459 
tact. When the temperature is raised until the bubble nearly 
disappears (which is seen by its contraction), the spirit-lamp is 
withdrawn, and the vanishing point carefully watched, and the tem- 
perature noted. The stopper of the retort is then withdrawn, and 
the stop-cock of the reservoir of water opened, so that the tempera- 
ture of the stage and mercury hath is soon reduced, and the ebulli- 
tion or reappearance of the bubble takes place, when the tempera- 
ture is again recorded. By this method the author felt confident 
that his results were correct, as they always were consistent when 
observing the same cavity. By means of this instrument the author 
had found fluid cavities in the trap tuffa of Arthur’s Seat, the 
greenstone of the Crags, and the basalt of Samson’s Bibs. He had 
also found that the porphyry of Dun Dhu in Arran, which most 
geologists assumed as of igneous origin, was full of fluid cavities 
contained in the doubly acuminated crystals of quartz for which 
this remarkable porphyry is distinguished. He also showed doubly 
acuminated crystals of quartz in the saliferous gypsums of India, 
both of which were full of fluid cavities, and the quartz impressed 
with the gypsum ; and as no geologist would hold that this forma- 
tion was of igneous origin, but that the quartz, if not contempo- 
raneous with the gypsum, must have been subsequent, and as the 
same phenomena were presented by the porphyry of Dun Dhu, he 
was forced to the conclusion that it was as much aqueous in its 
origin as the saliferous gypsum of India. The author exhibited a 
specimen of quartz which contained a crystal of iron pyrites, to 
which was attached a crystal of galena and also a small massy zinc 
blende, while over these three metals was laid a covering of gold. 
From this specimen he argued, that as all these metals were 
fusible at a much lower temperature than quartz, they must have 
aggregated during a gelatinous condition of the quartz ; and further, 
that as the sulphides of the three metals were in chemically com- 
bining proportions, any heat which would have fused the quartz 
would have made an alloy or a slag in which chemical combining 
proportions could not occur. 
He also exhibited specimens of schorl which he had obtained in 
the granite of Aberdeen, and drew the inference that schorl, which 
crackles and splits with a very small increment of temperature, 
could not have been present during a molten condition of the 
quartz ; and that it was crystallized prior to the solidifying of the 
