1864. | Mining, Mineralogy, and Metallurgy. 489 
vacuum, which is again filled on heating the crystal ; so that the tempera- 
ture of the crystals at the time of their formation may be approximately 
determined.” . . . Mr. Sorby has determined this temperature, and 
“represents the lowest temperature at which the consolidation could have 
taken place, which varies from 340° C, to 380°C, in the Vesuvian minerals, 
and 356° in the quartz of the trachyte of Ponza; to the mean of 216° in 
the Cornish granites, to 99° in those of the Scottish Highlands, and even 
descends to 89° in some parts of the granite of Aberdeen. Mr. Sorby has 
calculated the pressure in feet, of rock, which would be required to com- 
press the liquid so much that it would just fill the cavities at 360°C. The 
numbers thus obtained will therefore represent the actual pressure, pro- 
vided the rock was in each case consolidated at that temperature. It 
would thus appear that the trachyte of Ponza was solidified near the sur- 
face, or beneath a pressure of only 4,000 feet of rock ; while for the Aber- 
deen granite the pressure was equal to not less than 78,000 feet, and for 
the mean of the Highland granites 76,000. The Cornish granites vary 
from 32,400 to 63,000, and give, as a mean, 50,000 feet of pressure. In 
this connection Mr. Sorby remarks that, from Mr. Robert Hunt’s observa- 
tions on the mean increase of temperature in the mines of Cornwall, a 
heat of 360° C. would be attained at a depth of 53,500 feet.” 
In relation to this subject, the experiments made by Dr. Fairbairn, 
in Dukinfield Colliery, should be named. Ina lecture on “ Natural 
Laws’’* recently delivered by that gentleman, the results are thus 
given. It should be stated that Dukinfield is above 2,100 feet, or 
upwards of 350 fathoms in depth. “The amount of increase indi- 
cated in these experiments is, from 51° to 57° 40’, from 20 to 693 feet 
below the surface, or 1° in 99 feet; but, if we state the results which 
are more reliable, namely, those between 693 and 2,055 feet, we have 
an increase of temperature from 57° 40’ to 75° 30’, or a mean increase 
of 1° in 76°8 feet. This rate of increase is not widely different from 
that obtained by other authorities, such as Walfeoden and Arago, who 
found an increase of 1° in 59 feet. Other experiments have given an 
increase of 1° in 71 feet.” Dr. Fairbairn, with Dr. Joule, made an 
extensive series of experiments to ascertain at what depth beneath the 
surface of the earth the rocks would become fluid with this increase of 
temperature. “If,” says Dr. Fairbairn, “we assume the rate of in- 
crease to be continued to a depth of nearly 3 miles, we arrive at the 
temperature of boiling water ; at 39 miles we attain an amount of heat 
equivalent to 3,000°, which would melt the hardest rocks.” The ex- 
periments to which we have referred were specially to determine if the 
melting point of bodies was influenced by pressure—the result was, 
“an increase in the temperature of fusion proportional to the pressure to 
which the fused mass was subjected.” ‘* All these conditions tend to 
increase the solid thickness of the earth’s crust, and we may venture to 
state that ata depth of 100 miles we should find a pressure equal to 
1,200,000 Ibs., or nearly 600 tons on the square inch.” The ratio of 
increase in the temperature of fusion being 1° for every 500 Ibs. pres- 
sure; therefore, taking 2,000° F. as the temperature of fusion at the 
* Two Lectures on Iron and its Applications, &c., and on Natural Laws, 
delivered to the members of the Literary and Philosophical Society, Newcastle-on- 
Tyne, by William Fairbairn, C.E., LL.D., F-R.S., F.G:S. 
