carries off the carbon in the state of carbonic acid, or gaseous oxide of carbon, leaving a portion of pure iron for the metallic arsenic to combine with. Here it is obvious that the arsenic being oxydated on the surface only, it is only at the edge of the alloy that the carbon of the cast iron can be carried off, and therefore it is there only that the iron is at liberty to combine with the metallic arsenic. I should not think that it was owing to any peculiarity in the cast iron. I remain, Sir,

WHEN We described the mode of manufacturing this article, we did not state the purifying part of the process: we now mean to do this. In the distillation, the charcoal which remains is a product of considerable value; and it is found that this is better in proportion as the wood was dense; and that wood long exposed to the air gives more charcoal than green timber. This is a fact of some consequence in a practical point of view, as leading to the production of a better commodity, and in a theoretical point of view it seems to confirm the opinion oftimber being strengthened and made durable by exposure to the atmosphere a certain time before being worked up.

When the acid is first obtained, it is found coloured ofa reddish brown, from holding in solution a portion of the empyreumatic oil and tar that were formed at the same time with it; another portion of these ingredients is mechanically mixed with the acid. This latter is deposited by the acid, while it is in a state of rest, in the reservoir we have already described. A pump goes down to the bottom of this reservoir, and the tar, from its greater weight, sinking to the bottom, is occasionally pumped up. On the other hand, as the reservoir gets full, the clearest acid flows out from time to time into another reservoir, The acid thus separated

from the tar, which was merely mechanically mixed with it, is pumped into large caldrons made of bricks or tiles, where it is saturated with either lime or chalk, as circumstances may direct. By employing the former, there is a danger of doing more than saturating the acid, and then the lime acts on the empyreumatic oil; by employing the latter, there is no danger of this, but it requires a greater degree of heat, and forms, from the various substances mixed with the chalk, a crust, which must be removed. Whichever is employed, the tar separates during the saturation, and is skimmed off. To the solution of acetate of lime thus obtained and concentrated by evaporation, is added a concentrated solution of sulphate of soda; sulphate of lime is precipitated, and acetate of soda remains dissolved. The liquid is allowed to remain some time at rest, when it is drawn off. The residuum is preserved for further use. The acetate of soda is then evaporated till it has obtained a sufficient degree of concentration, when it is placed in large vats or dishes, and in three or four days a number of crystals are formed, of rhomboidal shape and dark colour. The motherwater is drawn off, again concentrated, and again allowed to deposit crystals; and this process is repeated till it will deposit no more. The acetate of soda is afterwards dried or roasted for twenty-four hours-an operation which requires considerable care, and then it is dissolved in water, either as it comes hot from the stoves, which is, however, dangerous, or when it is cooled. In a state of solution, the carbonaceous matter which has formed is separated, and then the acetate of soda is obtained in the state of white crystals. This salt is then decomposed by sulphuric acid, and acetic acid obtained in a state of purity. The French writer whose very long account of this process we have endeavoured to give in a few words, adds, and very properly, that this branch of chemical manufacture is yet in its infancy, and

needs many improvements. As it is now, however, carried on to a considerable extent in several parts of Europe, there can be no doubt that it will be very rapidly improved. Perhaps, however, we may remark, that it is one of those manufactories which set in a striking point of view the utility of chemical science. Following its indications, the products derived from converting wood into charcoal, which were formerly dissipated in the air, are now condensed into a most valuable acid,

just as the gas formerly dissipated from converting coal into coke now gives to our streets and houses a brilliant light, which has almost deprived villany of its power of concealment, and added to the security of society more than the most efficacious police regulations.

AN INSECT-DESTROYING

WASH.

MR. BRADDICK recommends the water through which coal gas is passed, as effectual for destroying the vermin on trees. He mixes one pound of flour of brimstone in three gallons of gas-water, and adds soft soap enough to make it adhere to the buds and branches when laid on with a painter's brush. The composition is not inflammable, and does no injury to the trees; and Mr. Braddick says he has proved this by the peach of China, the most delicate of all trees. Repertory of Arts, &c.

hand with a sensation similar to touching a piece of heated iron, the hand having the same appearance that it assumes when burnt.

TO CORRESPONDENTS. T.C. is informed that, in The Chemist, No. III. p. 40, we have given a re-· ceipt for what he inquires about; at the same time, there are so many methods recommended, that we will not answer for the one there described being the best.

A Subscriber and Reader had no always a pleasure in obliging our Coroccasion to offer us a bribe, as we have respondents when it is in our power. His Letter, however, came too late to receive any answer this week, and in our next Number ke will find what he asks.

Our vivacious friend Anodyne, who complains of our being dull, should attribute it not to us but to the times. Are not the newspapers dull? Are the magazines lively? Is not the world too busy to be cheerful? Are not all men so earnest in the pursuit of the means of enjoyment, that they forget to enjoy? Is there any thing novel, any thing brisk, spirit-stirring, or exciting in the world, except the horrid fanaticism in Ireland? and alas! this is any thing but cheerful. Is there even a good blazing fire or devastating flood of skill, and of that increase in knowto destroy half a city? It is the effect ledge, which we labour to propagate, to prevent all kinds of accidents, und make the course of the moral world tend to regularity like the movements of the planets. Without surprise there is no very lively emotion; and thus, what our Correspondent complains of as dulness, in reality equanimity, in the passions of men, which, we say, is common to the rest of the world with ourselves, is the result of knowledge; and he, as well as every other intelligent man, is anxious to increase that which, in his sense of the word, is to render the world still more dull.

* Communications (post paid) to be addressed to the Editor, at the Publishers'.

London Published by KNIGHT and LACEY, 55, Paternoster-row.-Printed by B. Bensley, Bolt-court, Fleet-street.

spots out of cloth, and does not, like many other salts, produce, with the staining acid, if it be one, a more destructive spot than that which it removes. It is one of the products of all animal decomposition at an elevated temperature, but it is then always accompanied with a fetid oil, from which it cannot be easily separated, on account of its great volatility. To obtain it in purity, recourse must be had to some other ammoniacal salt. The following is the method usually employed:-Take eight parts of sal ammoniac,(muriate of ammonia) add to it ten parts of chalk which has been previously well washed and dried; mix them, and place them in either an earthen or cast metal retort, on a small furnace, as seen in the plate; with this, a receiver, made of lead, or a com ́mon jug with a hole at the bottom, having also at its upper part a socket for a handle, is connected by means of a tube passing from the furnace to the receiver. The latter is to be placed in a tub in which a stream of cold water can be made constantly to flow. The joinings are all carefully luted, and a plug which can be removed at pleasure is put into the hole at the bottom. Heat is then applied, but very gradually at first, particularly if the retort is of earthenware. The best temperature is that which admits of the vapours being condensed as they are emitted; and the heat should always be lessened whenever, on moving the plug, vapour rapidly escapes. The operation goes on well when the vapour is not forcibly expelled, and a light cloud is seen in the receiver. As the operation goes on, the heat must be increased, and it is time to cease it when the vapour is forcibly expelled, and is no longer cloudy. The vapour of water then passes, and the operation should be stopped. The apparatus is allowed to cool, and the receiver, if of earthenware, is broken, and the upper part is found covered with the carbonate of ammonia, in white and dry crystals.

CHEMISTRY AS A SCIENCE. Art. XX.

PALLADIUM. RHODIUM. IRIDIUM. THESE three metals were all discovered in the years 1803 and 1804, and have never been found except in the ores of platinum. As they form but a small proportion of these, and these are not very abundant, the reader may be quite sure the three metals are not plentiful. They appear only to have been obtained in sufficient quantities to make experiments on, and ascertain their nature, and have been put to no use whatever. We shall therefore dispatch them in a few words: Palladium was discovered by Dr. Wollaston, in 1803. It may be obtained by adding to a solution of crude platina in nitromuriatic acid, prussiate of mercury, on which a precipitate generally takes place of a yellowish white colour. This is prussiate of palladium, and the acid may be expelled by heat, leaving the metal in a state of purity. It is white, resembling platinum. It is rather harder than wrought iron, and is very malleable.

A very violent heat is required to fuse it, and it is not altered by exposure to the air. It forms alloys with the other metals, but they have been put to no use, and their properties have scarcely been investigated.

Rhodium was discovered a year later than palladium, by the same chemist. Crude platinum is to be freed from mercury by a red heat, and from other substances by digesting it in a sand heat with a small quantity of nitro-muriatic acid till the whole is dissolved except a quantity of black sand from which the solution is separated. Into the solution, sal ammoniac, dissolved in hot water, is poured, which separates the platinum in the form of a yellow powder. Zine is immersed in the solution and allowed to remain as long as any precipitate is thrown down, which is treated with dilute nitric acid, in a gentle heat, to dissolve the copper and lead it contains. It is then dissolved in dilute nitro-muriatic acid, by the assistance of heat,

and common salt is added to the solution. This being evaporated to dryness, and the residuum washed with alcohol till it comes off colourless, there remains behind an oxide of rhodium, mixed with common salt. Dissolved in water, crystals are obtained by evaporation, which being again dissolved in water, and a plate of zinc immersed in the solution, a black powder precipitates. Heated with borax, this becomes white, and si assumes a metallic lustre. This is rhodium; and not more than the 250th part is found in the ores of platinum. It is of a white colour, nearly resembling platinum. It is brittle, comparatively infusible, and in hardness equal to iron. It is insoluble in acids; and it forms oxides and alloys. It differs from platinum in the colour which it imparts to gold: a very small quantity of platinum is sufficient to alter the colour of gold so as scarcely to be distinguished from platinum, while the same quantity of rhodium has little or no effect on the gold.

Iridium was discovered about the same period, 1803, by Mr. Smithson Tennant, in England, and by M. Descotils in France. When crude platina is dissolved in dilute nitro-muriatic acid by a small degree of heat, there remains undissolved a quantity of black shining powder, formerly supposed to be black lead. It consists, however, of two distinct metals, one of which Mr. Tennant called iridium, from the variety of colours its solutions exhibit, and the other osmium, from the peculiar smell of its oxides. To separate the two from each ach other, beat the black powder to redness, in a silver crucible, with its own weight of potash, and keep it in that state for some time. Then dissolve the potash by water, and a solution is obtained of a deep orange colour. Digest the powder that remains in muriatic acid, which becomes first blue, then olive green, and, lastly, deep red. The powder which is undissolved, is to be treated alternately with muriatic acid and potash, till the whole is dissolved;

and then two solutions are obtained; the alkaline of a deep orange colour, consisting chiefly of potash united to the oxide of osmium, and the acid solution of a deep red, consisting chiefly of muriatic acid united to the oxide of iridium. By evaporating the last to dryness, dissolving again in water, and again evaporating, crystals of muriatic acid, united with the oxide, are obtained. Being dissolved in water, a plate of any metal, except gold and platinum, precipi tates the metal in the form of a black powder, which, on being heated, becomes white, and of a metallic lustre. It has the appearance of platinum, and resists the action of heat and air like that metal. It is said to be even hea vier than platinum, but this is a point not so well ascertained. It resists the action of the most pow erful acids, and, like the other metals we have just mentioned, could it be obtained in sufficient quantities, would be well adapted for several chemical purposes. It possesses also, like them, the properties of uniting with oxygen, and forming alloys with other métals; but neither the oxides nor the alloys have been investigated. In fact, the metal is procured in such very minute quantities, and by such a tedious process, that hardly any thing is known of it; and unless some more plentiful means of obtaining it should be discovered, it will never reward the researches of the chemist, and never be put to any use. The metals next in suc cession are of a character so different from these, that we shall not mingle a description of both in the same article, and shall reserve Antimony for the next Number...

CIRCULATION OF THE SAP IN PLANTS.

DR. SCHULTZ, of Berlin, has lately examined the leaves of the Che lidonium Magus, in order to ascertain as near as possible the mode; in which the sap circulates in plants. The following he states as the result. There is at first an ascending motion, then a sort of trembling in