garden rhubarb, we can see, with the aid of a microscope, the fine
needle-shaped crystals of oxalate of potash lying among the fibres of
the plant,--a provision for an extra supply of the oxalic acid which is
the source of the intense sourness of this vegetable. When the sap of
the sugar-maple is boiled down to the consistence of syrup and allowed
to stand, it sometimes deposits a considerable amount of sand; indeed,
this is probably always present in some degree, and justifies, perhaps,
the occasional complaint of the grittiness of maple-sugar. But it is a
native grit, and not chargeable upon the sugar-makers. It is nothing
less than flint, which the roots of the maple absorbed, while it was
dissolved in water in the soil. The sap, still holding the flint in
solution, flows out, clear as water, when the tree is tapped; but when
it is concentrated by boiling, the silicious mineral is deposited in
little crystals, so that the bottom of the pan appears to be covered
with sand. We could not select a more interesting example of the very
wide diffusion of some compound substances than this one of silicic
acid. It is found in the mineral and vegetable kingdoms. Being a
mineral, it cannot be appropriated to animal uses, without being
decomposed and transformed into an organic condition; but in the
numerous species of plants whose stalks require stiffening against
the winds,--in the grasses and canes, including all our grains, the
sugar-cane, and the bamboo,--a silicate (an actual flint) is taken up by
the roots and stored away in the stalks as a stiffener. The rough, sharp
edge of a blade of grass sometimes makes an ugly cut on one's finger by
means of the flint it contains. Silex is the chief ingredient in quartz
rock, which is so widely diffused over the earth, and enters into the
composition of most of the precious stones. The ruby, the emerald, the
topaz, the amethyst, chalcedony, carnelian, jasper, agate, and garnet,
and all the beautiful varieties of rock crystal, are mostly or entirely
silex. Glass is a compound of silex and pearlash. One who is curious in