112 Transactions.— Miscellaneous. 
Without such extrancous aid it is difficult to account for the hardening of 
mortar in thick walls. The operation proceeds from the outside, conse- 
quently every advance made is a barrier to the next in so much that it 
excludes the air from the softer mortar inside. There is every reason to 
believe that hydrate of lime, when exposed to the atmosphere, will revert 
to its original carbonate ; but the process is such a slow one that it may be 
almost classed with the geological epochs. The oldest mortar in the world, 
that from a Pheenecian temple in Cyprus, is still far short of the ingredients 
it possessed when a limestone. 
The induration of hydraulic mortars is attributable in a small degree to 
the same causes as affect the rich ones, but principally to the formation and 
crystallization of complex silicates of lime and alumina, the precise nature 
of which is imperfectly understood. It is quite evident that the absorption 
of carbonic acid has very little to do with the setting of hydraulic mortar, 
for against its slow action already noticed we have the fact that large blocks 
of cement concrete harden uniformly to the consistency of stone in a few 
months under water, which proves that the setting property is inherent, 
and not the result of external influences. 
The treatment they receive in burning has a considerable effect on the 
quality of limes and cements of all kinds, but more particularly on those 
that are only moderately hydraulic. Under burning has been known to 
impart a spurious hydraulicity to rich limes, and over burning occasionally 
destroys that property in cement, but as a rule there is little trouble in 
obtaining maximum results with these extreme classes. The burning of 
hydraulic limes is a much more delicate operation, the niceties of which 
can only be acquired by long experience in the art generally, and con- 
siderable practice with the actual materials that are to be operated on. 
As already indicated, the ordinary aggregates are essential to the 
induration of rich limes, but in the higher hydraulic varieties and in cement 
they are simply diluents. As rich limes do not possess the faculty of 
expelling any excess of moisture with which they are in contact, it is ad- 
visable to employ a porous aggregate, such as the sands produced by 
aluminous and calcareous rock, but when the aggregate is only employed to 
weaken the mortar by making it go further, there is little danger in using 
hard silicious sand, provided it is free from earthy impurities; indeed, this 
is an indispensible condition in all aggregates. It has been ascertained by 
experiment that the best proportion of sand for rich limes is 21 to 1, and 
for ordinary hydraulic limes 13 to 1, This explains the partiality of 
builders to rich limes. It will, in their own phraseology, * carry more 
sand," which means that strength and comfort are sacrificed for an insig- 
nificant saving in cost, 
