Scientific amusements . When clay is submitted to the influenceof sulphuric acid and chloride of pota-sium, we obtain alum, which is a sul-phate of alumina and potash. Alumis a colourless salt, which crystallizeson the surface of water in beautifuloctahedrons of striking regularity. TheCalcined alum. |^g_ ^^ page 21 represents a group ofalum crystals. This salt is much used in the colouring offabrics ; it is also used for the sizing of papers, and theclarification of tallow. Doctors also use it as an astrin-gent and caustic substance. When alum is submitted tothe action of heat in an earthenwa
Scientific amusements . When clay is submitted to the influenceof sulphuric acid and chloride of pota-sium, we obtain alum, which is a sul-phate of alumina and potash. Alumis a colourless salt, which crystallizeson the surface of water in beautifuloctahedrons of striking regularity. TheCalcined alum. |^g_ ^^ page 21 represents a group ofalum crystals. This salt is much used in the colouring offabrics ; it is also used for the sizing of papers, and theclarification of tallow. Doctors also use it as an astrin-gent and caustic substance. When alum is submitted tothe action of heat in an earthenware crucible, it loses thewater of crystallization which it contains, and expands ina singular manner, overflowing from the jar in which it iscalcined (see above). Iron, the most important of commonmetals, rapidly unites with oxygen, and, as we know, whena piece of this metal is exposed to the influence of dampair, it becomes covered with a reddish substance. In thewell-known experiment of the formation of rust, the iron. IRON. 23 gradually oxidises without its temperature rising, but thiscombination of iron with oxygen is effected much morerapidly under the influence of heat. If, for example, weredden at the fire a nail attached to a wire, and give it amovement of rotation as of a sling, we see flashing outfrom the metal a thousand bright sparks due to the com-bination of iron with oxygen, and the formation of anoxide. Particles of iron burn spontaneously in contactwith air, and this property for many centuries has beenutilized in striking a tinder-box ; that is to say, in sepa-
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Keywords: ., bookcentury1800, bookdecade189, booksubjectscientificrecreations
