458 BUILDING [BRICKWORK. before 1825, it is only of late years that this scaffold has been much used. A couple of planks are secured side by side to form a platform, which is guarded by a railing all round to prevent the workmen falling off. To iron bands are secured pulleys and ropes, passing over other pulleys made fast to two or more beams projecting out of the upper windows, or secured to the roof-timbers, by means of which the workmen employed can raise or lower the scaffold to any position where it is wanted to get access to the work to be done. Brick-making. The manufacture of bricks forms the subject of a separate article. See p. 279 of the present volume. Mortars Mortars and Cements. A few observations on the com- and position of mortars and cements for bricklaying will be cements, necessary here. Mortar is of two kinds, common mortar, or that mixture of lime and sand ordinarily used in building ; and hydraulic mortar, or that which will set under water. Cement is a name given to the produce of certain argilla ceous stones, after calcination, which will set rapidly in the air, becoming a hard adhesive substance in a short time, and will also set under water, both without admix ture of any other substance. The name is given, too, to certain artificial imitations of these substances, possessing the same properties ; and besides, to various bituminous or oleaginous compositions, used in building for similar purposes. Lime. Pure lime, which is an oxide of a metal called calcium, does not exist in a natural state. It is, however, found abundantly in the conditions of carbonates and sub- carbonates, in chalk, and in the various other descriptions of limestones. Its chemical qualities and analysis will be described under the proper headings in this work. Limes are generally classed, since the publication of the work of Vicat, as (1) rich limes, (2) poor limes, (3) limes slightly hydraulic, (4) hydraulic limes, and (5) eminently hydraulic limes. In treating of mortar we have to deal with the first two of this division. The first operation is to drive off the water, which all limestones contain in a greater or less degree, and the carbonic acid gas, which is done by calcining or burning in a kiln at red heat ; this must be kept up for several hours, care being taken to avoid any approach to vitrification. By this process it is slightly diminished in bulk, loses nearly half its weight, and becomes caustic lime. The lime is next converted into a hydrate by a process called " slaking," or throwing pure water over it from time to time till it hisses and cracks with considerable force and some noise, gives off a large quantity of hot vapour, and falls into a powder. The rich limes, which are the purest oxides of calcium, increase to double their bulk in the process. The poor limes swell to a much less degree. The hydrates thus formed absorb water, and easily take the form of a paste. They contain rather less than one-third water to two-thirds lime. In this state, if treated with pure water, frequently renewed, every particle of the rich limes, and very nearly the whole of the poor limes, will be taken up in solution. In the pro cess of slaking too much water should not be used, as it " drowns " the lime, according to the expression of the workmen. When in the form of paste it begins to absorb carbonic acid, which is always present in air in considerable quantities, and gradually to crystallize again, and so to harden. If the air be excluded from the hydrate of pure lime, it may be kept for almost any length of time. Alberti (lib. ii. cap. 11) says that he once discovered some in an old ditch, which from certain indications must have been there . r >00 years, and was as soft as honey or marrow, and per fectly fit for use. Hkh lime- The rich limestones give a white lime, which easily slakes, and increases in bulk ; but it is curious that though stones. the stones differ so much in outward appearance and in texture, the lime, if they be well burned, is the same. The softest chalk and the hardest rag-stone or marble yield an equally good lime, the calcium which they contain being the same mineral. But as chalk generally contains water, irregularly distributed in some places and not in others, and as it does not exhibit the change that marble or stone does, it is frequently unequally burned, and therefore slakes imperfectly. It is said by Higgins (Mortars and Cements, p. 29), however, that lime made from chalk absorbs carbonic acid more rapidly than that made from stone ; but experience does not seem to warrant this con clusion. Poor limestones are those which contain silica, Poor lim magnesia, manganese, or metallic oxides. In consequence stoucs. of this they are more liable to vitrify in burning, and do not slake so freely. The lime is generally of a browner colour than that from rich limestones, which is said to be a proof of the presence of the above-named metallic oxides. If, however, they be ground so as to facilitate the slaking of every particle, arid if used immediately being made up, poor limes produce a mortar which becomes harder than that from the rich limes, and which resists water better. In fact, works where the latter have been used have been found to fail .entirely by the action of running water, which, as before has been said, will continue to remove the whole of a rich lime particle by particle. It is found that the mixture of some kind of hard matter Samls, iu particles or granules facilitates the setting of mortar, and renders it harder and more adhesive than when used alone, besides the saving of limestone and expense of burning. The harder this material and the sharper the particles the better, as the brick or stone has always some irregularities on the surface, into which these angles or sharp points may enter, and form what is called a key. The substance most generally used is sand, which is classed as river- sand and pit-sand. The former is usually preferred, as it is more free from earthy matters, particularly soft loams or clay. Mortar made with sand containing one- seventh or one-eighth part of fat clay moulders in winter like marl, a circumstance which proves the propriety of freeing from clay the sand used in mortar. If pit-sand be used it should be well washed. Scarcely any material is better than crushed quartz, or flint, from the sharpness of the angles of the particles ; in fact, it is said that very sharp sand, with an inferior lime, will make a more adhesive mortar than soft sand with the best lime. The practical mixing of mortar will be noticed further on. Where sand is scarce, other materials are sometimes used, the principal Other and cheapest of which is burned clay. The Romans used matcria this extensively in the form of pounded tile. At present Burned the custom is to throw up clay mixed with fuel in loose J heaps, to burn it slowly, and then to grind it in a mill with a proper quantity of lime. The French writers at one time asserted that burned clay, if not equal to pozzuo- lana, was very nearly so ; and large quantities were used as hydraulic mortars at various public works. Where the water was fresh, as at Strasburg, the work stood very well ; but where these mortars were exposed to the action of sea-water, they failed and went to powder in three or four years. Vicat gave great attention to the subject ; and though he attributed much of the fault to the imper fect carbonization of the materials, it appears with but little doubt there is some inherent difference between the pozzuolanas and other volcanic products and those produced artificially. After long investigation, Vicat was of opinion that this failure was due to the quantity of hydrochloride of magnesia always present in sea-water ; but in what way this affected the burned clay and not the volcanic products he was unable to explain.
A very excellent mortar, much used by engineers iu