204 
ME. EOBERT MALLET ON VOLCANIC ENERGY. 
Works of the Thames Plate-Glass Company at Blackwall; and the writer is indebted 
to the intelligent Manager, Mr. F. M. Waller, for extracts from his books for the 
manufacture of the year 1861, giving those measurements taken from about 40,000 
superficial feet of plate-glass. 
169. From a reduction of these it appears that the total surface of the hot glass being 
36172 square feet, that of the same at atmospheric temperature was 35692. Therefore 
the linear dimensions were as the square roots of those numbers, or as 190 : 189, or 
100 : 99-47, 
whence the lineal contraction of British plate-glass, between its very soft viscous 
condition, or very near to but below its temperature of perfect fusion, down to that of 
the atmosphere, say 50° as the mean of the year, is = 0-53 per cent. 
170. But three times this is guam proxime its cubic contraction, or 1-59 per cent. 
FlencelOOO in volume of glass very near its fusing-point became 984-10 at 50°, or the 
contraction in volume is thus yy{jy, which is somewhat (though probably not much) 
below the truth, were we to take the glass at the higher temperature of actual fusion. 
171. The toted contraction of the Barrow slag to that of the plate-glass is thus as 
1000 : 933 : 984, 
or the total contraction of the glass in passing through its whole range of temperature 
from somewhat below fusion is about equal to that of the slag in passing through about 
680°. The writer places much reliance on this result, based as it is upon so large an 
area of observation, and obtained by the repetition of the same sort of measurements by 
the same person for long periods, and being altogether of a character to cause minute 
errors to disappear from the final result*. 
172. We therefore may be permitted to conclude that rocks consisting of acid silicates 
contract still less than those of basic silicates, and that a terrestrial crust of the former 
is still more capable of floating upon the same in fusion beneath. 
173. As applied to our globe, it is highly probable that any inferences that may be 
drawn from either of these coefficients must he subject to the changes in volume that 
may arise in the mass cooled from changes in its molecular arrangement, such as that 
from the vitreous to the crystallized condition, data for which are unknown. Nor, in our 
ignorance of the proportions in which basic and acid silicates or other bodies constitute 
our globe, are we able to fix any mean coefficient for the whole. 
We have now, however, to attempt such applications of the numerical results arrived 
at as in the present state of our knowledge may enable us to illustrate the theory of volcanic 
action here propounded, and in some degree to test its feasibility, if not its truth. 
174. Sir W. Thomson has shown that in the agglomeration from the nebulous state of its 
particles from infinite distance there has been expended nearly 14 millions of foot-tons 
* It may be mentioned that the Manager of the Thames Plate-Glass Works has never been given by the 
writer any information as to the object of his inquiry as to glass contraction, nor has he to the present day any 
knowledge of the coefficients for slag obtained by the writer. 
