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of discovery; and hence it is that notes and appendices are multiplied to the certain confusion of the student, and the injury of the work, in simplicity and perspicuity of arrangement. In this point of view we are bound to consider another very able work"Dr. Ure's Dictionary of Chemistry," which, though different in arrangement,comes into close comparison with the work before us, both with respect to size and the class of readers for whom it is designed. Had Dr. Ure composed the Dictionary ab initio, we feel convinced that excellent as he has rendered it, he could have made it more worthy of approbation than it was possible to do with the Dictionary of Nicholson to work upon, in which it was requisite not only to have the whole of the old matter revised, that obsolete and refuted theories might be expunged, and more accurate and enlarged views inserted in their room, but to draw up a very great number of articles entirely new, of which the original Dictionary contained not even a hint. For the interest of chemistry, it is to be lamented that a man so able as Dr. Ure to do justice to the subject, was put upon the execution of this laborious and ungracious task. In all this Dr. Turner's work possesses most important advantages over its elder competitors, and the author has availed himself of them most amply; for it may be justly said to contain as complete a picture of the present state of chemistry in all its branches, as it was practicable to delineate within the bounds of a moderate volume.

To us, however, after a careful examination of his arrangement, and his method of treating the individual parts of his subject, there appears to be a considerable want of the simplicity requisite in a work intended for students beginning the science. For those who have already made some progress in chemical knowledge, the work is much more adapted, both as a book of study and of ready reference, upon the facts and principles which have been long established or recently discovered. For such purposes, we could not desire a better instructor than Dr. Turner; but this is as far as we can go in our estimation of his merits, for as an initiatory work, or first book on chemistry, we do not think its plan judicious, and feel assured that it will wear a repulsive aspect to the greater number of young men, who may peruse it with such a notion of its character. It may be remarked, accordingly, that the high praise which the work has obtained, and, unquestionably, well merits, has come from chemists who are capable of judging of its comprehensiveness and accuracy, and from students who, having acquired, by means of lectures or more elementary books, a certain acquaintance with chemical facts and chemical language, find Dr. Turner an admirable guide in their further progress. The title, indeed, of "System," would, in our opinion, be much more appropriate than Elements. We are aware that the author expressly says, the work is designed for persons who have attended, or are attending, Lectures on Chemistry;' but he must know well that the latter

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class in particular require something more simple and attractive than they will find in his Elements. He begins, for example, with the usual queue of metaphysical generalities respecting matter, extension, impenetrability, mobility, &c., which he goes on to explain thus:

Extension is the property of occupying a certain portion of space. A substance is said to be extended when it possesses length, breadth, and thickness. By impenetrability is meant that no two portions of matter can occupy the same space at the same moment. Every thing that possesses extension and impenetrability is matter.'-p. 1.

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Now we humbly submit, that though this manner of commencing a work on physical science can boast of the authority of the highest names, yet it is exceedingly repulsive, if not unintelligible, to young men commencing the study. Nay, he has not made the best, even of this plan, bad as we conceive it to be; for, in proceeding to explain the extreme divisibility of matter, he quotes Nicholson's Illustrations of the Extensibility of Gold, which is a mechanical, not a chemical example, and contents himself with saying, by chemical means, a still more minute division may be effected.' (p. 2). Now we may justly ask, why, in this case, Dr. Turner did not introduce a plain, easy, chemical experiment, rather than the inappropriate example from Nicholson? He tells us in his preface, that he wishes to make the student intimately acquainted with the theory, at the same time that he is acquiring a knowledge of the facts of chemistry, so that, by the establishment of fixed principles, the details may more easily be impressed on the memory, and excite an interest which they could not otherwise possess.

With due deference to the professor of chemistry, we are clearly of opinion, that this is an attempt to lay the foundation of an edifice in the air, and build downwards. It is the details and facts which alone can be learned; in the first instance, whatever attempts may be erroneously made to give the theory and the general principles a priority, because of their presumed importance. Does Dr. Turner require to be told, that every general is made up of particulars? and, consequently, that facts and details must always form the only foundation for principles and theories? He might as well endeavour to teach a manufacturer to bleach a piece of Irish linen, or dye a piece of silk green, by telling him generally, that a chemical substance has various affinities with other chemical substances, as to impart any real knowledge of the science by teaching what he calls fixed principles, while he makes the facts and the details a subsequent and minor consideration. Such remarks, to some, may appear too minute and hypercritical, but when authors will disregard the first principles of logic, we must remind them that they are amenable to its laws.

The work is divided into four parts, treating, in succession, of im

ponderable substances Inorganic Chemistry; Organic Chemistry and Analytical Chemistry, with an Appendix, containing certain tables. On each of those divisions, we shall now make such remarks as were suggested on perusing them.

The sections on electricity and galvanism, have been, the author informs us, very materially altered; but it strikes us forcibly, that though well written and interesting, they have more the air of historical sketches than of an elementary didactic treatise. If Dr. Turner composes his lectures upon these subjects according to this plan, he cannot hope to make his pupils, good chemical electricians or galvanists. We are much better pleased with his sections on caloric and light, though these again are too much like a dissertation, or a monograph. We select, as a favourable specimen of his manner, his remarks on the vapours of metals, earths, &c., supposed to be diffused in the atmosphere.

It has accordingly been supposed, that the atmosphere contains, diffused through it, minute quantities of the vapours of all the bodies with which it is in contact; and this idea has been made the basis of a theory of the origin of meteorites. But this doctrine has been successfully combated by Mr. Farraday, in his Essay on the Existence of a Limit to Vaporization, published in the Philosophical Transactions for 1826. The argument employed by Mr. Farraday, is founded on the principle by which Dr. Wollaston has accounted for the limited extent of the atmosphere. Since the volume of gaseous substances is dependent on the pressure to which they are subject, the air in the higher regions of the atmosphere must be much more rare than that in the lower, because the former sustains the pressure of a shorter atmospheric column than the latter; so that in ascending upwards from the earth, each successive stratum of air, being less compressed than the foregoing, is likewise more attenuated. Now it is found experimentally, that the elasticity or tension of any gaseous matter diminishes in the same ratio as its volume increases; and, accordingly, whenever the tenuity of a portion of air, owing to its distance from the earth's surface, or any other cause, is exceedingly great, its tension is exceedingly small. Reasoning on this principle, Dr. Wollaston conceives that at a certain altitude, probably at a distance of 40 or 50 miles from the surface of the earth, the rarefaction and consequent loss of elastic force is so extreme, that the mere gravity of the particles becomes equal to their elasticity, and thus puts a limit to their separation.

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What Dr. Wollaston suggests of aërial particles, Mr. Farraday supposes to occur in all substances; and this supposition is perfectly legitimate, because gaseous matter in general is subject to the same law of expansion, and is likewise under the influence of gravity. He infers that every kind of matter ceases to assume the elastic form whenever the gravitation of its particles is stronger than the elasticity of its vapour. The loss of tension necessary for effecting this object may be accomplished in o ways, either by extreme dilatation or by cold. For substances of great volatility, such as air and most gases, the former is necessary; because the degree of cold which we can command at the earth's surface diminishes their tension in a degree quite insufficient for the purpose. But the volatility of innumerable bodies is so small, that their vapour at com

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mon temperatives approximate in rarity to the air at the limits of the atmosphere, and a small degree of cold may suffice for rendering its elasticity a force inferior to its apparent gravity. In that case the vapour would be entirely condensed. Mr. Farraday found that mercury, at a temperature varying from 60° to 30°, yields a small quantity of vapour, but in winter no trace of vapour could be detected. Hence it is inferred, that at the former temperature the elasticity of mercurial vapour is slightly superior to the gravity of its particles, and that in cold weather the latter power preponderates, and puts an entire check to the evaporation of mercury. The earths and metals, which are more fixed than mercury, have vapours of such feeble tension, that the highest natural temperature is unable to convert them into vapour. Another force which co-operates with gravity in overcoming elasticity, is the attraction of aggregation, or the attraction exerted by a solid or liquid, to the contiguous particles of the same substance in the gaseous form. This argument affords very sufficient grounds for believing that the vapours of earthy and metallic substances are never present in the atmosphere.'---p. 70.

The facts relating to the doctrine of definitive proportion and atomic theory, are stated, in the second part, with so much perspicuity, that but for the extent of the subject, we should have extracted the whole. We should have preferred arranging this section in a subsequent part of the volume, but as it stands, in some degree, isolated, the student has it in his option to pass it over, till he obtains a knowledge of the elementary facts requisite for understanding it. The subject is confessedly difficult, but Dr. Turner has stated it in so clear a manner, as to remove much of the mystery encountered by beginners in some other elementary works. We particularly admire his marked distinctions between the ascertained facts, and the more hypothetical portions of the subject; the usual confounding of which together, has led to much misconception and mistake. Dr. Turner indeed, justly remarks, that it is not uncommon for persons commencing the study of chemistry, to entertain a vague notion, that this department of the science comprehends something uncertain and hypothetical in its nature, and to be thus led to form an erroneous estimate of its importance. But in the sketch which he has given of the laws of combination, he has stated only the pure expression of fact, founded on experiment alone, which is not necessarily connected with any speculation, and may be kept wholly free from it. To this sketch, however, he has appended a brief account of the theory of Dalton, the theory of volumes by M. Gay-Lussac, and the theory of Berzeling.

In his chemistry of the metals, Dr. Turner has given, in general, a very complete statement of the established facts, as well as of the newer discoveries. In his account of the tests of arsenic, so important in questions of medical jurisprudence, he objects to those proposed by Mr. Hume, of Long Acre, consisting of the ammoniacal nifrate of silver, and the ammoniacal sulphate of copper, particularly the latter, which is of little service, he thinks, when the

arsenic is mixed with tea, porter, &c. He, therefore, prefers the test proposed by Dr. Christison of Edinburgh.

When a current of sulphurated hydrogen gas is conducted through a solution of arsenious acid, the fluid immediately acquires a yellow colour, and in a short time becomes turbid, owing to the formation of orpiment, or the yellow sulphuret of arsenic. The precipitate is at first partially suspended in the liquid; but as soon as the free sulphuretted hydrogen is expelled by boiling, it subsides perfectly, and may easily be collected on a filter. One condition, however, must be observed, in order to insure success, namely, that the liquid does not contain a free alkali; for the sulphuret of arsenic is dissolved with remarkable facility by pure potassa or ammonia. To avoid this source of fallacy, it is necessary to acidulate the solution with a little acetic or muriatic acid. Sulphuretted hydrogen likewise acts on arsenic in all vegetable and animal fluids, if previously boiled, filtered, and acidulated. at of 0.202

But it does not necessarily follow, because sulphuretted hydrogen causes a yellow precipitate, that arsenic is present; for there are not less than four other substances, namely, selenium, cadmium, tin, and antimony, the sulphuret of which, judging from their colour alone, might be mistaken for orpiment. From these and all other substances whatever the sulphuret of arsenic may be thus distinguished: When heated with black flux, in the manner described for reducing the arsenic of lime, a metallic crust of an iron-grey colour externally, and crystalline on its inner surface, is deposited on the cool part of the tube; and by converting a portion of this crust into vapour, its alliaceous odour will instantly be perceived. Besides these circumstances, which alone are quite satisfactory, it is easy to procure additional evidence, by re-converting the metal into arsenious acid, so as to obtain it in the form of resplendant octohedral crystals. This is done by holding that part of the tube to which the arsenic adheres about three-fourths of an inch above a very small spirit lamp flame, so that the metal may be slowly sublimed. As it rises in vapours it combines with oxygen, and is deposited in crystals within the tube. The character of these crystals, with respect to volatility, lustre, transparency, and form, is so exceedingly well marked, that a practised eye may safely identify them, though their weight should not exceed the 100th part of a grain. This experiment does not succeed unless the tube be quite clean and dry.

It hence appears, that of the various tests for arsenic, the only one which gives uniform results, and is applicable to every case, is sulphuretted hydrogen: all the rest may be dispensed with. For this great improvement in the mode of testing for arsenious acid, we are indebted to Dr. Christison. By this process he discovered the presence of arsenious acid a when mixed with complex fluids, such as tea, porter, and the like, in the proportion of one-fourth of a grain to one ounce; and more recently, he has twice obtained so small a quantity as the 20th of a grain from the stomachs of people who had been poisoned with arsenic.'p. 4718ei

On some of the other subjects of metallic chemistry, connected with medicine, Dr. Turner is somewhat deficient. We find no mention, for example, of the compound salt, the chloride of gold and sodium, which has been prepared and employed on the Continent, particularly by M. Chrestien of Montpellier, and by M. M.