128 Transactions.— Miscellaneous. 
simply been ‘select the best.’’ After a long, laborious, and expensive 
investigation, and with the best possible intentions, the Commissioners 
selected ‘‘ the magnesian limestone, or dolomite of Bolsover,’’ one of the 
most worthless stones for the purpose in Great Britain. The sole reason 
for this untoward conclusion is in the fact that, at that time, the peculiar 
affinity of magnesian lime for sulphur was unknown, and the Commis- 
sioners had the strongest possible proof of the durability of the stone in 
Southwell Minster, where it had withstood the action of the weather for 
800 years. This was, however, in the pure air of a small country town— 
a condition that differs materially from that which the material occupied 
when exposed to the smoky and acidulous atmosphere of the metropolis. 
Tests.—Eixcept in rare cases, such as the arches of a long-spanned 
bridge, and the lower courses in a spire or chimney, the pressure on stones 
in a building never approaches their crushing weight ; their cohesive pro- 
perties may, therefore be disregarded in a popular investigation like the 
present one. I shall, however, consider shortly the proofs or tests of 
durability that should be observed in building with freestones. 
Generally speaking the hardest, heaviest, and least absorbent stones in 
a class such as sand and limestone are the best; but this is no criterion 
when comparing classes. In sandstones the chemical test is the maxi- 
mum amount of silica, and minimum of alumina; the proportions of the 
other ingredients being within certain limits apparently of no consequence. 
The best limestones are those that approach nearest the crystalline state ; 
uniformity of tint and homogeneity of structure are also favourable indi- 
cations. So far as strength and beauty, as well as durability under 
ordinary circumstances are concerned, the magnesian limestones are best 
when the lime and magnesia are in equal proportion. This, however, as 
already shown, seems the worst proportion for a smoky town. 
The absorbent properties of stones can be tested by subjecting them to 
the action of water under a slight pressure. With 14 Ibs. on the square 
inch English Sandstones absorb from one-seventh to one-fourth of their 
entire bulk ; Limestones, one-ninth to one-fifth ; Oolites and Dolomites one- 
fifth to one-fourth. 
The resistance of stone to disintegration can be tested by what is called 
Brard’s process ; this consists in boiling specimens in a solution of sulphate 
of soda (Glauber’s salts) and afterwards dipping them at intervals into the 
cold solution for a few days. The action of this salt closely resembles that 
of frost, and M. Vieat has calculated that the effect, after two days’ appli- 
cation, is equal to the force exerted by frost at 21° Far. on wet stone. The 
hardest granite is segregated by Brard’s process in thirty days. 
