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though the first set is caused by the same absorption of the water 
of mixture as in ordinary limes, the permanent hardness cannot, 
evidently, result from desiccation, either from absorption or evapora- 
tion. A chemical combination between the lime and some agent 
capable of resisting the softening action of water is necessary. 
Silica was proved to be the only substance found naturally in limes, 
which was capable of furnishing this protection ; and it was only 
useful when present in combination with alumina, as clay, in a 
hydrated form. Double silicates of lime and alumina form harder 
mortars than silicate of lime alone. Iron, manganese, magnesia, 
and alumina by itself, were shown to have no chemical effect in 
setting lime under water, though they furnish a kind of temporary 
mechanical protection from solubility. The iron in pozzuolana 
may even be withdrawn by an acid, without to any extent injuring 
the hydraulic properties of the earth. The author believed that in 
properly calcined hydraulic lime no union between the silica and 
lime took place in the kiln, but only on the completion of a hydrate 
by slaking, and then only when surplus water was present. This 
theory was founded on experiments upon the adhesion of mortar at 
different stages of setting, silicate of lime having little power of 
cementation. The degree of adhesion, marked tolerably exactly 
the progress of the silicification of the lime. Experiments on the 
quantity of silica derived from Portland cement, digested in a weak 
acid, both when fresh from the kiln and after having been set for 
six days, gave still more direct evidence of the truth of this theory, 
which was proved by the author to be of great practical importance 
in the application of limes. It was stated that, oxalic acid had 
been ascertained to be capable of setting limes permanently hard 
underwater, when mixed with the water used in slaking; but the 
oxalate of lime does not possess the same flinty hardness as the 
silicate, and of course is much more expensive — at the present price 
of oxalic acid. The paper closed with some remarks on “ cements,” 
both natural and artificial ; the properties of which, under the gene- 
ric types of Homan and Portland cements, were shortly explained. 
2. On Zinc-Methyl. By J. A. Wanklyn, Esq. 
Considerable difficulties attend the preparation of zinc-methyl. 
Frankland, who discovered the body, obtained it by heating pure 
