476 Royal Society :— 
particulars of these experimeuts are given, with diagrams showing 
the forms of sections, and the values as obtained from the formulee 
are stated. The value of the direct tensile strength of cast iron 
thus derived, falls between the limits of 1400 and 1700. 
In the Reports of the Commissioners of Inquiry into the “ Appli- 
cation of Iron to Railway Structures,” are given the results of about 
fifty experiments on the direct tensile resistance of one-inch square 
cast-iron bars, under the direction of Professor Hodgkinson. The 
bars consisted of seventeen different kinds of iron, each set of bars 
being of the like quality and manufacture ; and in several of these 
sets, which might have been expected to yield the same results, the 
difference is fully as great as in the cases here exhibited. From this 
fact an inference may be drawn in favour of the general applicability 
of the principles developed in the foregoing pages to cast-iron beams 
and girders of every variety of section. 
April 2.—The Lord Wrottesley, President, in the Chair. 
The following communication was read :— 
** Researches on Silica.”” By Colonel Philip Yorke, F.R.S. 
This communication is principally devoted to an attempt to de- 
termine the formula of silica, and to the relation of some remarkable 
results obtained in this research. After giving some account of the 
grounds on which the three different formulas now in use among 
chemists (viz. SiO;, SiO,, and SiO) had been advocated, the author 
proceeds to state, that it appeared to him that the direct method 
which had been followed by Rose deserved the preference. This 
method consists in determining the quantity of carbonic acid which 
is displaced from excess of an alkaline carbonate in fusion, by a given 
weight of silica. The number 22 being the equivalent for carbonic 
acid on the hydrogen scale, the equivalent of silicic acid is obtained 
. 22x weight of silica used 
ep proportion # =~ eisht of carbonic acid expelled’ 
Four experiments are detailed, made with carbonate of potash, 
which give as a mean result the number 30°7 for the equivalent of 
silica. This agrees with the formula SiO,, and nearly with the 
previous results of H. Rose. Then follow seven experiments made 
in a like manner with carbonate of soda, which give as a mean result 
the number 21°3 as the equivalent of silicic acid—a number agree- 
ing nearly with half that represented by the formula SiO,, or —. 
Some experiments are then related, which go to show that the 
increased loss resulting with carbonate of soda could not be caused 
by the action of heat alone.-—The author had next recourse to car- 
bonate of lithia, and obtained as the mean result of four experi- 
ments with this substance, agreeing well together, the number 14°99 
—a number which accords very closely with the formula SiO. 
These different numbers, obtained with silica, led the author to in- 
quire whether any other body acting as an acid produces similar re- 
sults with the fused carbonates of potash and soda. With this view, 
experiments were made with dry sulphate of magnesia, as a substi- 
