22. A close inspection should be made of all stone before it is used, to see that the specified quality is being delivered. A visit to the quarry is advisable when large quantities of stone are to be used, in order to make any necessary tests of the stone. The tests usually made to assist the architect in determining the qualities of stone as to durability, etc., are for compactness and hardness, absorption and solubility.

23. Compactness

Compactness. The densest and strongest stones are also, in general, the most durable. An idea of the compactness may be obtained by examining, through a good magnifying glass, the surfaces of freshly fractured stone, which should show a clear and bright surface, with the particles well cemented. A dull, earthy-looking fracture indicates liability to quick deterioration. A clear metallic sound, when struck with a hammer, is a good test of stone.

24. Absorption

Absorption. The tendency of a stone to absorb water should be considered as to the effect on the appearance of the building. While a dense, non-absorbent stone is restored to its original color by a heavy rain, one of open texture quickly absorbs the water, which carries into the pores of the stone dust and soot, that soon make it very dirty.

Generally the most durable stones are those which absorb the least water. In order to test the absorptive qualities of a stone, a good average specimen of it should be thoroughly dried, carefully weighed, and immersed in water for 24 hours. When taken out, the surface moisture should be dried off, and the piece weighed; from the gain in weight, a good idea of the value of the stone may be obtained. One that increases 10 per cent. in weight in 24 hours should be rejected, unless it can be shown that such stone has endured successfully, for an extended period, the tests of time and weather. Even one absorbing 5 per cent. of water, and containing a considerable proportion of clay, is unsafe to use.

25. Solubility

Solubility. To determine whether a stone contains much easily soluble earthy or mineral matter, crush finely a sample of the stone and place the pieces in a glass of water, letting the particles remain undisturbed for about half an hour; then give the contents of the glass a rotary motion by stirring. If the stone contains much earthy matter, the water will assume a turbid appearance, while if it has but little, the water will remain clear.

As before stated, the air in manufacturing- cities is very-likely to contain traces of various acids, which attack the stone when brought into contact with it by rain. To determine the probable effect of acids on stone, soak a piece in a dish of water containing a drop or two of muriatic or sulphuric acid. If there is a very noticeable action, it will be wise to further test the stone.