was thrown about the shafts as a refuse article, he was enabled for some time to collect considerable quantities without exciting 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 ani-. mal 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tedious and expensive mode of obtaining it by distillation from the sulphat of iron, or copperas of commerce, was first superseded in this country by the ingenious Dr.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 chambers 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 fourth 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 high 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 demand increased, by other persons in various parts of the kingdom, until the number of manufactories of sulphuric acid has become 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 foot 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 combus tion, 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 condensation 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 interval the plates are again charged, and preparation is made for a fresh combustion, which is thus repeated 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 reservoir 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 diluted 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 commerce.

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 10o. to 192°.

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 distinguished 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 his operations. The exact saturation of the citric acid with lime, for which purpose the carbonat of that earth should 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

consequence. Some attempts have consequently been made to facilitate and increase the supply, by combining the acid with lime, in those countries where this valuable fruit is indigenous, that it might be imported into this country in a less bulky form, and in a state not liable to be injured by the voyage, or by keeping. An attempt of this kind Mr. P. informs us, was made in Sicily a few years ago by a person who went thither from England for the purpose of conducting the operation, and Mr. P. has had access to his correspondence on the subject, with permission to avail himself of it for the public information. The undertaking, though apparently of very easy execution, seems to have been conducted with considerable difficulty; chiefly from the inconveniences which were met with in bringing the citrate into a perfectly dry state, and except this was accomplished, it was liable to heat, and was consequently not in a state fit for exportation. It was found necessary too, to send the carbonate of lime from England, and the operator met with unexpected embarrassments from the jealousy of the merchants, and the stupidity of the people, and their total inaptitude in all operations to which they had not been accustomed.

Mr. P. has given ample directions for the detection of any adulteration which may be practised on parcels of the liquid acid, and which those who consume it largely, will find extremely useful. Indeed, those who are interested in the preparation or employment of this acid on a large seale, will find much valuable information in this essay, which will repay them for the trouble of a careful and attentive perusal. It contains also several useful tables for ascertaining the proportion of pure acid, which may be obtained from different parcels of fresh juice, from which those who employ it in large quantities, and who have not much knowledge of chemistry, may derive great assistance.

The essay on the fixed alcalies, though less full and laboured than some of the others, contains a large proportion of useful information, communicated in a perspicuous manner. The different sources from which the alcalies are obtained, their employment in various manufactures, especially in the formation of glass and soap, the means of bringing them into a caustic state, the most direct methods of determining the proportion of alcali contained in the barilla of commerce, are stated in so clear and plain a manner, that a person not conversant with scientific chemistry, may readily avail himself of the information which the essay affords, and apply it to its particular object, or to his own individual pursuits. He very naturally laments, as every one must do, that our impolitic duties on salt, should prevent us from availing ourselves of

the means which chemistry has unfolded for obtaining an ample supply of soda, by the decomposition of common salt, at a rate which would entirely supersede the importation of barilla; but it is lamentable to reflect how frequently capital and ingenuity are turned aside from their natural direction towards pursuits which would be equally beneficial to the public, and the individuals engaged in them, by the operation of injudicious taxation. Mr. P. has not entered minutely into a consideration of the processes which have been adopted with more or less success, for preparing the alcalies by the decomposition of those neutral salts of which they form the base, though it is a subject of considerable importance, and one upon which, his acquaintance with the practical operations of chemistry we should expect, would have enabled him to offer some judicious remarks.

We have already trespassed so far on the time and attention of our readers, that we must bring our remarks to a close. Those who have occasion to consult or to study the work, will find in the other essays a pretty large proportion of pleasing or useful information. The essay on specific gravity contains very plain and ample directions for determining the specific gravity of bodies, whether solid or fluid; and that on edge tools contains some judicious discussion on the best means of giving the requisite temper to different cutting instruments. Mr. P. recommends the employment of metallic baths as the most accurate means of giving the requisite degree of temperature to the instrument to be hardened; and he has taken considerable pains to determine the melting point of different metallic mixtures of lead, zinc, and tin, that the artizan may be enabled to regulate the temperature of the bath in the most accurate manner to the precise object he has in view. Indeed we must do the Author the justice to say, that he has taken great pains to make his work really useful to those who are engaged in the different departments of useful industry on which he treats; and we believe he will be found to be with very few exceptions an intelligent and candid instructer. His style is easy, familiar, and free from affectation; and though there is reason to wish that the work had been more compressed, and that all the really useful notes had been incorporated into the respective essays to which they belong, yet Mr. P. has provided against one of the inconveniences which they necessarily produce, by a very copious and accurate index. Towards the poetical portion of the notes, we are inclined to be more severe, and to enter our protest against them entirely. We think they are misplaced in works such as this. The Plates of Apparatus, of which there are several in each volume, are executed with peculiar neatness and fidelity.