The agitator

— Fig. 91. gives a sufficiently exact idea of it, and may dispense with a lengthened description. It has ten cylindrical compartments, numbered from 1 to 10. The phials, after the solution of the alloy, are arranged in it in the order of their numbers. The agitator is then placed within reach of the pipette , intended to measure out the normal solution of sea salt, and a pipette full of this solution is put into each phial. Each is then closed with its gla stopper, previously dipped in pure water. They are fixed in the cells of the agitator by wooden wedges. The agitator is then suspended to a spring R, and, seizing it with the two hands, the operator gives an alternating rapid movement, which agitates the solution, and makes it, in le than a minute, as limpid as water. This movement is promoted by a spiral spring, B, fixed to the agitator and the ground; but this is seldom made use of, because it is convenient to be able to transport the agitator from one place to another. When the agitation is finished, the wedges are to be taken out, and the phials are placed in order upon a table furnished with round cells destined to receive them, and to screen them from too free a light. When we place the phials upon this table, we must give them a brisk circular motion, to collect the chloride of silver scattered round their sides; we must lift out their stoppers, and suspend them in wire rings, or pincers. We next pour a thousandth of the decime solution into each phial; and before this operation is terminated, there is formed in the first phials, when there should be a precipitate, a nebulous stratum, very well marked, of about a centimetre in thickne . At the back of the table there is a black board divided into compartments numbered from 1 to 10, upon each of which we mark, with chalk, the thousandths of the decime liquor put into the correspondent phial. The thousandths of sea salt, which indicate an augmentation of standard, are preceded by the sign +, and the thousandths of nitrate of silver by the sign -. When the a ays are finished, the liquor of each phial is to be poured into a large ve el, in which a slight exce of sea salt is kept; and when it is full, the supernatant clear liquid must be run off with a syphon. The chloride of silver may be reduced without any perceptible lo . After having washed it well, we immerse pieces of iron or zinc into it, and add sulphuric acid in sufficient quantity to keep up a feeble disengagement of hydrogen gas. The ma must not be touched. In a few days the silver is completely reduced. This is easily recognised by the colour and nature of the product; or by treating a small quantity of it with water of ammonia, we shall see whether there be any chloride unreduced; for it will be di olved by the ammonia, and will afterwards appear upon saturating the ammonia with an acid. The chlorine remains a ociated with the iron or the zinc in a state of solution. The first washings of the reduced silver must be made with an acidulous water, to di olve the oxide of iron which may have been formed, and the other washings with common water. After decanting the water of the last washing, we dry the ma , and add a little powdered borax to it. It must be now fused. The silver being in a bulky powder is to be put in succe ive portions into a crucible as it sinks down. The heat should be at first moderate; but towards the end of the operation it must be pretty strong to bring into complete fusion the silver and the scoriæ, and to effect their complete separation. In case it should be supposed that the whole of the silver had not been reduced by the iron or zinc, a little carbonate of potash should be added to the borax. The silver may also be reduced by exposing the chloride to a strong heat, in contact with chalk and charcoal. The following remarks by M. Gay Lu ac, the author of the above method, upon the effect of a little mercury in the humid a ay, are important :— It is well known that chloride of silver blackens the more readily as it is exposed to an intense light, and that even in the diffused light of a room, it becomes soon sensibly coloured. If it contains four to five thousandths of mercury, it does not blacken; it remains of a dead white: with three thousandths of mercury, there is no marked discolouring in diffused light; with two thousandths it is slight; with one it is much more marked, but still it is much le intense than with pure chloride. With half a thousandth of mercury the difference of colour is not remarkable, and is perceived only in a very moderate light. But when the quantity of mercury is so small that it cannot be detected by the difference of colour in the chloride of silver, it may be rendered quite evident by a very simple proce of concentration. Di olve one gramme of the silver supposed to contain 1 ⁄ 4 of a thousandth of mercury, and let only 1 ⁄ 4 of it be precipitated, by adding only 1 ⁄ 4 of the common salt nece ary to precipitate it entirely. In thus operating, the 1 ⁄ 4 thousandth of mercury is concentrated in a quantity of chloride of silver four times smaller: it is as if the silver having been entirely precipitated, four times as much mercury, equal to two thousandths, had been precipitated with it. In taking two grammes of silver, and precipitating only 1 ⁄ 4 by common salt, the precipitate would be, with respect to the chloride of silver, as if it amounted to four thousandths. By this proce , which occupies only five minutes, because exact weighing is not nece ary, 1 ⁄ 10 of a thousandth of mercury may be detected in silver. It is not usele to observe, that in making those experiments the most exact manner of introducing small quantities of mercury into a solution of silver, is to weigh a minute globule of mercury, and to di olve it in nitric acid, diluting the solution so that it may contain as many cubic centimetres as the globule weighs of centigrammes. Each cubic centimetre, taken by means of a pipette , will contain one milligramme of mercury. If the ingot of silver to be a ayed is found to contain a greater quantity of mercury, one thousandth for example, the humid proce ought either to be given up in this case, or to be compared with cupellation. When the silver contains mercury, the solution from which the mixed chlorides are precipitated, does not readily become clear. Silver containing mercury, put into a small crucible and mixed with lamp black, to prevent the volatilization of the silver, was heated for three quarters of an hour in a muffle, but the silver increased sensibly in weight. This proce for separating the mercury, therefore, failed. It is to be observed, that mercury is the only metal which has thus the power of disturbing the analysis by the humid way. A aying of Gold. —In estimating or expre ing the finene of gold, the whole ma spoken of is supposed to weigh 24 carats of 12 grains each, either real, or merely proportional, like the a ayer’s weights; and the pure gold is called fine. Thus, if gold be said to be 23 carats fine, it is to be understood, that in a ma , weighing 24 carats, the quantity of pure gold amounts to 23 carats. In such small work as cannot be a ayed by scraping off a part and cupelling it, the a ayers endeavour to ascertain its finene or quality by the touch. This is a method of comparing the colour and other properties, of a minute portion of the metal, with those of small bars, the composition of which is known. These bars are called touch needles, and they are rubbed upon a smooth piece of black basaltes or pottery, which, for this reason, is called the touchstone. Black flint slate will serve the same purpose. Sets of gold needles may consist of pure gold; of pure gold, 23 1 ⁄ 2 carats with 1 ⁄ 2 carat of silver; 23 carats of gold with one carat of silver; 22 1 ⁄ 2 carats of gold with 1 1 ⁄ 2 carat of silver; and so on, till the silver amounts to four carats; after which the additions may proceed by whole carats. Other needles may be made in the same manner, with copper instead of silver; and other sets may have the addition, consisting either of equal parts of silver and copper, or of such proportions as the occasions of busine require. The examination by the touch may be advantageously employed previous to quartation, to indicate the quantity of silver nece ary to be added. In foreign countries, where trinkets and small work are required to be submitted to the a ay of the touch, a variety of needles is nece ary; but they are not much used in England. They afford, however, a degree of information which is more considerable than might at first be expected. The attentive a ayer compares not only the colour of the stroke made upon the touchstone by the metal under examination, with that produced by his needle, but will likewise attend to the sensation of roughne , dryne , smoothne , or greasine , which the texture of the rubbed metal excites, when abraded by the stone. When two strokes perfectly alike in colour are made upon the stone, he may then wet them with aquafortis, which will affect them very differently, if they be not similar compositions; or the stone itself may be made red-hot by the fire, or by the blowpipe, if thin black pottery be used; in which case the phenomena of oxidation will differ, according to the nature and quantity of the alloy. Six principal circumstances appear to affect the operation of parting; namely, the quantity of acid used in parting, or in the first boiling; the concentration of this acid; the time employed in its application; the quantity of acid made use of in the reprise , or second operation; its concentration; and the time during which it is applied. From experiment it has been shown, that each of these unfavourable circumstances might easily occasion a lo of from the half of a thirty-second part of a carat, to two thirty-second parts. The a ayers explain their technical language by observing, that in the whole ma consisting of twenty-four carats, this thirty-second part denotes 1-768th part of the ma . It may easily be conceived, therefore, that if the whole six circumstances were to exist, and be productive of errors, falling the same way, the lo would be very considerable. It is therefore indispensably nece ary, that one uniform proce should be followed in the a ays of gold; and it is a matter of astonishment, that such an accurate proce should not have been prescribed by government for a ayers, in an operation of such great commercial importance, instead of every one being left to follow his own judgment. The proce recommended in the old French official report is as follows:—twelve grains of the gold intended to be a ayed must be mixed with thirty grains of fine silver, and cupelled with 108 grains of lead. The cupellation must be carefully attended to, and all the imperfect buttons rejected. When the cupellation is ended, the button must be reduced, by lamination, into a plate of 1 1 ⁄ 2 inches, or rather more, in length, and four or five lines in breadth. This must be rolled up upon a quill, and placed in a matra capable of holding about three ounces of liquid, when filled up to its narrow part. Two ounces and a half of very pure aquafortis, of the strength of 20 degrees of Baumé’s areometer, must then be poured upon it; and the matra being placed upon hot ashes, or sand, the acid must be kept gently boiling for a quarter of an hour: the acid must then be cautiously decanted, and an additional quantity of 1 1 ⁄ 2 ounces must be poured upon the metal, and slightly boiled for twelve minutes. This being likewise carefully decanted, the small spiral piece of metal must be washed with filtered river water, or distilled water, by filling the matra with this fluid. The ve el is then to be reversed, by applying the extremity of its neck against the bottom of a crucible of fine earth, the internal surface of which is very smooth. The annealing must now be made, after having separated the portion of water which had fallen into the crucible; and, lastly, the annealed gold must be weighed. For the certainty of this operation, two a ays must be made in the same manner, together with a third a ay upon gold of twenty-four carats, or upon gold the finene of which is perfectly and generally known. No conclusion must be drawn from this a ay, unle the latter gold should prove to be of the finene of twenty-four carats exactly, or of its known degree of finene ; for, if there be either lo or surplus, it may be inferred, that the other two a ays, having undergone the same operation, must be subject to the same error. The operation being made according to this proce by several a ayers, in circumstances of importance, such as those which relate to large fabrications, the finene of the gold must not be depended upon, nor considered as accurately known, unle all the a ayers have obtained an uniform result, without communication with each other. This identity must be considered as referring to the accuracy of half the thirty-second part of a carat. For, notwithstanding every po ible precaution or uniformity, it very seldom happens that an absolute agreement is obtained between the different a ays of one and the same ingot; because the ingot itself may differ in its finene in different parts of its ma . The phenomena of the cupellation of gold are the same as of silver, only the operation is le delicate, for no gold is lost by evaporation or penetration into the bone-ash, and therefore it bears safely the highest heat of the a ay furnace. The button of gold never vegetates, and need not therefore be drawn out to the front of the muffle, but may be left at the further end till the a ay is complete. Copper is retained more strongly by gold than it is by silver; so that with it 16 parts of lead are requisite to sweat out 1 of copper; or, in general, twice as much lead must be taken for the copper alloys of gold, as for those of silver. When the copper is alloyed with very small quantities of gold, cupellation would afford very uncertain results; we must then have recourse to liquid analysis. M. Vauquelin recommends to boil 60 parts of nitric acid at 22° Baumé, on the spiral slip or cornet of gold and silver alloy, for twenty-five minutes, and replace the liquid afterwards by acid of 32°, which must be boiled on it for eight minutes. This proce is free from uncertainty when the a ay is performed upon an alloy containing a considerable quantity of copper. But this is not the case in a aying finer gold; for then a little silver always remains in the gold. The surcharge which occurs here is 2 or 3 thousandths; this is too much, and it is an intolerable error when it becomes greater, which often happens. This evil may be completely avoided by employing the following proce of M. Chaudet. He takes 0·500 of the fine gold to be a ayed; cupels it with 1·500 of silver, and 1·000 of lead; forms, with the button from the cupel, a riband or strip three inches long, which he rolls into a cornet. He puts this into a mattra with acid at 22° B., which he boils for 3 or 4 minutes. He replaces this by acid of 32° B., and boils for ten minutes; then decants off, and boils again with acid of 32°, which must be finally boiled for 8 or 10 minutes. Gold thus treated is very pure. He washes the cornet, and puts it entire into a small crucible permeable to water; heats the crucible to dull redne under the muffle, when the gold a umes the metallic lustre, and the cornet becomes solid. It is now taken out of the crucible and weighed. When the alloy contains platinum, the a ay presents greater difficulties. In general, to separate the platinum from the gold with accuracy, we must avail ourselves of a peculiar property of platinum; when alloyed with silver, it becomes soluble in nitric acid. Therefore, by a proper quartation of the alloy by cupellation, and boiling the button with nitric acid, we may get a residuum of pure gold. If we were to treat the button with sulphuric acid, however, we should di olve nothing but the silver. The copper is easily removed by cupellation. Hence, supposing that we have a quaternary compound of copper, silver, platinum, and gold, we first cupel it, and weigh the button obtained; the lo denotes the copper. This button, treated by sulphuric acid, will suffer a lo of weight equal to the amount of silver present. The residuum, by quartation with silver and boiling with nitric acid, will part with its platinum, and the gold will remain pure. For more detailed explanations, see Platinum . ATOMIC WEIGHTS or ATOMS, are the primal quantities in which the different objects of chemistry, simple or compound, combine with each other, referred to a common body, taken as unity. Oxygen is a umed by some philosophers, and hydrogen by others, as the standard of comparison. Every chemical manufacturer should be thoroughly acquainted with the combining ratios which are, for the same two substances, not only definite, but multiple; two great truths, upon which are founded not merely the rationale of his operations, but also the means of modifying them to useful purposes. The discu ion of the doctrine of atomic weights, or prime equivalents, belongs to pure chemistry; but several of its happiest applications are to be found in the proce es of art, as pursued upon the greatest scale. For many instructive examples of this proposition, the various chemical manufactures may be consulted in this Dictionary.