15. The second musical science which belongs peculiarly to sculpture, (and to painting, so far as it represents form,) consists in the disposition of beautiful masses. That is to say, beautiful surfaces limited by beautiful lines. Beautiful _surfaces_, observe; and remember what is noted in my Fourth Lecture of the difference between a space and a mass. If you have at any time examined carefully, or practiced from, the drawings of shells placed in your copying series, you cannot but have felt the difference in the grace between the aspects of the same line, when inclosing a rounded or unrounded space. The exact science of sculpture is that of the relations between outline and the solid form it limits; and it does not matter whether that relation be indicated by drawing or carving, so long as the expression of solid form is the mental purpose; it is the science always of the beauty of relation in three dimensions. To take the simplest possible line of continuous limit--the circle: the flat disk inclosed by it may indeed be made an element of decoration, though a very meager one; but its relative mass, the ball, being gradated in three dimensions, is always delightful. Here[8] is at once the simplest, and, in mere patient mechanism, the most skillful, piece of sculpture I can possibly show you,--a piece of the purest rock-crystal, chiseled, (I believe, by mere toil of hand,) into a perfect sphere. Imitating nothing, constructing nothing; sculpture for sculpture's sake of purest natural substance into simplest primary form. 16. Again. Out of the nacre of any mussel or oyster shell you might cut, at your pleasure, any quantity of small flat circular disks of the prettiest color and luster. To some extent, such tinsel or foil of shell _is_ used pleasantly for decoration. But the mussel or oyster becoming itself an unwilling modeler, agglutinates its juice into three dimensions, and the fact of the surface being now geometrically gradated, together with the savage instinct of attributing value to what is difficult to obtain, make the little boss so precious in men's sight, that wise eagerness of search for the kingdom of heaven can be likened to their eagerness of search for _it_; and the gates of Paradise can be no otherwise rendered so fair to their poor intelligence, as by telling them that every gate was of "one pearl."