The microscope and its revelations . basaltic columns (Fig. 334, p). ThesheU is thus seen to be composed of a vast number of prisms, having a tolera-Fi&. 251. bly-uniform size, and usually pre-senting an ap-proach to thehexagonal are ar-ranged perpendi-cularly (or nearlyso) to the surfaceof the lamina ofthe sheU; so thatits thickness isformed by theirlength, and itstwo surfaces bytheir more satisfac-tory view of theseprismsis obtainedby grindiag-downa lamina until itpossesses a highdegree of trans-parency; and itis then seen(Eig. 251) thatthe prisms them-selves appear
The microscope and its revelations . basaltic columns (Fig. 334, p). ThesheU is thus seen to be composed of a vast number of prisms, having a tolera-Fi&. 251. bly-uniform size, and usually pre-senting an ap-proach to thehexagonal are ar-ranged perpendi-cularly (or nearlyso) to the surfaceof the lamina ofthe sheU; so thatits thickness isformed by theirlength, and itstwo surfaces bytheir more satisfac-tory view of theseprismsis obtainedby grindiag-downa lamina until itpossesses a highdegree of trans-parency; and itis then seen(Eig. 251) thatthe prisms them-selves appear tobe composed ofa very homoge-neous substance,but that they areseparated by definite and strongly-marked lines of such a lamina is submitted to the action of dilute acid,so as to dissolve-away the carbonate of lime,_ a tolerably firmand consistent membrane is left, which exhibits the prismaticstructure just as perfectly as did the original sheU (Fig. 252);the hexagonal divisions being apparently the walls of cells. Fig. 251. Section of Shell of Pinna, transversely tothe dii-ection of its 252. Membranous basis of the same. PRISMATIC SHELL-STEUCTUEE OF BIYALYES. 591 resembling those of the pith or bark of a plant, in which thecells are frequently hexagonal prisms. In very thin naturallaminse, the nuclei ofthe cells can often be Fig. 253. ^^^^^ mmmmmmM:^ of the shell perpen- - - . dicularly to its sur- face, we obtain a viewof the prisms cut inthe direction of theirlength (Fig. 253);and they are fre-quently seen to be marked by delicate I ,>:=5--i tz^i-i^ i~- - i Fi l ^^;i.^^ /3-i r/ij stransverse striee (Fig. \^ ^iFp|=ite^;i N1^ f|lf?i =; im^254), closely resem- „ ^. • ^^^ o, ,, ^ ^. , ,. 11- ^ ,1 ^ 1 Section of the Shell of Piw?za, m the direction bhng those obser- of its prisms. vable on the prisms of the enamel of teeth, to which this kind of shell-structui-e may be considered as bearing a very close resemblance. Fig. 254. \ \,
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