me. eobert mallet on volcanic energy. 
187 
reduced to powder is to that which would have been necessary to reduce the entire cube 
to powder as the weight of the crushed part to the weight of the entire ; so that the 
weight of those large uncrushed fragments having been ascertained subsequently, this 
correction was applied. 
11S. As the total descent of the plunger is in all cases small, less than l - 5 inch, the 
acceleration of its descent is not considerable, the rather as by the construction of the 
testing-machine a certain proportion of the crushing-pressure was relieved from the 
lever as soon as the plunger began to descend considerably, so that for the remainder 
of the descent the plunger was urged downwards principally by the weight of the heavy 
end of the large lever above it. Moreover, as each cube broke up at first into irregular 
prisms and wedge-shaped pieces closely adjacent to each other, which afterwards crunched 
down rapidly to powder, so the resistance to the plunger was not far from constant during 
its descent. 
In this manner the entire series was gone through, only varied by very careful balancing- 
off of the weight of the lever itself in the case of the very friable rocks, such as the 
Oolites. 
119. The final pressure at the moment of crushing gives the value of W, and the descent 
of the plunger, subject to the correction described, that of h in the preceding equation. 
120. In all the harder rocks, notwithstanding the close contact of the large mass of metal 
in the plunger and base-plate, the heat produced by the crushing down was easily per- 
ceptible by the hand, and was so great in the case of some of the granites and porphyries 
as to heat the plunger and base so much as to oblige a delay to let them cool to the 
same temperature as before, and as they were at in all the experiments, viz. about 57° Fahr. 
Had it been practicable to exclude the daylight, there can be little doubt but that a flash 
of light would have been found to attend each such crushing. 
121. The results, both immediate or direct, and those deduced from this laborious train 
of experiments, are comprised in the accompanying large Table, 'No. I. The specific gra- 
vities, column 3 in this Table, were obtained by the writer by weighing one of the spare 
cubes of each rock, first in air and then in water contained in a thin cylindrical glass 
jar with a ground circular glass cover, the weight of which, as also of its contents of 
distilled water at GO 0 Fahr., was known. This method, which is very much more rapid 
than the usually prescribed methods, is to be commended for all specific gravities of 
solids heavier than water. 
122. The specific heats of these several rocks (column 27) were obtained by the method 
of mixtures operating also upon these cubes, which were heated in boiling water, and then 
immersed in distilled water at temperatures of 50° to 54° Fahr., applying the usual cor- 
rections for the glass vessel containing the fluid, the thermometer, &c. 
So few specific heats have previously been determined for rocks that these may be 
found to possess some general interest. 
123. In column 8 and column 10 are given the mean loads per square inch at which 
the first signs of disintegration and at which the final crushing to powder occurred, 
mdccclxxiii. 2 c 
