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 such things may make glass out of a straw, by burning it and heating the ashes with a blowpipe.  A little globule of pure glass will form as the ashes are consumed.  The following curious instance, quoted by that interesting physiologist, Dr. Carpenter, shows the same effect upon a large scale.  A melted mass of glassy substance was found on a meadow between Mannheim and Heidelberg, in Germany, after a thunder-storm.  It was, at first, supposed to be a meteor; but, when chemically examined, it proved to consist of silex, combined with potash,—­in the form in which it exists in grasses; and, upon further inquiry, it was ascertained that a stack of hay had stood upon the spot, of which nothing remained but the ashes, the whole having been ignited by the lightning.