If the contents of the blue and white papers in a Seidlitz powder are mixed, no chemical action follows, but if dissolved separately in glasses of water, and then poured together, a violent effervescence takes place. If a small amount of potassium chlorate and a _little_ piece of sulphur be put together in a mortar, and then pressed by the pestle, sharp detonations follow. Dynamite, which is nitro-glycerine mixed with infusorial earth, sugar or sawdust, is quite harmless when free from acid, unless struck. The above instances illustrate the various influences that stimulate chemical combination. Almost all the crust of the earth is formed of three substances—quartz, lime, and alumina. Wherever we stand on the round globe, it is safe to say that one or all of these are beneath our feet. QUARTZ. [Illustration: QUARTZ CRYSTALS.] This mineral comprises about one half the earth’s crust. Its symbol is SiO₂, being a compound of silicon and oxygen, in the proportions indicated. It is very hard, easily scratching glass, of which it forms an important constituent, is acted upon by only one acid—hydrofluoric; this attacks it eagerly, as may be shown by the following interesting experiment: Take a little lead saucer, or in the absence of this, spread lead foil carefully over the inside of an ordinary saucer, and in this place some powdered fluor spar. This mineral is quite abundant in nature, and is always to be obtained, in the form of a powder, from dealers in chemicals. Have a pane of glass covered by a thin film of wax. Now trace upon this surface with a sharp point, anything you may desire, verse or picture. Pour into the saucer containing the fluor spar, sufficient sulphuric acid to make a paste. Place over this the plate of glass, with the waxed side down, and let it remain for twenty-four hours. Remove the wax by heating, and on the glass you will find a perfect etching, the HF having removed the silica. The same effect may be produced in a few moments by applying to the bottom of the saucer a moderate heat. Care should be taken not to inhale the fumes, as they are highly corrosive. Quartz can be melted at a high temperature, and may be dissolved in certain hot solutions. It is still a question in dispute, whether the numerous quartz veins found in rocks were introduced there in melted form or in solution. Probably, sometimes in one state and sometimes in the other. Any visitor to a glass manufactory can see how easily glass in a melted state is manipulated; and travelers often bring from the geysers[1] fine specimens of silica called geyserite, derived from the material held in solution in the hot water, and deposited on the edge of the “basin.” [Illustration: SIDE AND TOP VIEW OF THE REGENT OR PITT DIAMOND (REDUCED IN SIZE)—CUT IN THE FORM OF THE “BRILLIANT.”]