mould, bearing on its interior all the minutest traces of its late vegetable occupant.  When this process is completed, the mould being still kept at nearly a red heat, receives the fluid metal, which, by its weight, either drives the very small quantity of air, which at that high temperature remains behind, out very through the airholes, or compresses it into the pores of very porous substance of which the mould is formed.

108.  When the form of the object intended to be cast is such that the pattern cannot be extricated from its mould of sand or plaster, it becomes necessary to make the pattern with wax, or some other easily fusible substance.  The sand or plaster is moulded round this pattern, and, by the application of heat, the wax is extricated through an opening left purposely for its escape.

109.  It is often desirable to ascertain the form of the internal cavities, inhabited by molluscous animals, such as those of spiral shells, and of the various corals.  This may be accomplished by filling them with fusible metal, and dissolving the substance of the shell by muriatic acid; thus a metallic solid will remain which exactly filled all the cavities.  If such forms are required in silver, or any other difficulty fusible metal, the shells may be filled with wax or resin, then dissolved away; and the remaining waxen form may serve as the pattern from which a plaster mould may be made for casting the metal.  Some nicety will be required in these operations; and perhaps the minuter cavities can only be filled under an exhausted receiver.

110.  Casting in plaster.  This is a mode of copying applied to a variety of purposes:  to produce accurate representations of the human form—­of statues—­or of rare fossils—­to which latter purpose it has lately been applied with great advantage.  In all casting, the first process is to make the mould; and plaster is the substance which is almost always employed for the purpose.  The property which it possesses of remaining for a short time in a state of fluidity, renders it admirably adapted to this object, and adhesion, even to an original of plaster, is effectually prevented by oiling the surface on which it is poured.  The mould formed round the subject which is copied, removed in separate pieces and then reunited, is that in which the copy is cast.  This process gives additional utility and value to the finest works of art.  The students of the Academy at Venice are thus enabled to admire the sculptured figures of Egina, preserved in the gallery at Munich; as well as the marbles of the Parthenon, the pride of our own Museum.  Casts in plaster of the Elgin marbles adorn many of the academies of the Continent; and the liberal employment of such presents affords us an inexpensive and permanent source of popularity.