for him 

 many friends and adherents amongst the members of the 

 Edinburgh academy. The most distinguished of these were 

 Sir James Hall and the mathematician John Playfair. 

 Hall (1762-1831) contested the validity of the opinion held by 

 some of Hutton's opponents, that the melting of crystalline 

 rocks would only yield amorphous glassy masses. Hall 

 followed experimental methods; he selected different varieties 

 of ancient basalt and lavas from Vesuvius and Etna, reduced 

 them to a molten state, and allowed them to cool. At first he 

 arrived only at negative results, as vitreous masses were pro- 

 duced; but he then retarded the process of cooling, and 

 actually succeeded in obtaining solid, crystalline rock-material 

 (Nicholson's Journal, No. 38, 1800). By regulating the tem- 

 perature and the time allowed for the cooling and consolidation, 

 Hall could produce rocks varying from finely to coarsely 

 crystalline structure. And he therefore proved that under 

 certain conditions crystalline rock could, as Hutton had said, 

 be produced by the cooling of molten rock-magma. Hall then 

 put to the test Hutton's further hypothesis, that limestone also 

 was melted and re-crystallised in nature. To this hypothesis 

 the objection had been made that the carbonic acid gas must 

 escape if limestone were brought to a glowing heat, and the 

 material would be converted into quicklime. This was Hall's 

 first experience; then he devised another experiment. He 

 introduced chalk or powdered limestone into porcelain tubes 

 or barrels, sealed them, and brought them to a very high 

 temperature. The carbon dioxide gas could not escape under 

 these conditions. The calcareous material was thus subjected 

 to the enormous pressure of the imprisoned air, and carbonic 

 acid was converted under this pressure into a granular substance 

 resembling marble. Hall calculated from a series of successful 

 experiments that a pressure equivalent to fifty-two atmospheres, > 

 or to a depth of sea-water 1,700 feet below sea-level, was neces- 

 sary for the production of solid limestone, 3000 feet of depth for 

 that of marble, and 5,700 feet of depth in order to reduce 

 carbonate of lime to a molten state. 



These results were afterwards confirmed by other experi- 

 mentalists. Thus Werner's theory that crystalline rock repre- 

 sented in all cases a precipitate from water was shown to be 



