The variety of machiner7 and apparatus used in dressing ores is very great, and they pass under different names in different districts, but they are all very similar in principle to those we have described.
DrETALS—KETALLOIDS. Although each metal is considered in a separate article, there are various points regarding the general physical and chemical characters of these bodies, and the method of classifying the,m, which require notice.
It is not easy to define a metal. All the elements are usually divided by chemists into two groups--viz., the non-metallic bodies or metalloids, and the metals; the list of non-metallic bodies containing all those elements in which the characteristic properties of the bodies popularly known as metals (such as silver, gold, iron, etc.) are wanting; these characteristic properties being their metallic luster, their opacity, and their capacity of eonductincr heat and electricity. The non-metallic elements are 14 in number— viz., oxygen, t'hydrogen, nitrogen, sulphur, selenium, tellurium, phosphorus, chlorine, bromine, iodine, fluorine, carbon, boron, and silicon, of which five are gases, one a liquid, and the rest are solids at ordinary temperatures.
The division of the elements into these two great groups is, however, not based upon any definite scientific grounds, and it is still an open question whether some of the metalloids, as, for example, tellurium and silicon, should not be placed among the metals. The non-metallic bodies or metalloids being only remarkable as a group for their negative properties, require no special consideration, and we therefore proceed to notice the general properties of the metals.
The following are The most important of the physical properties of the metals: 1. All metals, unless when they are in a finely. pulverized form, exhibit more or less of the characteristic luster termed metallic. Two of the non-metallic elements, iodine and carbon, in some forms, present also a metallic luster. 2. All metals are good oon -ductors of heat and electricity, although in very unequal degrees. 3. With the excep tion of mercury, all the metals are solid at ordinary temperatures. With the exception -of gold, copper, calciuin, aud strontium, the metals are more or less white, with a tendency to blue or gray. MOst of them have been obtained in crystals, and probably all of them are capable of crystallizing under certain conditions. 4. Metals are remarkable for their
opacity, and, with the exception of gold, do not transmit light, even when they are reduced to extremely thin leaves. 5. All the metals are fusible, although the temperatures at which they assume the fluid form are very different (see PITSHCG POINTS); and some of them, as mercury, arsenic, cadmium, zinc, etc., are also volatile. 6. Great weight, or a high specific gravity, is popularly but erroneously regarded as a characteristic of a metal; while platinum, osmium, and iridium (the heaviest bodies known in nature) are more than 20 times as heavy as water, lithitun, potassium, and sodium are actually lighter than that fluid. 7. Great differences are observable in the hardness, brittleness, and tenacity of metals. While potassium and sodium may be kneaded with the finger, and lead may be marked by the finger-nail, most of them possess a considerable degree of hardness. Antimony, arsenic, and bismuth are so brittle that they may be easily pulverized in a mortar; while others, as iron, gold, silver, and copper, require great force for their disintegration. Taking iron and lead as representing the two extremes of tenacity, it is fouud that an iron wire will bear a weight 26 times as heavy as a leaden wire of the same diameter. See DUCTILITY, MALLEABILITY. 8. It is a remark able property of the metals that none of them axe capable of being dissolved -vithout undergoing chemical change. Sulphur, phosphorus, iodine, etc., may be dissolved, and after the evaporation of the solvent, may be reobtained with all their original properties; but this is never the case with metals.
Amongst the chief chemical properties of metals we next notice: Their strong affinities to certain of the non-metallic elements. All the metals, with out exception, combine with oxygen, sulphur, and chlorine, and often in several propor tions, forming oxides, sulphides (formerly terrued sulphurets), and chlorides. Many of them combine with bromine, iodine, and fluorine. The other compounds of this nature, excepting carbide (formerly carburet) of iron, or steel, and the hydrides of arsenic and antimony (commonly known as arseniuretted and antimoniureted hydrogen), which are of importance in toxicology, may be passed over without notice.