106.  Of casting iron and other metals.—­Patterns of wood or metal made from drawings are the originals from which the moulds for casting are made:  so that, in fact, the casting itself is a copy of the mould; and the mould is a copy of the pattern.  In castings of iron and metals for the coarser purposes, and, if they are afterwards to be worked even for the finer machines, the exact resemblance amongst the things produced, which takes place in many of the arts to which we have alluded, is not effected in the first instance, nor is this necessary.  As the metals shrink in cooling, the pattern is made larger than the intended copy; and in extricating it from the sand in which it is moulded, some little difference will occur in the size of the cavity which it leaves.  In smaller works where accuracy is more requisite, and where few or no after operations are to be performed, a mould of metal is employed which has been formed with considerable care.  Thus, in casting bullets, which ought to be perfectly spherical and smooth, an iron instrument is used, in which a cavity has been cut and carefully ground; and, in order to obviate the contraction in cooling, a jet is left which may supply the deficiency of metal arising from that cause, and which is afterwards cut off.  The leaden toys for children are cast in brass moulds which open, and in which have been graved or chiselled the figures intended to be produced.

107.  A very beautiful mode of representing small branches of the most delicate vegetable productions in bronze has been employed by Mr Chantrey.  A small strip of a fir-tree, a branch of holly, a curled leaf of broccoli, or any other vegetable production, is suspended by one end in a small cylinder of paper which is placed for support within a similarly formed tin case.  The finest river silt, carefully separated from all the coarser particles, and mixed with water, so as to have the consistency of cream, is poured into the paper cylinder by small portions at a time, carefully shaking the plant a little after each addition, in order that its leaves may be covered, and that no bubbles of air may be left.  The plant and its mould are now allowed to dry, and the yielding nature of the paper allows the loamy coating to shrink from the outside.  When this is dry it is surrounded by a coarser substance; and, finally, we have the twig with all its leaves embedded in a perfect mould.  This mould is carefully dried, and then gradually heated to a red heat.  At the ends of some of the leaves or shoots, wires have been left to afford airholes by their removal, and in this state of strong ignition a stream of air is directed into the hole formed by the end of the branch.  The consequence is, that the wood and leaves which had been turned into charcoal by the fire, are now converted into carbonic acid by the current of air; and, after some time, the whole of the solid matter of which the plant consisted is completely removed, leaving a hollow