In selecting stone for constructive purposes, it is necessary to ascertain its qualities with regard to the following characteristics.

Durability

The power of resisting atmospheric and other external influences, is the first essential in a stone for almost any purpose. The durability of a stone will depend upon its chemical composition, its physical structure, and the position in which it is placed; and the same stone will greatly vary in its durability according to the nature and extent of the atmospheric influences to which it is subjected. To make sure that a stone will "weather," - that is, will wear well under exposure to the weather - many points have to be inquired into.

The chemical composition of the stone should be such that it will resist the action of the atmosphere, and of the deleterious substances which, especially in large cities, the atmosphere often contains. These destroying substances are taken up by the moisture in the air, or by the rain, and are thus conveyed into the pores of the stone. The sulphur acids, carbonic acid, hydrochloric acid, and traces of nitric acid, in the smoky air of towns, and the carbonic acid which exists even in the pure atmosphere of the country, ultimately decompose any stone of which either lime carbonate or magnesia carbonate forms a considerable part. The oxygen even in ordinary air will act upon a stone containing much iron, and the fumes from bleaching works and factories of different kinds very soon destroy stones whose constituents are liable to be decomposed by the particular acids which the fumes respectively contain. In addition to the direct chemical action of the sulphuric and sulphurous acids upon the constituents of stones, sulphates are sometimes formed by them which crystallize in the pores of the stone, expanding and throwing off fragments from the surface.

The durability of a stone depends, therefore, to a great extent upon the relation between its chemical constituents and those of the atmosphere surrounding. A stone which will weather well in the pure air of the country may be rapidly destroyed in the smoke of a large town.

A stone will weather very differently according to the nature and extent of the atmospheric influences to which it is subjected. Obviously most stones will stand a pure atmosphere better than one which is charged with smoke, or with acids calculated to attack the constituents of the stone; and the stone will be less attacked in dry weather than during rain: the destructive acids cannot penetrate so deeply, and the frost has no influence whatever when the stone is dry. Therefore the number of days on which there is rain in any district ha3 a great influence on the durability of stone used there. Wind has a considerable effect upon the durability of stone. A gentle breeze dries out the moisture, and thus favours the lasting qualities. A high wind, however, is itself a source of destruction; it blows sharp particles against the face of the stone, and thus grinds it away. Moreover, it forces the rain into the pores, and may thus cause a considerable depth to be subject to the effects of acids and frost.

Variation of temperature, apart from the action of frost, is also a source of decay, the expansion and contraction due to it causing the opening of undetected natural joints; but its effect must be comparatively slight as a destructive agent.

The position of a stone in a building may very much influence its durability. The stone in that side which faces the prevailing rain is most liable to decay. Faces that are sheltered altogether from the sun and breeze, so that the moisture does not quickly dry out, are very liable to decay. This may be noticed especially in buildings of an inferior stone situated in a bad atmosphere. In these it will be seen that the soffits of arches and lintels, the shady sides of window jambs, and parts of carvings which the sun never gets at, are always the first portions to suffer. Stone exposed to very different degrees of heat on its different faces is liable to crack from unequal expansion and contraction.

The physical structure of a stone is of great importance, for upon it largely depends its power of resisting the action of the atmosphere. Chalk and marble are of the same chemical composition - both nearly pure lime carbonate - yet the latter, especially when polished, will resist an ordinary atmosphere for a long time, while the former is rapidly disintegrated and destroyed. Hence stones which are crystalline in structure are found to weather better than those that are non-crystalline. No stone intended for the exterior of a building should have a porous surface, otherwise the rain conducts the acids from the atmosphere into the pores of the stone, which soon becomes decomposed. Also in winter the wet penetrates the pores, freezes, expands, and disintegrates the surface, leaving a fresh surface to be similarly acted upon, until the whole stone is gradually destroyed. If the other qualities of two stones are the same, then that which has the closer and finer grain is likely to be the more durable. It is important that a stone be homogeneous in its structure. If the grains and the cement uniting them are both of lasting material, the stone will be very durable.

If the grains be easily decomposed and the cementing material remains, the stone will become spongy and porous, and then liable to destruction by frost. If the cementing material is destroyed, the grains will fall to pieces. Stone should contain no soft patches or inequalities; unequal weathering leaves projections which catch the rain, etc, and hasten decay.

Alexis A. Julien, of the School of Mines, Columbia College, sums up the results of a series of papers read before the New York Academy of Sciences on the decay of building-stones as follows: - If a rough estimate be desired, founded merely on the observations made of the comparative durability of the common varieties of building-stone used in New York city and vicinity, there may be found some truth in the following approximative figures for the "life" of each stone, signifying by that term, without regard to discoloration or other objectionable qualities, merely the period after which the incipient decay of the variety becomes sufficiently offensive to the eye to demand repair or renewal.

Life in years.

Coarse brownstone..............

5-15

Laminated fine brownstone........

20-50

Compact fine brownstone........

100-200

Limestone, coarse fossiliferous......

20-40

" fine oolitic(French)......

30-40

Marble (dolomite), coarse..........

40

" " fine..........

60-80

Marble, fine ..............

50-200

Granite...........

75-200

Gneiss.............

50 years to many centuries.