To determine the effect of the atmosphere of a large city, where coal is used for fuel, soak clean small pieces of the stone for several days in water which contains one per cent each of sulphuric and hydrochloric acids, and agitate frequently. If the stone contains any earthy matter likely to be dissolved by the gases of the atmosphere, the water will be more or less cloudy or muddy. The following results were obtained by this method.
Some experimenters heat the specimens to 500° to 600° F., and plunge them while hot into cold water. The results are supposed to show the relative resistance to frost action, and also to indicate something as to the fire resisting qualities of the stone. The following comparative results were obtained by
method :t Resistance to Fire. Stones differ greatly in their ability to resist the heat of a burning building. Of course, the injurious effect of a conflagration is greater in proportion as the temperature is higher; but there are no reliable data for estimating accurately the effect of different temperatures.
Experiences with great conflagrations have established the fact that granite is less fireproof than either limestone or sandstone. The susceptibility of granite to the effect of heat is doubtless partly due to the fact that it has a compact and complex structure, and that each of its constituent minerals has different degrees of expansibility; and possibly partly also to the water in minute cavities which upon be coming highly heated is converted into steam and causes an explosion.
Up to the point at which limestones and marbles are converted into quicklime, i.e., between about
and
Fahr., they are not much injured by heat. Limestones and marbles seldom crack from heat alone; but crumble when water is thrown on them. It should be remembered that the sudden cooling of the surface of a heated stone due to repeated clashes of cold water, often has more to do with its disintegration than heat alone.
Sandstones are not usually injured by a conflagration, except for the discoloration caused l:y the smoke. The great durability of sandstone under fire, and incidentally the relative resistance of sandstone and granite, was shown at the burning of St. d'eter's
Church at Lamerton, England. "The church itself, which was built in great part of granite, was completely ruined; while the tower, built of local sandstone, around which the heat of the fire was so great as to melt six of the bells as they hung in the belfry, was left intact, although the granite window jams and sills were destroyed."* Chemical Analysis. A chemical analysis of a stone is of very little value in itself, since the analysis alone does not show whether any particular constituent is contained in the grains where it is not easily attached, or in the cement that holds the grains together where it is easily attacked by the acid gases of the atmosphere. But sometimes a chemical analysis is important in connection with a microscopical examination.
is now held that the best method of determining the probable durability of a building stone is to study its surface, or thin transparent slices, under a micro scope. This method of study in recent years has been most fruitful in developing interesting and valuable knowledge of a scientific and truly practical character. An examination of a section by means of the microscope will show, not merely the various substances which compose it but also the method according to which they are arranged and by which they are attached to one another. For example, "pyrites is considered to be the enemy of the quarryman and con structor, since it decomposes with ease, and stains and discolors the rock. Pyrites in sharp, well defined crystals sometimes decomposes with great difficulty. If a crystal or grain of pyrites is embedded in soft, porous, light colored sandstones, like those which come from Ohio, its presence will with certainty soon demonstrate itself by the black spot which will form about it in the porous stone, and which will permanently disfigure and mar its beauty. If the same grain of pyrites is situated in a very hard, compact, non-absorbent stone, the constituent minerals of which are not rifted or cracked, this grain of pyrites may decompose and the products be washed away, leaving the stone untarnished."