MR. ROBERT MALLET OX VOLCANIC ENERGY. 
193 
grounds) that a solidified shell equal in thickness to half the diameter of the neck, 
or to 7 ^, had formed all round the interior of the globe, so that at that moment a b was 
the diameter of the still liquid ball of fused rock within this crust. Cavities were formed 
within this by the final cooling and consolidation of the whole, which Bischoff supposes 
represent its total contraction. If, he says, the temperature of the liquid rock running 
into the sphere were exactly known, with the weight of the whole ball and that of a b 
and the specific gravity of the whole, we should have enough to determine the total 
contraction. As it is he applies to his assumed ball a b the unreliable coefficient of 
contraction deduced from his crucible experiments, and so arrives at a result. 
In one instance, at least, even this result is further vitiated by an extraordinary over- 
sight. 
His calculation is thus : — 
Diameter of the sphere N M 21 inches. 
Thickness of the crust ^r=N a or 5M— 1^ or 2 inches. 
Diameter of the liquid ball ab— 21 — 1^ or 2 inches=19, or 19|- inches; average 
19 J inches. 
Cubic content of this ball = 3733 cubic inches. 
Contraction =37 3 3 X 0*06 (the coefficient of the crucible experiment) = 22 4 cubic inches. 
Now on Bisciioff’s own data the calculation is as follows : — 
Diameter, as before, 21 inches. 
Diameter of liquid ball ab — 21 — 2 (1-J or 2 inches) 
= 21 — 3 or 4 inches, 
= average 17 or 18 inches, or mean 17^ inches. 
Cubic content of ball of that diameter =2786 '87 cubic inches. 
Contraction 2787x0*06 = 167 cubic inches. 
136. Mr. David Forbes (‘ Chemical News’ for October 1868) has noticed, though not 
in any detail, the general unreliability of Bisciioff’s results. He, however, endeavours to 
substitute for these deductions of his own from experiments still more fallacious, in 
support of his notion that the acid or basic silicates of which our earth’s rocks consist 
scarcely contract at all in passing from the liquid to the solid state. 
137. His facts are derived from the supposed dimensions of fused Eowley Bag stone 
(basalt) in the liquid state, and when again solidified after having been cast into moulds at 
Messrs. Chajstce’s glass-works to form thin patent artificial stone, and from melted slags 
cast by himself into cast-iron ingot moulds of moderate capacity. 
As regards the first, the moulds into which the fused rock was cast were formed of 
what founders call “ dry sand,” i. e. loamy sand, in which the impression of a wood 
“pattern” or model is made, the moulds being then dried and heated to something like 
a red heat, at which temperature the liquid rock is poured into them to fill the cavity. 
