SAND. The fine angular materials derived from the disintegration of rocks, and deposited according to the gravity of the various particles under the action of running water, are technically known by the generic term "smiths" They are, for the purposes of classification, subdivided into river sand, sea sand, and pit sand; volcanic, cal careous, argillaceous, or silicious sand ; red, yellow, or white sand ; according to their manner of occurrence, their qualities, or their colour. In the arts they are used for many purposes, but as their applications in the manufacture of mortar, of plastic building materials, and of glass, are, perhaps, of the greatest practical importance, attention will be principally called to these applications ; but before doing so it may be of interest to observe that the sea-sands on some parts of the shores of the Atlantic are largely used in agriculture, on account of the largo quantity of calcareous and nitrogenous matters they contain. This application of sand, of the description known locally by the name of " tangue," prevails in the departments of the Calvados, Manche, Cotes du Nord, &c., in France, to a very great extent, and it is found to be very beneficial.
For mortar making, sands may be selected according to the pro perties of the lime in connection with which they are employed, as under some circumstances they play a very important part in the chemistry of the hardening of the mortars. As was said under MORTAR, the value of that clam of materials depends upon the rapidity and the energy with which the double silicate of lime and alumina is formed; and it therefore follows that the sands which contain the various ingre dients hi a state susceptible of entering into combination with tho lime, are those which are of the greatest value. The sands derived from the destruction of the purely silicious rocks are for the most part totally inert for the purposes under consideration, because the silica they contain is in a permanent crystalline form ; but the sands derived from the destruction of gneiss, grattwack6, felspathic granites, and other rocks, in which the officious acid does not exist in a stable com pound, are easily acted upon by pure caustic lime. Thus it is found that in the north of Spain, and in the granitic district of the north of France, the of the felspathic granites of those districts has produced a sand in which the Rilicato of lime of the felspar exists in a state able to combine with the pure hydrate of lime, there obtained by slaking the caustic lime derived from the more pure crystalline lime stones. The volcanic sands, known technically by the names of
pozzsolanoa and tram, act oven more energetically than the sands from the felspathic rocks; and when mixed with the pure hydrate of lime in proper proportions, they even communicate to it properties nearly similar to those of natural cements. On the other hand, sands con taining plastic clay in its natural state are positively injurious ; not only because the clay does not exist in them in a state able to form any stable compound with the lime, but also because it prevents the latter from performing one of ita most useful functions in a mortar ; namely, that of presenting a nucleus around which crystallisation could take place in the limeeitself. At the present day it is believed that with the energetic cements a sharp, angular, crystalline sand, one whose chemical properties would be totally inert, is the best adapted for mortar making; with hydraulic limes the sands ought to possess the faculty of slowly forming new compounds with the hydrates of lime ; and with rich limes, especially if they are required to be used in sea water, sands exercising an energetic action on the hydrates, must exclusively be used.
These remarks point to a rough practical subclassification of sands, into the classes of the inert, lightly energetic, and decidedly energetic sands ; and Vicat, who was the first to call attention to this description of action, states that when sands aro treated by acids and by lime water they may easily be distinguished from one another. Thus the inert sands resist the action of acids, and are totally without action upon even boiling limewater. The slightly energetic sands yield in a trifling degree to the acids, and take up a small proportion of lime from the lime water. The decidedly energetic sands are powerfully affected by acids, and they take up it large proportion of the lime presented to them in solution. Care must, however, be taken in the experiments on the nature of this class of materials to allnw for the action of the acids upon the lime present in the sand ; for all purposes connected with the formation of the insoluble silicate of lime and alumina by the reciprocal actions of the caustic lime and the sand, the calcareous matters of the latter will remain practically inert.