iron, or contains much muriate of silver, the heat generated is too great, and is diminished by a further addition of lime. If much sulphate of lead is present, or pyrites difficult to decompose, chemical action is faint, or does not take place, and must be promoted by augmenting the temperature. Then also what is called magistral, or a mixture of sulphate of iron and copper, is added. The operation is thought to go on well when a portion of the mixture, on being placed in the hand, causes a sensation of heat. A few days after wards, about six times as much mercury as the tourte is supposed to contain silver, is added, as well as more magistral. If the mercury acquires a leaden colour, the operation goes on properly. In order to expose every part, however, to an equal action, the mass is stirred about in rather an extraordinary way. Twelve or twenty mules are driven round in it for several hours, or a considerable number of barefooted workmen march backwards and forwards in this metallic mud. Every day, the overseer ascertains the progress of the work by washing a portion of the mixture in a wooden bowl, and judging by the aspect of the amalgam. When the mercury assumes the colour of ashes, separates in a grey powder, and sticks to the fingers, the heat is too great, and more lime is added. If, on the contrary, the mer cury retains its metallic lustre, or is covered with a red or golden scale, if it does not appear to act on the n

mass, the heat is increased by adding magistral. Thus the operation, naturally modified by the temperature of the season and by the different species of ore, may be almost directed at the will of the workman. It is, however, a very te dious process, and sometimes lasts five months. When the amalgamation is judged complete, the mud is thrown into troughs made either of stone or wood, in which the arms of a mill revolve, so as to stir it, and a stream passes through it; the earths and the oxides are carried off by the water, and the amalgam

remains at the bottom, which is put into bags and subjected to genle pressure. It is then placed in a pyramidical form, is covered with pots of the same shape, and heat is applied all around; the mercury distils off in vapour, and is conducted into trenches and condensed by cold water, to be afterwards used. Mr. Humboldt says, for about every 2lbs. of silver obtained by this process, 3lbs, of mercury are lost. In Saxony, however, where the same process is carried on, though not on so large a seale, about one-fifth only of mercury, for the silver gained, is lost, while the operation seems to be performed in the two-hundredth part of the time.

The principal place where the amalgamation is carried on in Saxony is Freyberg, and there the process is different from that in use at Mexico. After the mineral has been bruised and washed, and carefully separated from all extraneous substances, it is mixed with salt in the proportion of 10 to 100. The salt is first carefully sifted, and the two substances are inti mately mixed together, after which they are dried in a room over the furnace. When dry, the schlich is thrown into one end of a furnace, the fire being at the other, and the calcination taking place in the middle. Above the furnace are vaulted chambers, divided into several compartments, where the dust which rises is deposited. After the schlich has been in the furnace about half an hour, it is raked into the centre, where the heat is great er, care being necessary not to make the heat too strong at first, the mixture being constantly stirred, so that no lumps form. The heat is gradually increased, 'the salt decrepitates with some violence, the sulphur begins to burn, and at length the whole mass is incandescent, and continues so though the fire is withdrawn. When the schlich ceases to burn, the fire is again kindled, and a considerable degree of heat produced. At this period the mixture gives out a great quantity of chlorine, the

smell of which it is difficult to support. The workman takes up a portion of the material from time to time with an iron ladle, and when the smell of chlorine is very strong, he stops the operation and rakes the residuum of the calcined mass bye to cool. In this process, it seems, according to M. Rivero's account, that the sulphurets of the mineral are converted into sulphates, and being decomposed by the heat, the sulphuric acid attacks the salt, forms sulphate of soda, and sets the muriatic acid at liberty, which, acting on the oxides, converts them into chlorides; these are afterwards decomposed, and give out the quantity of chlorine which then becomes so perceptible. When the schlich has been thus roasted, it is again sifted through iron sieves, and the parts found not sufficiently calcined are mixed with more salt and calcined a second time. The part of the schlich which passes through the sieve is convey ed by funnels to the amalgamating casks. In general there are twenty of these in one room, and they are made to turn round by means of machinery adapted to the purpose. About 300lbs. of water is poured into each cask, and then about 1000lbs. of schlich and some pieces of forged iron are put in. The casks are fastened up and turned round for about one hour; 500lbs. of mercury are then put into each cask, and it is firmly closed. The machinery is immediately put in motion, so as to make each cask turn round about 15 or 20 times in a minute. Every four hours the amalgamation is examined; and in about sixteen hours the operation is completed. The casks are then filled with water, and are turned round gently for an hour, when the contents are separated into two parts, the amalgam and the residuum, which latter is again washed, more completely to separate the remaining mercury. The former is subjected to gentle pressure, and the mercury then separated by distillation, when the silver remains. Our plate represents the room where the amalga

mation is carried on. AA is the moving wheel; B B is another wheel with 108 teeth, on the axle of AA; CC are smaller cog wheels working to correspond with B, and each of the axles on which they are placed gives motion, by means of eog wheels, DD, to a number of amalgamating casks, proportionate to the size of the room and the quantity of work to be done; EEE are the casks; FF show the half circular piece of iron used in conjunction with a screw to secure the opening of the cask after the materials are put in; GG are also screws, the purpose of which is to enable the workmen to move any of the casks from the contact of the teeth of the wheels, DD, so that it may be worked at without stopping the movements of the other casks; HH shows the situation of an iron funnel or conductor, composed of several pieces, which can be moved in any direction-it serves to convey the mercury into any one of the casks at pleasure; I shows one of the holes through which the residuum, from the amalgamation, passes into troughs to be washed, and have the mercury separated from it; M M are the holes in which funnels are placed to receive the amalgam from the casks, and which convey it into the basins, LL, whence it is carried by the pipes, KK, to the place where the mercury is to be separated by distillation. To give a clearer idea, this plate has been divided into two parts: on the left the casks are shown as they work; on the right, in the middle of the figure, the manner in which the funnel is placed above the casks is shown; while, on both sides, the floor is represented, above which the casks work.

There is another method of obtaining silver from native ores, which is practised at the rich mine of Konsberg, in Norway, and which is extremely simple. The ore is cleansed from the earthy matter as much as possible, and is then melted with an equal weight of lead, from which it is separated by

the process of cupellation, as described in our No. XVI. Where the ore is naturally mixed with lead, which is very often the case, no addition is necessary; in general, however, it contains sulphur and other substances, which are got rid of by roasting after it has been cleaned. As mercury itself is a costly substance, till some other method is found out of purifying silver, however abundant it may be in the Andes, it will never become very cheap.

We need not, we believe, describe the appearance and properties of silver: they are known to our readers from what they possess of it, and perhaps even still more by what they desire. In consequence of money being the representative of all things, it is very generally confounded with wealth, and few men wish for what money will buy, compared to the number who wish for money itself. Silver is equal in brilliancy to any of the metallic bodies except polished steel; it is softer than copper, and harder than gold, and in. ferior to none of the metals except the latter in malleability and ductility. It can be beat into leaves the thousandth part of an inch thick, and drawn into wire as fine as human hair. Silver can be burnt and dissipated, being converted into an oxide. The various uses, also, to which silver is put are too well known for us even to allude to them. Slightly alloyed with copper, to add to its durability, it constitutes the great mass of the coin of all Europe; and we cannot on the present occasion better take leave of our readers, than by wishing their pockets may never be destitute of this precious metal.

that disposition which has always existed, to receive any theory, however speculative and theoretical, having the irresistible charm of novelty for its recommendation. Is it not surprising that experience should not have had the effect of teaching us greater caution, and checking, in some measure, this unfortunate disposition? That the phlogistic theory of Stahl should have been so strenuously adopted, just emerging as the science then was, from alchemical obscurity, ought not to excite our surprise ; nor should the fact of its having remained uncontroverted for such a length of time, considering the necessarily slow progress of philosophical knowledge, incur our cen

sure.

Modern philosophers, in judging the talents and discoveries of their predecessors, are too apt to forget the disadvantages under which they laboured. . Stahl, considering the time in which he lived, is as much entitled to our admiration and gratitude, as a contributor to science, as he who, with the advantages of subsequent experience, proved his theory to be untenable; and, be it remarked, the admiration and attachment which was displayed by the philosophers of that day, towards the phlogistic hypothesis, was not more enthusiastic than that with which the antiphlogistic theory (so called) of oxygen was received at at its first announcement. The latter was certainly conclusive as to the fact of combustion being an act of combination and not of separation, and consequently decisive, as far as Stahl's explanation of his hypothesis went; the theory, however, which it was attempted to establish on Lavoisier's principle, has been unable to stand the test of experi ment-both have been proved to be equally failacious. Oxygen can no longer be allowed to be the necessary supporter of combustion. As to the real explanation of the phenomenon we are, therefore, still in ignorance. We have also another proof of this speculative and theorising disposition in the substance oxygen, which for many

years was asserted to be the acidi fying principle; subsequent experiments have proved that there are substances possessing all the distinguishing characteristics of acids, which are totally devoid of it. Are not such examples sufficient to teach us the necessity of caution? Are they not, in fact, almost sufficient to justify scepticism itself? Yet it appears we are now expected to subscribe to a doctrine, to understand which appears to me to be beyond the limits of the reasoning faculties of man,-I mean the present prevailing fashionable "Atomic Theory." Instead of indulging in personal invective and abuse against those who happen to differ in opinion with him, I wish its strenuous advocate, the great leviathan of philosophical literature, would condescend to favour us with his definition of an atom; I, for one, lamentable as my ignorance may appear, candidly confess myself unable to form an idea of what an atom can be. Erroneous theories, we have seen, often appear extremely plausible; the most absurd, we have also seen, will be sure to find its advocates; and it may be generally observed, that in proportion to its absurdity is the enthusiasm or fanaticism of its supporters. Convinced of the truth of this observation, the manner in which the publication alluded to supports this doctrine tends rather to strengthen my disbelief of it. It is much to be regretted that the illiberal tone of its criticisms, and the constant practice of indulging in ridicule and abuse, should deprive it of that respect with which, as the professed organ of the first scientific Society in Europe, we should be inclined to look upon it. I am happy in being able to congratulate the "Chemist" on the courage and independent spirit it has shown in expressing its censure on such degrading and unphilosophical conduct. ANTI-STAHL.

DICTIONARY OF CHEMISTRY. ASSIMILATION. The process by which the materials forming the

nourishment of animals or plants are appropriated to the different parts of the body, or converted into the different substances of which it consists. Partly this is a chemical process, as the decomposition of the materials can be traced to chemical agents; but the adapting or assimilating power belongs exclusively to life.

ASTRINGENT PRINCIPLE, tannin. A vegetable principle, distinguishable by the taste: it is found in bark, the husks of nuts and walnuts, in green tea, in nut-galls, and various other substances. It is the active agentin converting hides into leather, hence its name of tannin.

ACTAMITE, a muriate of copper, which is found native,

ATHANOR. A furnace now fallen into disuse. It was constructed on the principle of supplying itself with fuel.

ATOMETER. An instrument contrived by Professor Leslie, for measuring the quantity of exhalation from a humid surface in a given time.

ATMOSPHERE. The aerial fluid surrounding the earth; chemically, a compound of four volumes of azot and one of oxygen, with a small quantity of various other substances.

ATOMIC THEORY. A prevailing theory of the chemical nature of bodies. It seems to consist in supposing that all simple substances are formed by the union of an indefinite number of precisely similar particles or atoms; and that particles or atoms of simple substances only unite with the particles or atoms of other simple substances in certain definite proportions; and that whenever one simple substance unites with two proportions of any other simple substance, the last proportion is always a multiple of the first. The latter part of the theory, namely, that all substances in combining with one another, unite in certain proportions only, that is, unite according to some general rule, or law, and not at random or by chance, is what the philosopher would expect à priori, and seems

confirmed by experience. The former part of the theory, namely, the existence of atoms, is a remnant of a very ancient system of philosophy, and ought, in the country which gave birth to BACON, and in which the philosophers pretend that the senses are the source of all our knowledge, and that for us there are no facts but what they inform us of, to have been long since extinct. All the phenomena we witness may be as well oxplained without as with the assumption of the atomic existence of substances; and we hold the present prevalent theory to be a proof, (unfortunately there are too many extant,) of the continued influence of portions of philosophical theories long after those theories are generally denied and abandoned.

ATROPIA One of the newly discovered vegetable principles, which probably belongs to the class of vegetable alkalies. It is so named from having been ob tained from the Atropa Belladona, or deadly nightshade, and is the poisonous ingredient in this plant. ATTRACTION, See Affinity.

METHOD OF CLEANING GOLD TRINKETS, AND OF PRE SERVING ENGRAVED COPPER-PLATES..

BOIL the trinkets in water of ammonia, which dissolves the metallic copper of the alloy to a certain depth, so that after the operation, the metal is, in fact, gilded, nothing but pure gold being visible. In this process the waste of gold, which takes place by the application of neutral salt to disengage nitric acid, the usual method of cleaning trinkets, is avoided. Hi therto chemists have neglected to observe, that metallic copper is soluble in ammonia. Dr. M'Cullóch has shown, that the solution takes place rapidly at a heat sufficient to boil the water of ammonia. He says, copper-plates are apt to be injured by lying by, a coat of oxide forms on the surface, which is rubbed off by the hand of the workman on the first inking when

IT has been sometimes asserted, and sometimes denied, that the evaporation from the surface of the human body was the cause why it remained at nearly the same temperature, that of blood heat, though the air around it was much hotter. The following experiment seems to place this matter in a clear point of view:-One of those porous vessels, called an alcarrazas, which have already been described at page 205, and which permits evaporation from every part of its surface, together with two wet sponges and a frog, were all placed in an oven, between 126° and 142° of Fahr. the temperature of which varied Before being placed in the oven, the temperature of the alcarrazas and of the sponges was somewhat above blood heat, while that of the frog, like all cold-blooded animals, was considerably below it. In the course of a quarter of an hour the two sponges, the earthen jar, and the frog had all nearly the same temperature, and what is very remarkable, this was about the ordinary temperature of hot-blooded animals; and during two hours all these substances remained at the same temperature. The temperature of the frog, as might have been expected, rose; but the tem perature of the sponges and of the jar, quite contrary to expectation, fell; all remained for a considerable period much below the temperature of the stove, and the temperature which they preserved was