To gain a true knowledge of the structure of bone, sections must be cut as in wood, both transversely and longitudinally; but with _fossil_ bones, without the lapidary’s wheel, &c., it is a laborious task, and indeed can seldom be properly accomplished. In this place, also, it may be mentioned that by submitting bone to the action of muriatic acid diluted ten or fifteen times with water, the lime, &c., is dissolved away and the cartilage is left, which may be cut into sections: in _caustic potash_ the animal matter is got rid of. Both of these preparations may be mounted in fluid. The method of cutting thin sections of bone may be also employed with the stones of fruit, vegetable ivory and such like substances; many of which show a most interesting arrangement of cells, especially when the sections are transverse. Most of these objects present a different appearance when mounted _dry_ to that which they bear when _in balsam_, owing to the cells becoming filled; and to arrive at a true knowledge of them we must have a specimen mounted in both ways. To those who study polarized light, few objects are more beautiful than the sections of the different kinds of horn. We will briefly inquire into the best method of cutting these. There are three kinds of horn, the first of which is hard, as the stag’s, and must be cut in the same manner as bone. The second is somewhat softer, as the cow’s. The third is another and still softer formation, as the “horn” (as it is termed) of the rhinoceros. In cutting sections of the two last we should succeed best by using the machine invented for these purposes which I shall shortly describe when the method of cutting wood is considered. To aid us in this when the horn is hard it must be boiled for a short time in water, when the cutting will be more easily accomplished. The sections should be both transverse and longitudinal, those of the former often showing cells with beautiful crosses, the colours with the selenite plate being truly splendid. Of this class the rhinoceros horn is one of the best; but the buffalo also affords a very handsome object. The cow’s, and indeed almost every different kind of horn, well deserves the trouble of mounting. Whalebone, when cut transversely, strongly resembles those of the third and softer formation. All these are best seen when mounted in Canada balsam, but care must be taken that they have been thoroughly dried after cutting, and then steeped in turpentine.