tions of those which were retained at the first quartering. The mass has now been reduced to one-fourth that with which the assayer started. It is now crushed to a size that will pass through a sieve having 4-inch mesh, thoroughly mixed, and quartered down to half its bulk, an eighth of that at the commencement; the sample reserved is crushed to pass through an 8 mesh sieve, that is having 8 meshes to the linear inch. It is again thoroughly mixed, and sampled down in the same way to one-sixteenth the original bulk, and weight; crushed to pass through a 12 mesh sieve, again mixed, and sampled down to one thirtysecond of the original, and again divided. The sample is then crushed to pass through a 30 to 40 mesh sieve, and sampled down to from Toth to th of the original, a split shovel being used in the later stages. 00 500 This shovel is constructed with several teeth, each formed like a gutter, and the ore which falls between the teeth is taken for the first sample, then that which is caught by the teeth, and so on. After the last process, the sample is again crushed down to pass through a 100 mesh sieve, or in some cases smaller, and the residue, that which passes through, is spread out on a sheet of glazed paper, oiled silk, or something similar, and thoroughly rolled over and over again at least 100 times. It is then spread out in a thin layer, and divided up into squares. A small sample is now taken, by means of a spatula, a tool provided for the purpose, from each of the squares, the sampler being very careful not to take the sample from the surface, but to dip into each, right to the bottom. The final samples are put up in accurately gauged bottles, and separately estimated. The separate assays should agree within a small percentage. It is important not to lose any dust in any of the above processes, and to weigh very carefully at every step, particularly before the sample goes through the sieve. final samples, which weigh a few ounces each, should, it is claimed, fairly represent the ore to be assayed. ESTIMATING. The There are, broadly, two groups of methods of estimating the metal contained in the sample. By dry methods, in which heat is employed, the ore being fused. By wet methods, in which the ore sample is treated chemically, and usually a certain insoluble salt of the metal produced, which is weighed, and from its weight, the weight of the metal in the sample deduced, and thence the percentage, or other quantities required. Gold and Silver are always estimated in ounces or pennyweights per ton of ore, other metals by percentages of the metal in the ore. Dry, or Fusion Methods.-Only some of the methods used with Lead, Copper, Tin, and Iron will be dealt with in this section. The fusion methods all consist in adding a flux to the ore, submitting the mixture to the action of heat, and thereby separating the metal assayed for by gravity. By a flux is meant a substance or a mixture of substances that when added to ore, forms with some of the components of the ore a fusible compound, and hence causes the whole mixture to run, when heat is applied, the metal sought being heavier than the other substances in the ore, easily separating from the slag. The substances contained in the mixture submitted to fusion, other than the metal sought, are called the slag, and it is important, in all cases of assaying by fusion, to obtain a liquid slag, the metal then easily separating out. For Lead, the flux consists of a mixture of the Carbonates of Potassium and Sodium, with some Borax, and Flour, and to this is added some iron, in the form of nails. If the fusion is carried out in an iron pot, the iron of the pot will do as well. The object of the iron is to carry away the sulphur present in so many lead ores, by the formation of Sulphide of Iron. Cyanide of Potassium may also be used as a flux, the cyanide forming with the sulphur a compound of Potassium, Sulphur, and Cyanogen. The process is, however, dangerous, as the fumes of Cyanide of Potassium, or Prussic Acid, are very poisonous. The fusion is carried out in a furnace, usually of the muffle pattern, designed for the purpose, but sometimes in an open furnace. Other Methods with Lead.-There are two wet methods of assaying lead ores, known as the Gravimetric, and the Volumetric. In the Gravimetric, the ore is treated with Nitric Acid, the Sulphur contained in it being thereby converted into Sulphuric Anhydride, and combining with the lead to form the insoluble Lead Sulphate, from which the amount of the lead is estimated, by the fact that pure Lead Sulphate contains 68.3 per cent. of Lead. The salts of Iron and Copper have to be dissolved out separately with this method. In the Volumetric method the ore is treated as in the Gravimetric process, Lead Sulphate being formed as before. The Lead Sulphate is dissolved in Ammonium Acetate, and treated with Ammonium Molybdate, the White Lead Molybdenate being formed, which is carefully estimated in a specially graduated burette. Copper.-In the dry or fusion process for assaying Copper, a flux is employed, consisting of Borax, Soda, and Bi-Tartrate of Potash, the process being very much the same as with Lead. The principal wet method, with Copper, is the Electrolytic, the ore being dissolved in a solution of Sulphuric Acid, and subjected to the action of an electric current, the Electrodes being Platinum, the increased weight of the Cathode, the negative Electrode, measuring the weight of Copper in the solution, and therefore in the sample, if all the Copper has been brought down. A favourite form for the Electrodes is, two concentric cylinders, the inner one being the Cathode. Care must be taken, in using the Electrolytic method, to eliminate the Silver, Arsenic, and Cadmium, which would come down with the Copper. and crushed sufficiently to pass through a 40 mesh sieve, and concentrated, the concentrates being roasted in an iron roasting dish in a muffle furnace. The product is treated with aqua regia; the product of this, after filtering, is ground to an 80 mesh sieve, and is then assayed with a flux, by heat in a muffle furnace, Cyanide of Potassium being used for a flux by some assayers; a mixture of the Bicarbonates of Potash, and Soda, together with some Borax, and some Charcoal, by others; and Sodium Carbonate, with Lime, by others. The process is similar to that with the other metals. Iron.-Iron may be assayed by the fusion method, and by two wet methods. In both wet methods, the iron in the ore is all reduced to the Ferrous state, and in one is then treated with Permanganate, in the other with Bichromate of Potassium, the Iron being thereby raised to the Ferric state, the volume of the standard solution required to bring all up being the measure of the Iron in the ore. With Permanganate of Potash, the salt being of a deep red colour, the Iron solution turns yellow, as the Permanganate is added, till all the Iron has been raised to the Ferric state, when the first drop in excess causes some of it to turn pink. With the Bichromate, the solution turns green, and the end of the process is known by placing a drop of the solution on a white tile, and adding Ferrocyanide to it. As long as any Ferrous salt remains, the solution turns blue. In all cases the metal, or the salt of the metal, is carefully weighed in specially delicate balances, from which even the air is excluded, and which are kept scrupulously free from dust. The percentage of metal in the ore is found by a simple arithmetical calculation, from the weight of ore in the sample, and the weight of the metal or the salt of the metal finally obtained. But it is necessary to be careful, as the sampling proceeds, to see that credit is given for all the metal obtained. Sometimes minute pellets of the metal are obtained in the crushing process, before any particular sieve. These are carefully collected, weighed, and credited, not to the final sample, but to the sample and weight at which they occurred. Thus, if say a pennyweight of metal were obtained at the sample Tin.-A certain quantity of the ore is taken, going through the sieve at which the sample There are two Volumetric methods for assaying Copper. In one, Potassium Cyanide is added to an Ammoniacal salt of Copper, the insoluble salt Cyanide of Copper being formed, which is estimated in the usual way. The ore is treated with a mixture of Nitric and Sulphuric Acids, and then with Ammonia. In the other method, Potassium Iodide is added to Cupric Acetate, prepared from the ore, Cupric Iodide being formed, and it is then treated with Hyposulphite of Sodium, Hydriodic Acid being formed. The quantity of Copper in the sample is estimated from the quantity of Iodine liberated; one atom being liberated for each atom of Copper. was reduced from to, the pennyweight would be credited as a pennyweight in so many pounds the weight of the sample at that stage. Assembling. This is a comparatively recent practice in its application to engineers' work, being only possible when an interchangeable system is adopted. The term signifies that all the parts of which a motor or mechanism is built up are brought together finally without any correction by hand fitting; that any parts which are identical in shape and dimensions may be taken at random from a pile, and put together without the assistance of cutting or of scraping tools, differing therein from mechanisms which require the numerous corrections of the fitter. This ideal is not always fully realised even in shops where parts are nominally assembled, but it is absolutely so in large numbers of mechanisms. Without it, cheap production, and the replacement of worn and broken parts as required by customers, would not be possible. In order to the realisation of a perfect system of assembling, the machinery employed in the production of the parts must be so designed that the tools and appliances used shall both cut and size, that is embody the dimensions as well as the formation of all similar pieces. If this is not done on one machine, it must be on another at a later stage, i.e. a grinder succeeding the lathe, or planer. A familiar illustration is afforded by the Automatic Screw Machines, in the use of Box Tools, which size as well as shape, and the cutting of screw threads by fixed dies. And then, to prevent loss of time when the parts come into the hands of the assemblers, the separate pieces are all gauged as they leave the machines. Fixed gauges of various kinds are used, and lads or girls frequently handle them. Even here things are often so arranged that the pieces automatically size themselves, according as they fit, or do not fit gauges, which lie in a course along which they are compelled to travel. Assembling in its strict and absolute sense is only practicable with the smaller mechanisms, of which Small Arms afford the best illustration. As dimensions increase, the devices used for small work are no longer practicable, neither can the effects of spring, of temperature, and other variables be eliminated. In all machines of medium and large sizes some fitting and adjustments become more or less necessary. Then it is a question whether the work is sufficiently often repeated to make it worth while incurring the expense of working very closely to absolute gauged dimensions, when mutual fitting might be equally well adapted to the case. That is a question which is answered differently by different firms. The operation of assembling is done on special benches set apart for the work, and it may happen that the assemblers will put together the whole of a small mechanism, or a separate section of a mechanism will be entrusted to each worker, which is more desirable and economical in some instances. Where parts are put into stock largely, the complete fittings may only be made up as required, and the assemblers kept busy on detached portions, which they assemble to the exclusion of everything else. Special devices are employed in connection with assembling, such as boards or trays carrying the detached components of the mechanism, for convenience of picking them out, and special stands to rest the partly completed pieces upon, holding them steady while the assembler is at work. Devices such as indicators and various gauges are also employed, for getting correct distances and positions of related parts, not because the latter are not machined correctly, but because such adjustments are unavoidable in many mechanisms, though not necessarily involving cutting or working by tools. When, however, the latter practice is involved, we step from assembling at once into fitting. Fig. 150, Plate XII., is an example of a modern assembling shop. Assistant Cylinder.-A small cylinder designed by Mr Joy to relieve the strain on the eccentrics, and the valve gears of heavy marine engines. The dead weight of the slide valve, its inertia, and the friction of the valve on its seating, make up a total which stresses the eccentrics and gears severely, and produces tensile and compressive strains in the valve rod. The assistant cylinder sets up forces equal and |