of lime. Nodules of this character are found in Mass., and in some other States; and it is probable they would yield, if suitably calcined and ground, an article in nowise inferior to that imported.

30. GYPSUM, or PLASTER of PARIS. This stone is a sulphate of lime, and has received its name from the extensive use made of it at Paris, and in its neighborhood, where it is quarried and sent to all parts of the world; being of a superior quality, owing, it is stated, to a certain portion of carbonate of lime which the stone contains. Gypsum is a very soft stone, and is not used as a building stone. Its chief utility is in furnishing a beautiful material for the ornamental casts and mouldings in the interior of edifices. For this purpose it is prepared by calcining, or, as the workmen term it, boiling the stone, until it is deprived of its water of crystallization. In this state it is made into a thin paste, and poured into moulds to form the cast, in which it hardens very promptly. Calcined plaster of Paris is also used as a cement for stone; but it is eminently unfit for this purpose; for when exposed, in any situation, to moisture, it absorbs it with avidity, swells, cracks, and exfoliates rapidly.

Gypsum is found in various localities in the United States. Large quantities of it are quarried in New York, both for building and agricultural purposes.

31. DURABILITY OF STONE. The most important properties of stone, as a building material, are its durability under the ordinary circumstances of exposure to weather; its capacity to sustain, without change, high degrees of temperature; and its resistance to the destructive action of fresh and salt water.

The wear of stone from ordinary exposure is very variable, depending, not only upon the texture and constituent elements of the stone, but also upon the locality and position it may occupy in a structure, with respect to the prevailing driving rains. The chemist and geologist have not, thus far, laid down any infallible rules to guide the engineer in the selection of a material that may be pronounced durable for the ordinary period allotted to the works of man. In truth the subject admits of only general indications; for stones having the same texture and chemical composition, from causes not fully ascertained, are found to possess very different degrees of duration. This has been particularly noted in feldspathic rocks. As a general rule, those stones which are fine-grained, absorb

least water, and are of greatest specific gravity, are also most durable under ordinary exposures. The weight of a stone, however, may arise from a large proportion of iron in the state of a protoxide, a circumstance generally unfavorable to its durability. Besides the various chemical combinations of iron, potash and clay, when found in considerable quantities, both in the primary and sedimentary silicious rocks, greatly affect their durability. The potash contained in feldspar dissolves, and, carrying off a considerable proportion of the silica, leaves nothing but aluminous matter behind. The clay, on the other hand, absorbs water, becomes soft, and causes the stone to crumble to pieces. Iron in the form of protoxide, in some cases only, discolors the stone by its conversion into a peroxide.— This discoloration, while it greatly diminishes the value of some stones, as in white marble, in others is not disagreeable to the eye, producing often a mottled appearance in buildings which adds to the picturesque effect.

32. Frost, or rather the alternate actions of freezing and thawing, is the most destructive agent of Nature with which the engineer has to contend. Its effects vary with the texture of stones; those of a fissile nature usually splitting, while the more porous kinds disintegrate, or exfoliate at the surface.When stone from a new quarry is to be tried, the best indication of its resistance to frost may be obtained from an examination of any rocks of the same kind, within its vicinity, which are known to have been exposed for a long period. Submitting the stone fresh from the quarry to the direct action of freezing would seem to be the most certain test, were the stone destroyed by the expansive action of the frost; but besides the uncertainty of this test, it is known that some stones, which, when first quarried, are much affected by frost, splitting under its action, become impervious to it after they have lost the moisture of the quarry, as they do not re-absorb near so large an amount as they bring from the quarry.

33. M. Brard, a French chemist, has given a process for ascertaining the effects of frost on stone, which has met with the approval of many French architects and engineers of standing, as it corresponds with their experience. M. Brard directs that a small cubical block, about two inches on the edge, shall be carefully sawed from the stone to be tested. A cold saturated solution of sulphate of soda is prepared, placed over a fire, and brought to the boiling point. The stone, suspended from a string, is immersed in the boiling liquid, and kept there during thirty minutes; it is then carefully withdrawn; the liquid is decanted free from sediment into a flat

vessel, and the stone is suspended over it in a cool cellar. An efflorescence of the salt soon makes its appearance on the stone, when it must be again dipped into the liquid. This should be done once or more frequently during the day, and the process be continued in this way for about a week. The earthy sediment, found at the end of this period in the vessel, is weighed, and its quantity will give an indication of the like effect of frost. This process, with the official statement of a commission of engineers and architects, by whom it was tested, is minutely detailed in vol. 38, Annales de Chimie et de Physique, and the results are such as to commend it to the attention of engineers in submitting new stones to trial.

34. From more recent experiments by Dr. Owen it was found that the results obtained by exposing the more porous stones to the alternate action of freezing and thawing during a portion of a winter were very different from those resulting from Brard's method, owing to the action of the salts being chemical as well as mechanical.

35. By the absorption of water, stones become softer and more friable. The materials for road coverings should be selected from those stones which absorb least water, and are also hard and not brittle. Granite, and its varieties, limestone, and common sand-stone, do not make good road materials of broken stone. All the hornblende rocks, porphyry, compact feldspar, and the quartzose rock associated with graywacke, furnish good, durable road coverings. The finegrained granites which contain but a small proportion of mica, the fine-grained silicious sand-stones which are free from clay, and carbonate of lime, form a durable material when used in blocks for paving. Mica slate, talcose slate, hornblende slate, some varieties of gneiss, some varieties of sand-stone of a slaty structure, and graywacke slate, yield excellent materials for flag-stone.

36. The influence of locality on the durability of stone is very marked. Stone is observed to wear more rapidly in cities than in the country; and the stone in those parts of edifices exposed to the prevailing rains and winds, soonest exhibits signs of decay. The disintegration of the stratified stones placed in a wall is mainly effected by the position which the strata or quarry bed receives, with respect to the exposed surface; proceeding faster when the faces of the strata are exposed, than in the contrary position.

37. EFFECTS OF HEAT.-Stones which resist a high degree of heat without fusing are used for lining furnaces,

and are termed fire-stones. A good fire-stone should not only be infusible, but also not liable to crack or exfoliate from heat. Stones that contain lime, or magnesia, except in the form of silicates, are usually unsuitable for fire-stones. Some porous silicious limestones, as well as some gypsous silicious rocks, resist moderate degrees of heat. Stones that contain much potash are very fusible under high temperatures, running into a glassy substance. Quartz and mica, in various combinations, furnish a good fire-stone; as, for example, finely granular quartz with thin layers of mica, mica slate of the same structure, and some kinds of gneiss which contain a large proportion of arenaceous quartz. Several varieties of sand-stone make a good lining for furnaces. They are usually those varieties which are free from feldspar, somewhat porous, and are uncrystallized in the mass. Talcose slate likewise furnishes a good fire-stone.

38. RESISTANCE TO ATTRITION.—Hardness is an essential quality in stone exposed to wear from the attrition of hard bodies. Stones selected for paving, flagging, and steps for stairs, should be hard, and of a grain sufficiently coarse not to admit of becoming very smooth under the action to which they are submitted. As great hardness adds to the difficulty of working stone with the chisel, and to the cost of the prepared material, builders prefer the softer or free-stones, such as the limestones and sand-stones, for most building purposes. The following are some of the results, on this point, obtained from experiment:

Table showing the result of experiments made under the direction of Mr. Walker, on the wear of different stones in the tramway on the Commercial Road, London, from 27th March, 1830, to 24th August, 1831, being a period of seventeen months. Transactions of Civil Engineers, vol. 1.

The Commercial Road stoneway consists of two parallel lines of rectangular tramstones, 18 inches wide by 12 inches deep, and jointed to each other endwise, for the wheels to travel on, with a common street pavement between for the horses.

The following table gives the results of some experiments on the wear of a fine-grained sand-stone pavement, by M. Coriolis, during eight years, upon the paved road from Paris to Toulouse, the carriage over which is about 500 tons daily, published in the Annales des Ponts et Chausées, for March and April, 1834:

M. Coriolis remarks, that the weight of water absorbed affords one of the best indications of the durability of the finegrained sand-stones used in France for pavements. An equally good test of the relative durability of stones of the same kind, M. Coriolis states, is the more or less clearness of sound given out by striking the stone with a hammer.

The following results are taken from an article by Mr. James Frost, Civ. Engineer, inserted in the Journal of the Franklin Institute for Oct. 1835, on the resistance of various substances to abrasion. The substances were abraded against a piece of white statuary marble, which was taken as a standard, represented by 100, by means of fine emery and sand. The relative resistance was calculated from the weight lost by each substance during the operation.