Bram.—On the Building Materials of Otago. 109 
cable to either of them. The cementing ingredients are the same in 
both eases, the only difference being in the proportions in which they 
occur. Pure lime is practically worthless for building purposes ; it never 
acquires the necessary cohesive strength in any situation, and never hardens 
at allin a damp place. In order to make good mortar the limestone 
must contain a mixture of clay—the proportion varies from 8 per cent. in 
ordinary building lime to 85 per cent. in strong hydraulic cement. If it 
were necessary to have a clear dividing line between limes and cements, the 
best place to strike it would be at the neutral point where the adhesive 
and cohesive forces are equal. The particles of rich and moderately 
hydraulic limes adhere more readily to a foreign substance than to each 
other, but the conditions are reversed with strong hydraulic limes and 
cements. This gives the only tangible difference I can imagine between 
the two articles, but as it does not admit of a practical application, the dis- 
tinction would only be valuable from a scientific point of view. 
Limes and cements are usually divided into four classes, according to 
their properties and strength— 
1st. The Common or Rich Limes that contain less than 10 per cent. of 
clay or other impurities. 
9nd. Poor Limes, in which the impurities consist of from 10 to 25 per 
cent. of sand or other insoluble ingredients that will not enter 
into chemical combination with the lime. 
8rd. Hydraulic Limes, such as contain from 10 to 30 per cent. of 
alumina and soluble silica. 
4th. Hydraulic Cements, containing from 80 to 40 per cent. of alumina, 
soluble silica, and other impurities. 
In addition to the ingredients named, each of the above classes fre- 
quently contains small quantities of iron, manganese, magnesia, potash or 
soda, with sulphuric and other acids, which do not seem to have an 
injurious effect on the cementitious properties of the article; on the con- 
trary, some of them, such as iron and soda and some of the acids, are 
always present in the best cements ;—the quantity, however, of all foreign 
substances, except silica and alumina, seldom exceeds 5 per cent. We may 
therefore assume, shortly, that our mortars are simply lime and clay in 
varying proportions. 
As already stated, the common or rich limes are comparatively useless 
where strength is required, and absolutely worthless in a damp situation. 
They are easily burned and slaked, swell to a great extent in slaking, shrink 
in drying, and are soluble in water when set. Their adhesive properties are 
stronger than their cohesive ones, consequently they cannot be used without 
a large admixture of sand. It is ccmmon to hear the expression that 
