Platinum is a metal of a white color, and is the heaviest substance known, being more than twenty-one times heavier than water. It is capable of being welded and wrought out by the hammer like iron; it is not acted on by any of the acids, but, like Gold, is dissolved by chlorine or a mixture of hydrochloric and nitric acid; it does not tarnish by exposure to air, and is extremely difficult to melt, requiring the very highest heat that can be produced; these qualities render it one of the most useful of the metals to the chemist, furnishing him with retorts, crucibles, and evaporating dishes suitable for many purposes; its high price, however, renders it not so generally used as it would otherwise be. NICKEL. This metal is of a white color and very difficult to melt, it is about eight times heavier than water and is chiefly obtained from the ore known as kupfer-nickel, found very plentifully in Germany; this kupfer-nickel is a native arseniuret of nickel, that is, nickel in chemical combination with arsenic. It is very difficult to separate these two metals, but an effectual and cheap process has lately been devised to do so. Nickel is attracted by the magnet similar to iron but in an inferior degree. Till lately nickel was but little used, but it now forms the basis of those compound metals known as nickel-silver, German-silver, and British-plate, all which varieties are generally employed as an economical substitute for silver; it is also used largely as a foundation on which to deposit pure silver by the electro-plating process, for which purpose it is most admirably suited being very superior to copper in consequence of its color, as the silver always wears off unequally and exposes the ground work of metal beneath. German-silver is composed of copper, nickel, and zinc, in various proportions. Nickel always forms one of the constituents of meteoric iron, those mysterious masses called aerolites, which sometimes fall to the earth. COAL. Illustration: MINERALS (‡ INDUSTRIAL SCENE). This, the richest product of the mine, the well-known fuel used in almost every branch of industry as well as for warming our houses, is got from the depths of the earth, where it exists in certain localities forming what are called “coal fields” or “basins.” It is the result of changes produced during many ages upon vegetable matter buried during the various convulsions which the earth has undergone, and pressed into layers or strata of various thicknesses. To raise this valuable fuel, powerful machinery is used and deep shafts are sunk at an enormous expense. Coal exists in various forms: the following are the most easily recognised:-- 1.--Cubical coal; shining and easily broken into squarish fragments. It burns brightly. 2.--Slate coal; dull in color, splits like slate. Burns well.