Qualities of Building Stone.—The principal qualities of a good building stone are—(1) Strength, (2) hardness, (3) durability, (4) appearance, (5) facility for working. There are also other minor points; but stone possessing one or more of the above qualities, according to the purpose for which it is required, may be regarded as good for that purpose.
Strength of Stone.—Stone should only be subjected to a compressive strain. It is occasionally subject to a cross strain, as in lintels over doors and windows; these are, however, contrary to the true principles of construction, and should not be allowed except a strong relieving arch is turned over them. The strength of stone in compression is about 120 tons per square foot for the weakest stones, and about 750 tons per square foot for the strongest. No stones are, however, subjected to anything like this amount of compressive force; in the largest buildings it does not amount to more than twelve or fourteen tons per square foot.
Hardness of Stone.—This is of more importance than its strength, especially in pavements or steps, where it is subject to great wear; also in plinths and quoins of buildings where it is desired to preserve a good face and sharp arris. The order of strength and hardness of stone is—(1) Basalt, (2) granite, (3) limestone, (4) sandstone. Granite, seinite, and gneiss take the first, place for strength, hardness and durability, but they will not stand a high temperature. "Stones which are of a fine, uniform grain, compact texture and deep color are the strongest; and when the grain, color, and texture are the same, those are the strongest which are the heaviest; but otherwise the strength does not increase with the specific gravity." Great hardness is objectionable when the stone has to be worked with a chisel, owing to the labor required to work it. Hard stones, also, generally wear smooth, and become polished, which makes them unsuitable for some purposes. Brittleness is a defect which frequently accompanies hardness, particularly in coarse-grained stones; it prevents them from being worked to a true surface, and from receiving a smooth edge at the angles. Workmen call those hard stones which can only be sawn into slabs by the grit saw, and those soft which can be separated by a common saw.
Expansion of Stone by Heat.—Rocks are expanded by heat and contracted by cooling. Variation in temperature thus causes some building stones to alternately expand and contract, and this prevents the joints of masonry from remaining close and tight. In the United States with an annual thermometric range of more than 90 deg. Fah., this difficulty led to some experiments on the amount of expansion and contraction in different kinds of building stones. It was found that in fine-grained granite the rate of expansion was .000004825 for every degree Fah., of increment of heat; in white crystalline marble it was .000005668; and in red sandstone .000009532, or about twice as much as in granite. In Western America, where the climate is remarkably dry and clear, the thermometer often gives a range of more than 80 deg. in twenty-four hours. This great difference of temperature produces a strain so great that it causes rocks to crack or peel off in skins or irregular pieces, or in some cases, it disintegrates them into sand. Dr. Livingstone found in Africa (12 deg. S. lat., 34 deg. E. long.) that surfaces of rock which during the day were heated up to 137 deg. Fah. cooled so rapidly by radiation at night that unable to stand the strain of contraction, they split and threw off sharp angular fragments from a few ounces to 100 lbs. or 200 lbs. in weight. According to data obtained from Adie "Trans. Roy. Soc. Edin.," xiii., p. 366, and Totten the expansion of ordinary rocks ranges from about 2.47 to 9.63 millionths for 1 deg. Fah.
BLUNDERS AND ABSURDITIES IN ART.
In looking over some collections of old pictures, it is surprising what extraordinary anachronisms, blunders, and absurdities are often discoverable.
In the gallery of the convent of Jesuits at Lisbon, there is a picture representing Adam in paradise, dressed in blue breeches with silver buckles, and Eve with a striped petticoat. In the distance appears a procession of Capuchin monks bearing the cross.
In a country church in Holland there is a painting representing the sacrifice of Isaac, in which the painter has depicted Abraham with a blunderbus in his hand, ready to shoot his son. A similar edifice in Spain has a picture of the same incident, in which the patriarch is armed with a pistol.
At Windsor there is a painting by Antonio Verrio, in which the artist has introduced the portraits of himself, Sir Godfrey Kneller, and May, the surveyor of the works of that period, all in long periwigs, as spectators of Christ healing the sick.
A painter of Toledo, having to represent the three wise men of the East coming to worship on the nativity of Christ, depicted three Arabian or Indian kings, two of them white and one black, and all of them in the posture of kneeling. The position of the legs of each figure not being very distinct, he inadvertently painted three black feet for the negro king, and three also between the two white kings; and he did not discover his error until the picture was hung up in the cathedral.