426 Proceedings of the Royal Society 
read, undeterred by the taunts of his friend, he determined to subject 
these opinions to the test of experiment. The issue was his essay 
in 1798, entitled “ Experiments on Whinstone and Lava.” He 
first by fusion and quick cooling obtained a black vitreous mass 
from basalt, greenstone, porphyry, and greywacke ; and on again 
fusing these glassy bodies, and cooling them very slowly, he re- 
covered stony masses, “ entirely crystalline, with facets appear- 
ing in the solid parts,” much resembling the original rocks. In 
the case of basalt from our Castle rock, the resemblance was “ so 
strong in colour and texture, that it would be difficult to distinguish 
them.” Extending these experiments to lava, he tells us, in the 
first place, that travellers have given rise to erroneous ideas of the 
characters of lava, by bringing away with them only the superficial 
scoriae, and that the deeper parts present very much the appearance 
and texture of our trap rocks. Specimens of this kind he accord- 
ingly found to comport themselves exactly like greenstone and 
basalt, according to the rate of cooling. The lava of Mount Etna, 
near Catania, he found to resemble closely the columnar basalt of 
Arthur Seat, and that near Santa Yenere was very like the basalt 
of the Castle rock; and both of them presented the same varying 
phenomena as these rocks, when fused and then cooled quickly or 
slowly. 
But Sir James Hall’s greatest triumph was his subsequent ex- 
perimental inquiry, not produced to the Society till 1805, and con- 
sequently a little beyond the period included in my present sketch, 
“ On the Effects of Compression in modifying the Action of Heat.” 
By a series of difficult, dangerous, costly, but skilfully contrived 
experiments, he ascertained that carbonate of lime is a fusible 
body, if, while exposed to intense heat, it be also subjected to 
powerful pressure, so as to prevent the conversion of its carbonic 
acid into gas. He also found that, according to the degree of heat, 
and consequently of approach to perfect fusion, he could produce, 
under proportionally high pressures, varying from 52 up to 173 
atmospheres, the latter of which corresponds to a mile of sea, 
every essential character which carbonate of lime variously assumes 
in the mineral world, from the slightly cohesive chalk to the firm 
solid structure of opaque secondary limestone, the crystalline struc- 
ture of translucent marble, and even the transparency and rhom- 
