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and at a rate which would effectually prevent the importation of sal ammoniac from any part of the East.'

Mr. P. has here fallen into a very important error, which it is the more necessary to notice, that persons inclined to enter on speculations of this kind, may not be misled by the prospect of advantages thus delusively held out from the want of more correct information. In Scotland, salt for domestic purposes, is obtained from sea water by evaporation; and there is consequently a very abundant residuum of the nature which Mr. P. has stated. This residuum however is not homogeneous ; it consists of sulphat of magnesia, which we believe goes under the appellation of bittern, and which when crystalized and purified, is the Epsom salt of commerce, and the muriate of magnesia, which being uncrystallizable, goes under the name of oil of salt, and is the material made use of by Mr. Astley in his process for preparing sal ammoniac. But it happens very unfortunately for Mr. Parkes' recommendation, that at the salt works in Cheshire and Worcestershire, so far from these substances being thrown away as a useless residuum, they do not occur at all; the brine from which the salt is obtained not containing any sulphat of magnesia ; and of the muriate of magnesia a quantity too small to separate during the evaporation so as to form a residuum. That this is the case of the salt works at Droit wich, we know from personal inquiry.

Mr. Horner, in his account of the salt springs at Droit wich, published in the second volume of the Transactions of the Geological Society, estimates the proportion of muriate of magnesia, at only 1 76 grains in each pint of brine, or .07 per cent. of the whole saline ingredients ; so that it does not bear the proportion to the marine salt of 1 to 1600. We have not at hand the neans of stating the proportion of this salt in the Cheshire brine, but we believe it is equally pure as that of Droit wich. Hence, the salt manufactured at these places is dry and clean, and does not deliquesce at all; while the Scotch salt is quite the reverse, and always requires to be kept in a dry, warm situation, to be at all fit for use There are we believe a few situations on the English coast where sea water is evaporated for the making of salt, as at Lymington, and there the recommendation of Mr. P. might be adopted with advantage; but it does not apply at all to the salt works in the interior of England, so far as our acquaintance with them goes Whether the proprietors of the works on the coast, are prohibited from availing themselves of the advantages which Mr. P has pointed out, by the excise laws, we do not know; but the manufacture of Epsom salt is carried on there we believe to a pretty considerable extent. We apprehend, however, that these estabLishments are not of sufficient extent to make the question of

any importance in a national point of view, as it regards the manufacture of sal ammoniac; though certainly there can be no equitable reason assigned for imposing limitations on the capital and ingenuity of one part of the Empire, which are open and free in others.

We have pointed out these instances of carelessness and inaccuracy, not in the spirit of uncandid severity, but because the last is especially too important to be passed over without notice, and because we are persuaded Mr. P. would be himself the first to wish for their correction. The more pleasing part of our duty remains, to point out by a reference to a few of the essays, the species of information, which Mr. P. has selected for the gratification and instruction of his readers. The essay on burytes contains a good deal of curious and useful inforination on the native salts of that earth, more especialiy its carbonat. It is well known, that if the carbonat of this earth could be procured in sufficient quantity at a cheap rate, it might be introduced into the arts with great advantage, especially for the purpose of obtaining soda by the decomposition of common salt. Mr. P. a few years ago, visited the principal mines from which it has been obtained, situated near Chorley, in Lancashire; and he gives an interesting detail of the inforination which this visit enabled bim to produce. A century ago it seems these mines were worked with success for the lead ore with which they abound. The carbonat of barytes being the matrix in which it was imbedded, was left in the mine as a useless production. The late Sir F. Standish, however, the proprietor, discontinued the working of these mines about five and twenty years ago, from no other cause as it would seem, than that he was defrauded by the persons in his employment, and from that time they have been abandoned entirely. About that period the nature and properties of the barytic carbonat, were investigated and made known; and our present knowledge of the useful purposes to which it may be applied, would now augment the value of the produce very considerably. It is on this account much to be regretted, that the present proprietor is not induced by these considerations to have them worked again ; for it does not appear from the information obtained by Mr. P. that any deficiency of lead ore had been felt before the working was given up.

It would seem, however, that even at that period, the carbonat was known to be applicable to some useful purposes in the arts. Mr. P. was informed that about thirty years ago, these mines were visited by two Frenchmen, who collected and carried away a pretty considerable quantity of this mineral ;, and that subsequently a man who occupied a small farm on the estate, had been engaged in a clandestine commerce with it, for, as it

was thrown about the shafts as a refuse article, he was enabled for some time to collect cousiderable quantities without exeiting observation. What he thus collected, was sent privately to Liverpool, and from thence exported to Germany. This man was informed by the agent at Liverpool, that it was employed in the manufacture of Porcelain, and though any information on the subject which he was likely to procure, must be liable to considerable uncertainty and suspicion, yet this hint may deserve the attention of the manufacturers of this costly production, in our own country, if this earth does not already enter into the composition of their Biscuit ware. Mr. P. learned that the inhabitants of this sequestered district had found by experience, that this mineral was a poison to their cattle and poultry, probably long before its deleterious effects on the animal economy were known to physicians and physiologists.

The essay on sulphuric acid contains an interesting account of the progressive improvements by which the manufacture of this important chemical agent, has been brought to its present state of perfection and extent. The tedious and expensive mode of obtaining it by distillation from the sulphat of iron, or copperas of commerce, was first superseded in this country by the ingenious Dr. Ward, who formed the sulphuric acid by the direct combination of its constituent principles, though the process does not appear to have been his own invention. That gentleman, however, obtained a patent for this process, by which sulphur and nitre were burnt together in large glass globes, of the capacity of forty or fifty gallons each, each globe having a proper quantity of water introduced into it, to absorb the acid as it was formed during the combustion. By this means, he had for some time the monopoly of the manufacture of this acid, until the celebrated Dr. Roebuck, of Birmingham, introduced the capital improvement of conducting the combustion of the mixture of sulphur and nitre in cha: bers constructed of sheet lead. This plan at once removed the great source of expense in the breakage of the glass vessels, and speedily reduced the price of the acid to about one fourthi of its former cost; and thus contributed in the most direct and essential manner to its extensive introduction into various processes of the arts, from which its former bigh price must have nearly excluded it.

The first establishment of the leaden apparatus was effected at Birmingham, by Dr. Roebuck in conjunction with the late Mr. Samuel Garbett; and this original work still continues to be carried on. The situation of this work however, and the difficulty and danger of transporting it at that period, (about 1746,) when our internal navigation was so incomplete, confined the consumption of the acid principally to the neigh

bourhood of Birmingham. Other establishments were consequently formed, in the first instance at Prestonpans, by the original proprietors; and afterwards the de nand increased, by other persons in various parts of the kingdom, until the number of manufactories of sulphuric acid has becoine now very considerable. These chambers were in the first instance cubes of about six feet, but they have been gradually enlarged according to the judgement or caprice of individuals engaged in the business; for experience does not seem to have proved that the dimensions of the chamber are of any importance. They now vary from twelve to twenty feet in width; and from twenty to forty in length, and there is one, Mr. P. informs us, in Lancashire which is 120 by 40 feet, and contains a space of 96,000 cubic feet

• The process, however, whatever may be the size of the chambers, is generally conducted similarly, and in this way. A quantity of common brimstone, coarsely ground, is carefully mixed with crude salt petre in the proportion of seven or eight pounds of the former to one pound of the latter; and this mixture is afterwards divided into separate charges, containing quantities proportioned to the size of the chamber in which they are intended to be burnt. The best method for apportioning this mixture appears to me to be this: to allow one pound for every 300 cubic feet of atmospheric air contained within the chamber. The mixture of sulphur and nitre is usually spread upon several plates either of iron or of lead, and these are afterwards placed upon stands of lead within the chamber at some distance from each other, and at a fout or two above the surface of the water Things being thus arranged, the sulphur is lighted by means of a hot iron, and the doors are then closed. If well mixed the brimstone and nitre will soon be in rapid combustion, which will continue for 20 or 30 minutes, during which the chamber will become entirely filled with gas. Three hours, calculating from the time of lighting, are generally allowed for the coirdensation of this gas; and then it is customary to throw open the doors for three quarters of an hour, for the free admission of atmospheric air and the expulsion of all the incondensable gas, in order that the house may be thoroughly sweetened, as it is called, for the next burning. During this intervall the plates are again charged, and preparation is made for a fresh combustion, which is thus re- , peated every four hours, day and night, without intermission, till the water at the bottom of the chamber is thought to be sufficiently acidified, when it is drawn off, by means of a syphon, into a rescrvoir of lead, conveniently placed for its reception, and the floor of the chamber replenished with water for another making.' Vol. II. P.

414. The acid obtained in this manner is still largely dilated with water, which it is necessary to remove by evaporation, that the acid may be brought to the degree of concentration in which it is met with -as an article of coinmerce.

This is generally performed in boilers of lead; but if required to be very pure, it is concentrated in vessels of glass. In its most concentrated state, it is commonly of the specific gravity of 1.847, and still, according to Dalton, contains about 22 per cent of water. As however it is almost always necessary to dilute it with water, before it is applied to the various purposes for which it is wanted, Mr. P. has constructed a very useful table (the result of actual experiment) of the specific gravity of the concentrated acid when diluted with different proportions of water, which, though it has been published in the Philosophical Magazine, is very properly reprinted in this Essay. There is also another table added, shewing the change of the specific gravity produced in the concentrated acid at various degrees of temperature from 10°. to 1920.

In the essay on citric acid, Mr. P. gives a minute and circumstantial account of the manipulations requisite to be practised, to obtain this most useful acid in a crystallized form. Before Scheele's time some unsuccessful attempts to purify it had been made ; but it was this listinguished chemist who first devised the process for separating the foreign substances with which it is combined in the fruit, and thus enabling the pure acid to assume the state of crystals. The process which Mr. P. recommends, is that of Scheele; but as he has himself practised it on a scale of considerable magnitude, he has pointed out many circumstances which the operator will find it useful to attend to, in order to ensure complete success in bis operations. The exact saturation of the citric acid with lime, for which purpose the carbonat of that earth_sbould be employed, the complete decomposition of the citrat of lime by the sulphuric acid, which, combining with the lime, sets the ci ric acid free, and the proper management of the evaporation, so as to bring the liquid citric acid into a state of concentration favourable to the formation of crystals, are important steps in the process, upon each of which Mr. P. gives some judicious and useful directions.

Mr. P. states as the result of his own experience, that twenty gallons of good lemon juice, will generally give eighteen pounds of dry citrate of lime, and this, if the process is well conducted, will yield ten pounds of pure colourless crystals of citric acid. The many important purposes to which this acid has been found applicable, not only for domestic and medicinal purposes, but also in the arts, more especially in the delicate operations of the calico printer, and its extensive consumption in the Navy, where its daily use by every sailor has almost entirely preserved that important class of men from the ravages of the scurvy during the late war, have rendered a plentiful supply of it an object of great and even of national

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