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THEORY AND PRACTICE OF ENGINEERING.

Rook I f.

The Cipollino has greenish rings or zones produced by green talc; the fracture isgranular and shining.

Alabaster—Sulphate of Lime. — There are two varieties, the gypseous, which is a semi-crystalline sulphate, and the calcareous, which is a carbonate. The word alabaster isderived from the Greek, and signifies vases of perfumes, called at first alabastra, fromhaving no handles, and this kind of stone being often used for the purpose the namebecame applied to it. The Oriental alabaster is a carbonate of lime, frequently variegatedwith beautiful colours, and is susceptible of a high polish.

The common alabaster is composed of sulphuric acid and lime, though some varietieseffervesce with acids, and contain a portion of the carbonate; it is much softer than marble,and usually forms the lowest bed of the gypsum quarries. At Carrara it is worked intovases, statues, and ornamental models, &c. ; the hardest variety is preferred, particularlythat which has a granular texture and a pure white colour ; it does not polish so readilyas marble, but it is easier to work. When carved it is smoothed down with pumice-stone,and afterwards polished with a mixture of chalk, soap, and milk, the hand being used togive it the last finish.

Granite ranks as the most durable among all the stones selected for the purposes ofbuilding, but it is difficult and expensive to work. 'The Egyptians employed this materialin enormous masses: we have an account of an edifice hewn out of a single mass of granite,and moved from the city of Sais to the Isle of Elephanta, which, after deducting the void,is stated to have been upwards of 1222 cube cubits; its weight therefore would be upwardsof 200 tons. The temple of Latona at Euto, mentioned by Herodotus , had the sides of asingle stone, and the roof also in one piece.

The obelisks at Rome were quarried in Egypt , and transported at a great cost to theimperial city: of twelve which remain eleven are erect; they are a large-grained redgranite, covered with hieroglyphics with the exception of three, and the granite of whichthey are composed is of so imperishable a nature, that after exposure to the action of theatmosphere for 3000 years, the hieroglyphics are still entire; they are in relief, but thiswas effected by sinking the surface of the granite around them, the sinking forming theoutline; so that the figure, not projecting beyond the ordinary surface, is not exposed todecomposition, but in some degree protected. Poggio mentions that in 1430, these obeliskswere all prostrate, with the exception of that of the Vatican, which was in one single block,unbroken, and without hieroglyphics; its height is about 84 feet, and its weight may betaken at 300 tons. It was moved by Fontana to the Piazza of St Peter’s, and is thelargest mass of granite known to have been applied to purposes of art, with the exceptionof that at St. Petersburg for the statue of Peter the Great. Granite was also largelyemployed by the Romans for the shafts of columns; those of the Pantheon are of onesingle piece with statuary marble, capitals, and base. Egypt , as well as the island of Elba,supplied them with abundance of this material, not only for their temples, baths, and fori,but also for fountains, the basins of which were hollowed out of large masses.

In England, granite does not seem to have been brought into use for the purposes ofconstruction much before the commencement of the present century. The bridges atLondon over the Thames , erected by the Messrs. Rennie, may perhaps be mentioned asinstances of its first application upon a large scale: its beauty and durability render itsuperior to all other material for works considered national, or intended as monuments ofart to be bequeathed to future generations. In the selection of granite, attention shouldbe paid to its quality, which evidently varies as it is produced from different quarries,that obtained from the surface being usually more or less in a state of decomposition :the quartz, mica, and felspar, composing the granite, do not seem to be held together byany base or cement, and the size of the crystals or grains differs in the various specimens,as do the proportions of the ingredients themselves. As the quartz, mica, and felspar,contain several elementary substances, to determine their constituents it is necessary toreduce a given quantity of granite to a powder, and then to fuse it in a platinum cruciblewith three or four times its weight of alkali, which will decompose it by uniting with oneor more of the constituents. The mass so fused is soluble in dilute muriatic acid bythe application of different re-agents; the constituents are precipitated, after which it maybe filtered, carefully dried, and weighed.

The quartz may be considered pure silex ; it is an indestructible body, sufficiently hardto scratch glass, and when struck with steel will produce fire; it is also infusible beforethe blow-pipe ; it is whitish, yellowish, or greyish white. The form of its crystals aregenerally six-sided prisms or pyramids; they may be cleared parallel to the faces of theprimitive crystal, which is a rhomboid ; its specific gravity is 2-6 to 2*7.

Mica , as found in granite, is confusedly crystallised, and its primary form is said to bea right rhomboidal prism ; its colour varies from white, yellow, green, brown to almostblack ; in some instances it is so very transparent that it is used as a substitute for glass ; itsspecific gravity is about 2*0 to 2*5. The analyses of mica vary, but Klaproth gives thatof two varieties, the first from Zinnwald, the latter from Siberia .