200 
ME. EOBEET MALLET ON VOLCANIC ENEEG-Y. 
157. The solid conical blocks of slag cooling within the iron cones reached the atmo- 
spheric temperature in 12 hours from filling, parting with 3680° in that time; but 
consolidation commenced in 20 minutes (or -jg- of 12 hours) from running into the 
cones. If, therefore, we admit that the rate of cooling in the first 20 minutes was fiveAr 
six times as great as the mean rate (which the laws of cooling would seem to justify), 
then the heat at the stage of incipient consolidation was 
3680°— (6 x 103°)=:3062 o , or say 3000°. 
158. After each cone of slag was sufficiently solidified, the cone of iron (with the slag 
within) and the base-plate were lifted together, and in the vertical position, by a crane on 
to a waggon and carried off on the furnace tramway and deposited vertically upon a level 
iron platform within range of another crane, and so left to cool. After 24 hours the iron 
cone was gently lifted off the cone of slag it contained ; and owing to the contraction of 
the slag and to the taper form, widest at bottom, the parting took place very readily 
and without materially fracturing the cones of slag, which were found sufficiently 
regular in form and smooth in surface to admit of accurate measurement. 
159. The admeasurements were then made by means of steel callipers and scales for 
diameters and heights, the former being controlled by circumferential measurements 
with a well graduated steel flexible strip or tape*. 
1G0. On breaking up these cones no large segregated central core of matter different 
from the general mass was found, nor any large cavities. The exterior, where it had 
come into direct contact with the iron base or cone, was vitreous, and of a more or less 
bluish tint; but all the remainder showed itself, on examination with the naked eye or 
lens, as a tolerably uniform mixture of ash-grey crystals, more or less distinct, enveloped 
in a light greenish-yellow or pale-brown glass, the proportion of the crystalline to the 
glassy matter being very great, and greatest as we approached the centre, where in 
some places the grey crystals (resembling VVollastonite) were very clearly developed. 
The material operated on thus shows itself as a true crystallized rock, and not a mere 
vitreous mass. 
* [Since the reading of this paper it has been asked why the writer did not determine the volume of these cones 
of slag by weighing in air and in water, or, more directly, by making water-tight the base of the iron cone with 
the cone of slag within it, and ascertaining the volume of water requisite to fill up the vacancy between them. 
Both these methods occurred to me, and, indeed, were those employed (with mercury) by Bischoff, but were aban- 
doned by the writer, on the grounds of many practical difficulties, in favour of the method of direct measurement. 
The cones of slag being intersected by large though closed cracks in various directions could not be disturbed 
or weighed in air, still less in water, without much hazard, and the apparatus needed must have been powerful 
and expensive to make. The making the cast-iron cones water-tight at the inner edge of their bases would have 
been extremely difficult, as the iron cone, as soon as it was raised a few inches, could not be lowered again by 
reason of numerous minute fragments of slag detached having fallen down between ; some water under so large 
a head would have penetrated these cracks; and for either method so many sources of error would have arisen, 
and so many corrections would have had to be applied, that simple measurement appeared to promise the 
better results.] 
