Some mechanical distinctions between the various kinds of atones may here be alluded to, on account of their important influence upon the cost of masonry. Thus the granites, and the ailicious conglomerates, are not susceptible of being sawn, but can only be detached in masses by means of wedges; the sandstones, and the harder oolites, can only be sawn by means of the framed, or plate saw, and sharp gravel and water; whilst such stones as the Bath, Caen, ke., are worked by the toothed saw. The first class above-described, or the stones unable to be sawn, are worked by means of points, or axes ; and in remarkably fine works they are occasionally polished : the second class is worked by the hammer, the mallet point and chisel, and sometimes "rubbed fine " the third class, or that of the soft oolites, is worked by axes or by tools slightly resembling those used by joiners, and the finished surface is, in England, obtained by the use of " the drag "—a tool very much like a short hand-saw. Rubble masonry is usually left rough from the tool, whilst Ashlar work, especially in the decorative parts of a building, such as strings, cornices, window and door dressings, kc., is either rubbed, or brought to a smooth face by the drag : in some cases however, as in what are called rusticated courses, the atones are occa sionally worked with a straight chase round the joints leaving the remainder rough from the tool. (Roams.] The above remarks have been almost exclusively applied to the manner in which masonry is executed In London ; but the leading principles they Involve apply equally to that art in other countries; for the mode of bonding the material, the precautions to be observed In order to avoid unequal settlements when rubble and ashlar are com bined, or when cementing materials of variable powers of expansion in setting are used, and the principles to be observed in selecting the stones and in placing them in a building, must be identical in every locality. It may be interesting, however, to the student of the arts of construction, to mention that the practice of the ancients in this particular branch has been elaborately described by Vitniviva; and that the practice of the moderns of recent times may be studied in detail in such works as Rondelet's ' Art de Ratir ;' or in Gwilt's ' En cyclopedia of Architecture.' Practically there is no difference between the styles of masonry adopted in former times and those now used, for the various systems of bonding used by the ancients, and described by Vitnivius under the names of the opus insertum,rctieulaSum, imbrica tun, or rerinnum, the isodomum, pseudisodomum, or the emplecton, In fact, only varieties of what would at present be described as coursed rubble, or small ashlar masonry. The Egyptians seem, however, to have attached great importance to the use of stones of large dimension, and even occasionally to have executed monolithic structures of con siderable importance; and the taste for this peculiar element of sub limity of expression in buildings may also be said to have pervaded nil the Eastern nations directly concerned in the early progress of civilisa tion, for it may be discovered in the monuments of Persia, '3Iedia, and in those of Tadmnor and Palmyra. The character of the materials used
by the Greeks (that is to say, the large straight block), which they obtained with facility from their marble rocks, had a marked influence also on the expression of their architecture, insomuch as their masons were able to ;Tan large openings by straight beams or lintels ; whereas the Romans, and, generally speaking, the inhabitants of the West of Europe, have been driven, by their lack of materials for the execution of such tmbeated masonry, to resort to the use of bricks, or of small stones, used as arches for the purpose of spanning the openings so much more easily dealt with when long lintels could be obtained. The Cyclopean masonry, as the irregular rubble work executed with stones of enormous dimensions, is called, and of which antique specimens are to be seen in the countries inhabited by the Pelasgic tribes, or in the modern structures of parts of Germany and of Spain, is again little else than a modification of the styles of masonry already described.
In some cases, where coloured marbles are obtained at small expense, they are applied as ornamental decorations by being let into panels in the external walls, or even in some instances they are exclusively used for the face work. Nearly all marbles are pure carbonates of lime of a highly crystalline character, and they are therefore exposed to the destructive actions before alluded to as being likely to affect the more earthy carbonates, although in an inferior degree. For internal masonry, however, marble may be used without hesitation in situa tions at least where the following objections would not apply; that is to say, that when used for chimney-pieces marble is exposed to a slow calcination if placed too near the fire-place, and it loses its crystalline to assume a saccharoid texture in proportion as the carbonic acid gas is driven off by the heat. The marbles, again, which contain veins of a different nature to the mass of the material, are exposed to unequal contractions, and the Italian black and gold, for instance, or the ser pentines (though strictly speaking the latter can hardly be called marbles), frequently crack upon the lines of the veins. Marble is in some countries, as in Holland, much used for lining the walls, and paving entrance halls ; but unless great care is taken to maintain an equable temperature, and a great degree of dryness in the atmosphere, a very unpleasant condensation of vapour takes place upon the face of the marble, in consequence of its powers of rapid conduction, and of its non-absorption. At the present day the fashion is to use foreign marbles for ornamental masonry, and the English market is supplied from Belgium and Italy ; but there are some singularly beautiful materials of this description to be obtained from our own country, and their neglect must be a cause for regret to artists and to economists. Marbles are worked by the mallet and chisel, and sawn by the plate saw ; indeed, on account of their uniform character, they are suscep tible of being worked by steam machinery.