110 "Transactions.—Muüiscellaneous. 
mortar is injured by too much sand, but the chances are that its bad quali- 
ties are the result of the opposite condition, and, above all that, the sand 
and lime are not properly mixed. If greater care was exercised in this 
behoof, so that an approach could be made to the theoretical maximum of 
an atom of sand between each atom of lime, the result would be an im- 
mediate doubling of the strength of rich lime mortar. 
Poor Limes possess all the bad qualities of the rich ones, and have an 
additional drawback im irregularity of consistency through not slaking so 
readily, which necessitates grinding. 
Hydraulic Limes are frequently sub-divided into three or four sections, 
ranging from slightly to eminently hydraulic, the former being practically a 
rich lime, and the latter a cement. These limes do not slake readily, nor 
do they expand much in the process. The higher kinds slake so slowly and 
so imperfectly that they are always pulverized by grinding. Hydraulic 
limes set under water in from three to fourteen days, according to the 
strength of the sample. 
Hydraulic Cements cannot be slaked by water in the usual way ; they are, 
properly speaking, not calcined, but vitrified. The produce of the kilns 
resembles slag from a blast furnace, and it requires the aid of stone- 
breakers, iron rollers, and French burr mill-stones to convert it into the 
cement of commerce. In common with the higher kinds of hydraulic lime, 
cement does not require any admixture of sand to make it into mortar ; the 
maximum strength is obtained by using it in a pure state. Some hydraulic 
cements set under water in a few minutes, but the best kinds take a few 
hours. In seven days the latter attain a tensile strength of 250 pounds to 
the square inch. 
The quality of limestones cannot be determined by a knowledge of the 
geological formation in which they occur, nor by their general appearance. 
Hydraulic limes are perhaps more plentiful in what may be called the 
medieval rocks—cretaceous to carboniferous—than in any others ; but, as 
they are frequently met with in the formations above and below those 
named, we may give them an almost universal range of locality. The 
character of the stone seems to be determined chiefly by its immediate sur- 
roundings—the outer beds are argillaceous or silicious, according as the 
adjoining stratum is clay or sand, and the whole rock is influenced by the 
manner in which it was deposited, and the subsequent changes to which it 
was subjected. If the lime had been deposited in still, clean water on a 
rocky bottom, and had attained a considerable degree of hardness before 
being disturbed by convulsions from below, or pressure from above, we 
might expect it to be comparatively pure ; but if deposited in an estuary 
where the water is muddy and the bottom soft, and where floods leave 
