Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/68

Rh 58 is understood to include ; the second column in each case gives the chemical characters of the native compounds utilized, italics indicating ores of subordinate importance. The term &quot; oxide &quot; must be understood to include carbon ate, hydrate, and occasionally (when marked in the table with *) silicate. Metal. Character of Ores. Iron Oxides, sulphide. r, ( Complex sulphides, also oxides, C Pr er j Jtal. Sulphide and reguline metal, Sllvcr I chloride. Gold Reguline metal. ( Sulphide and basic-carbonate, ml- Lead I phate.kc. Zinc Sulphide, oxide.* Tin Oxide. Mercury Sulphide, reguline metal. Antimony Sulphide. Bismuth lleguline metal. Nickel and cobalt... ...Arsenides. Platinum and platinum metals... Reguline. Aluminium Oxide, * sodio-fluoride. We have separated the last two from the rest because the methods used for their preparation are more of the character of laboratory operations, and because we do not mean to include these in our general exposition of metal- lurgic principles. The history of metallurgy, up to the most recent times, is obscure. It is only since about the beginning of this century that the art has come to be at all scientifically criticized ; and in the case of the most important processes all that science has been able to do has been merely to put her stamp upon what experience has long found to be right. Great and brilliantly successful scientific efforts in the synthetic line are not wanting, but they all belong to recent times. Science, by its very nature, aims at publicity ; empiricism at all times has done the reverse ; hence a history of the development of the art of metallurgy does not and cannot exist. A few historical notes on the discovery of certain of the useful metals are given in the introduction to METALS (q.v.). General Sequence of Operations. Occasionally metallic ores present themselves in the shape of practically pure compact masses, from which the accompanying matrix or &quot; gangue &quot; can be detached by hand and hammer. But this is a rare exception. In most cases the &quot; ore,&quot; as it comes out of the mine, is simply a mixture of ore proper and gangue, in which the latter not unfrequently predomi nates so much that it is not the gangue but the ore that really occupies the position of what the chemist would call the impurity. Hence, in general, it is necessary, or at least expedient, to purify the ore as such before the libera tion of the metal is attempted. Most metallic ores are specifically heavier than the impurities accompanying them, and their purification may be (and generally is) effected by reducing the crude ore to a fine enough powder to detach the metallic from the earthy part, and then washing away the latter by a current of water, as far as possible. In the case of a &quot;reguline&quot; ore, such as auriferous quartz, for instance, the ore thus concentrated may consist substan tially of the metal itself, and require only to be melted down and cast into ingots to be ready for the market. This, however, is a rare case, the vast majority of ores being chemical compounds, which for the extraction of their metals demand chemical treatment. The chemical operations involved may be classified as follows : 1. Fiery Operations. The ore, along in general with some kind of &quot;flux,&quot; is exposed to the direct action of a powerful fire. The fire in most cases has a chemical, in addition to its obvious physical function. It is intended either to burn away certain components of the ore in which case it must be so regulated as to contain a sufficient excess of unburned oxygen ; or it is meant to deoxidize (&quot;reduce&quot;) the ore, when the draught must be restricted so as to keep the ore constantly wrapped up in combustible flame gases (carbonic oxide, hydrogen, marsh-gas, &c.). The vast majority of the chemical operations of metallurgy fall into this category, and in these processes other metal- reducing agents than those naturally contained in the fire (or wind) are only exceptionally employed. 2. Amalgamation. The ore by itself (if it happens to be a reguline one), or the ore plus certain reagents (if it does not), is worked up with mercury so that the metal is obtained ultimately as an amalgam, which can be separated mechanically from the dross. The purified amalgam is subjected to distillation, when the mercury is recovered as a distillate while the metal remains. 3. Wet Processes. Strictly speaking, certain amalgama tion methods fall under this head ; but, in its ordinary acceptance, the term refers to processes in which the metal is extracted either from the natural ore, or from the ore as it is after roasting or some other preliminary treatment, by means of an aqueous acid or salt solution, and from this solution precipitated generally in the reguline form by some suitable reagent. Few methods of metal extraction at once yield a pure product. What as a rule is obtained is a more or less impure metal, which requires to be &quot; refined &quot; to become fit for the market. We now pass to the individual con sideration of the several steps referred to. Comminution of Ores. Assuming the ore to be given in the shape of large lumps, these must first be broken up into small stones (of about the size of those used for macadamizing a road) before they can go to the grinding-mill. This formerly used to be done by hand work; nowadays it is preferably effected by means of an American invention called the stone-breaker (fig. 1). This consists essentially of two substantial vertical iron plates; one is fixed, the other is connected with an excentric worked by an engine so as to alternately dash against and recede from the former. The lumps of ore, in passing through this jaw-like contrivance, are broken up into smaller fragments fit for FIG. 1. American Stone-Breaker. the mill. For the production of a coarse powder revolving cylinders are often employed. Two cylinders of equal diameter and length, made of iron, steel, or stone, are suspended by parallel axes in close proximity to each other. The width of the slit between them can be made to vary according to the requirements of the case. The cylinders are made to revolve in opposite directions, so that the stones when run into the groove formed by their upper halves are drawn between them and are crushed into bits of a size depending on the least distance between the two surfaces. Exceptionally hard stones might bring the machine to a standstill or cause breakages ; hence only one of the two axes of rotation is absolutely fixed ; the cushions of the other are only held in relatively fixed positions, each between a couple of guiding rails, by means of powerful springs at their backs. The springs are made of alternate disks of india- rubber and sheet-iron, and yield appreciably only to very strong pressures. When an exceptionally hard stone comes on, they yield