154 Transactions.— Miscellaneous. 
concrete brick, composed of one of cement to six of sand, will, when six 
days old, resist a pressure of eighteen tons, which is about double the 
strength of ordinary red bricks. 
Conerete is cast into blocks for arch-stones, quoins, sills, lintells, steps, 
and mouldings of all kinds. 
In view of the interest taken in concrete as a building material, I shall 
devote a few remarks to the consideration of its properties. The cementing 
ingredient in concrete is generally hydraulic lime, or cement, or a mixture of 
the two. The former has not yet been discovered, or, atleast, used as such 
in Otago, neither has the latter been manufactured, so they cannot be called 
native ; but, as the raw materials for making cement exist in large quanti- 
ties, there is no doubt its manufacture will become a colonial industry at no 
distant day. The proportion of cement to the aggregates varies from @ 
fourth to a tenth, according to the nature of the work, the strength of the 
cement, and the character of the other materials; for house-building 1 to 6 
is weak enough, particularly here, where the cement may have deteriorated 
by exposure on the voyage. The best aggregate is one in which the pieces 
are of all sizes, from two inch metal to fine sand, adjusted in such regular 
gradations that the cement will exactly fill the vacuities. Large metal and 
fine sand, with other materials of an intermediate size, does not make good 
concrete. The ingredients should be mixed dry, and water added in infini- 
tesimal quantities, through a fine rose, or otherwise in the shape of spray. 
This is an important point, for a wash of water enriches one portion of the 
mass at the expense of another. No more water should be put in than 
sufficient to damp the cement; as a certain limited quantity only is required 
in setting, the excess evaporates, and leaves cavities for the reception and 
retention of moisture. Mixing, the next operation, is also equally im- 
portant ; it must be done in a thorough systematic manner, so that every 
piece of stone, or particle of sand, is completely coated with cement. It is 
almost impossible to get this work done properly by manual labour, and 
although machinery is constantly employed on large works, the necessity 
for it in ordinary house-building is not yet fully recognised. 
The manner of depositing the material in the moulds, or frames, has 
given rise to a difference of opinion ; some authorities hold that the con- 
crete should be placed loosely, as pressure impairs the setting properties of 
cement, while others advocate excessive ramming. If Roman, or any other 
quick setting cement is used, pressure will undoubtedly do harm ; but, with 
ordinary heavy Portland Cement, or hydraulic limes, in the proportions 
usually adopted, there is no risk in ramming, and the quality of the con- 
crete is so much improved by it, that, if necessary, it would be better to 
