When considered with reference to their sources of supply, it may generally speaking be regarded that sea sands aro purer and better fitted for mortar making than any other; provided always that the efflorescence of the muriate of soda they contain should not be likely to affect injuriously the character of the work. This would be the case with all internal decorations ; and sea sand must therefore be carefully excluded from all ornamental structures. River sand is also very likely to contain the muriate of soda when obtained from the tidal portions of a stream, and in addition to this inconvenience there is almost a certainty that tho river sand will contain a large proportion of nitrogenous elements, which would in a building give rise to the formation and elliorescence of the nitrate of lime, which in its turn would be fatal to any coloured decorations or paintings exposed to its effects. Pit sand is in fact the most generally fitted for building pur poses ; but care must be taken with it to secure that it should be free from argillaceous ingredients, and only that sand should be used whose grains are sharp and angular. The specific gravity of a sand is a primtl facie evidence of its goodness.
In brick and tile making, and in some processes of the higher ceramic arts, sand is occasionally used when the clay is to use the workman's phrase, "too rich ; " that is to say, when it is exposed to run together in the kiln in an irregular manner, in consequence of an excess in the proportion of alumina the clay may contain. A clay is considered, in fact, to be rich when the proportions of silica and alumina are respectively 00 and 40 per cent. ; it is considered to be poor when those proportions are 80 and 20 per cent. ; and according to the usage intended to be made of the clay a greater or less proportion of pure silicious sand must be added to the richer varieties. See
Brogniart, Trait4 des Arts Ceramiques.' The very pure and perfectly white silicious sands which are found in Norfolk and in Alum Bay, Isle of Wight, used formerly to be raised in large quantities for glass making. The supply has been nearly exhausted ; and latterly some of the best sands for this important branch of manufacture have been imported, in ballast, from Australia ; rich ferruginous sands for steel making are also imported from Now Zealand.
It may be desirable to add, that the fine sands charged with organic matters to be found in the embouchures of rivers, or in some bays on the sea-shore, are technically known by the name of silt, and that they are very rarely of any commercial value. Pit sands, or large deposits of sand in the interior, are occasionally found to be so charged with water as to remain constantly in motion. and are then called running sands. Under these conditions they form one of the most dangerous class of foundations, for if an escape should be found for the waters they may contain, the sands would in all probability be removed by thorn, and the foundations would be exposed to lateral displacement. The precautions to be taken in such cases are, if possible, to intercerft the supply of water to the sand; if that cannot be effected, to keep it constantly charged ; and, under either of those conditions, to isolate the immediate surface of the foundations in such a manner as to allow any movements which may be produced to take place vertically. Dry sand has, in its dry state, been often used in foundations instead of concrete; and if it be prevented from spread ing laterally, that material is practically incompressible. It is also used for tamping round the charges of mines.