operation of the blowpipe is conducted. By directing a stream of air against a flame, the blue part is as it were driven into the interior of the flame, and terminating now in a point, instead of as before forming a circle round the flame, the effect is most intense at, or just beyond, that point. This effect is easily explicable on the principles of Sir H. Davy, deduced from a variety of experiments on explosive mixtures of gases. He has shown, that generally speaking the heat of a flame is greater, the more perfect the combustion of the inflammable matter while in a gaseous state. On the other hand the illuminating power results from an opposite cause; namely, the decomposition of part of the gas towards the interior of the flame where the air is in the smallest quantity, and the deposition of solid charcoal, which, first by its ignition, and then by its combustion increases in a high degree the intensity of the light. This he shewed very decisively by the simple experiment of placing a piece of wire gauze at a small distance above a stream of coal gas, and igniting it only above the gauze. The further he moved the gauze from the pipe the more air mixed with the gas, and the feebler was the light of the flame, while the heat proportionally increased; as appeared from its effects in heating a fine wire of platinum. In a flame urged by a blowpipe at the point of the blue flame the greatest quantity of oxygen is supplied to the gases issuing from the wick, and therefore at that point their combustion is more perfect, produces more heat and less light, whereas in the outer parts of the flame, where a solid product of charcoal is deposited in the form of smoke, the combustion is less perfeet, the heat less and the light greater.

Our author after giving several directions for obtaining a proper sort of flame, proceeds to describe the two principal operations in which the blowpipe is employed: these are the oxidation of metals, and the reduction of oxides. The account of these we will give in his own words. P. 28.

"Oxidation ensues when we heat the subject under trial before the extreme point of the flame where all the combustible particles are soon saturated with oxygen: the farther we recede from the flame, the better the oxidation is effected, (provided we can keep up sufficient heat): too great a heat often produces a contrary effect, especially when the assay is supported by charcoal. Oxidation goes on most actively at an incipient red heat. The opening in the beak of the blowpipe must be larger for this kind of operation than in other cases.'

"For reduction, a fine beak must be employed, and it must not be inserted too far into the flame of the lamp; by this means we

obtain a more brilliant flame, the result of an imperfect combustion, whose particles as yet unconsumed, carry off the oxygen from the subject of experiment, which may be considered as being heated in a species of inflammable gas.. If in this operation, the assay become covered with soot, it is a proof that the flame is too smoky, which considerably diminishes the effect of the blast. Formerly the blue flame was considered as the proper one for the reduction of oxides; but this idea is erroneous; it is in reality the brilliant part of the flame which produces deoxidation: it must be directed on the assay, so as to surround it equally on all sides, and defend it from the contact of the air."

The substance on which the mineral to be examined is to be supported, comes next under discussion. Charcoal is, in most cases to he used; that of the lightest wood, and most free from cracks, and even-grained, is the best. In a note the translator recommends that from the alder. Supports of platina are also occasionally used with advantage. Sometimes the substance is to be placed in a glass tube flask, or mattrass. After this follows an enumeration of all the various implements, and articles of different descriptions which are useful in applying the blowpipe to mineralogical purposes. These descriptions, it appears have been in some instances curtailed by the translator. Thus after an observation of the author, that order in the arrangement of the different instruments, &c. is very advantageous, the translator adds in a note,

"Here follows a long detailed description of a table with a drawer at each side, and four in front, divided into moveable compartments of tinned iron to hold various instruments, &c. not forgetting a hook with a towel fixed to the right leg of the table. Next comes an equally elaborate description of a red morocco case to hold a travelling blowpipe apparatus." P. 42.

The next section is devoted to the description of the different re-agents, or chemical tests, employed with the blowpipe, and directions for using them. These are few in number, and applicable in a great variety of cases, to assist in reducing oxides, and effecting fusion, and for other purposes connected with these. We shall not, however, attempt to give our readers any account of these processes. This section is closed by some general rules for conducting experiments with the blowpipe; which display evidently the result of a minute practical acquaintance with all the varied forms under which these phenomena present themselves, and the various methods in which the different operations are performed; they will doubtless be found of infinite value to the young experimenter, and are given in a clear, simple,

+

and interesting manner. We will quote as a specimen part of the conclusion of these directions:

According as they are exposed to the outer or inner flame of the lamp, and fused alone, or with fluxes, mineral substances present numerous phænomena, which must be carefully noted, and form, when taken together, the general result of the trial to which each individual has been submitted. The minutest circumstance of these phenomena must be attentively observed; because it may often lead to the detection of elements, whose presence was not suspected.

"Whenever we would record the result of an experiment with the blowpipe, either for our own instruction, or that of others, wẹ must always make two experiments, note down separately the result of each, and then compare the two together; for it often happens, that something which escaped us on a first observation, strikes us on a second. The safest mode is for two persons to make and note down separately a similar set of experiments, and compare their results: if they agree, they may be considered as accurate, otherwise the cause of discrepancy must be sought for. A little difficulty sometimes attends this sort of association, from two persons not always seeing and denominating colors alike. For instance, there were certain shades which John always called yellow, or dull yellow, and which I persisted in calling red; although we agreed as to their fundamental colours, pure yellow and pure red."

We conceive all our scientific readers will concur in admitting the excellence of these cautions and directions.

The largest and most important part of the work now commences: this is a detailed account of the different characteristics exhibited by each species of minerals when exposed to the blowpipe. Of this part we shall not attempt any account; but only wish to notice its great importance, and the able and luminous manner in which the subject is treated. We will, however, just give one instance, taken at random, to shew the manner in which the characteristics developed by a mineral before the blowpipe, are described.

"7. FELSPAR.

"Alone in the mattrass, transparent Felspar gives off no water. The cracked opaque felspar often affords a large portion of water, which was contained mechanically in the interstices of the mineral.

"On charcoal, in a bright heat, it becomes vitreous, semi-transparent and white, and fuses with difficulty on the edge into a blebby semi-transparent glass. It is a mineral of very difficult fusion.

"With borax, fuses very slowly without effervescence into a diaphanous glass.

"Salt of phosphorus attacks it with great difficulty; with the pulverized mineral it gives a globule which becomes opaline on cooling, and leaves a silica skeleton,

"With soda the solution is slow, and attended with effervescence; it gives a transparent glass very difficult to fuse and obtain free from blebs.

"With solulion of cobalt, only the fused edges are coloured blue."

Then follow two "remarks" relative to the composition, &c. of the mineral, and the difference between it and another, which" behaves" in every respect like it. A similar method is adopted with each species: these are also arranged systematically, according to the chemical method, which was fully explained by the translator in the prefatory part of the work, where also the advantages attending it are pointed out. At the end is given an account of the appearances exhibited by urinary calculi before the blowpipe, which will be no doubt highly useful to those engaged in the chemical examination of them.

The author has evidently been at great pains to procure pure specimens to operate upon. He mentions the names of Hauy, Bournon, Gillet de Laumont, Brongniart, Brochant and others, well known in the mineralogical world, as having supplied him with specimens. The circumstance of the purity of the specimens, as well as that also of the re-agents employed is obviously of the first importance towards laying down with precision the real characteristics of minerals.

His detail of facts is in many instances not confined to the appearances exhibited before the blowpipe: he sometimes extends his descriptions to many other particulars belonging to the substance under examination. Thus the description of the Amphiboles and Pyroxenes is full of interesting information respecting their composition. And upon the whole, a fund of instruction may be derived from the work besides what immediately relates to the blowpipe..

One of the most useful parts of the whole work, we consider to be, a table by the translator, exhibiting at one view the effects exhibited by a variety of substances before the blowpipe. The operator with this instrument will doubtless duly appreciate its merits; it will afford a very convenient means of reference in many situations where large books may be inconvenient: in a vertical column at the side are ranged the earths and metallic oxides; and on a line with each, are expressed the phenomena it presents with each of the different re-agents before the blowpipe, which stand at

the tops of several more vertical columns. This table is given on a sheet at p. 118. which may be easily taken out, so as to be ready for reference on all occasions.

We have now brought our remarks to a close; and only wish to conclude by expressing in the strongest manner, our recommendation of the work to all such of our scientific readers as may not yet have met with it: it is indeed a work indispensable in the collection of every chemist, mineralogist, and philosophical student.

ART. V.

Prospectus. View of London, and the surrounding Country, taken with Mathematical Accuracy from an Observatory purposely erected over the Cross of St. Paul's Cathedral, to be published in Four Engravings, by Thomas Hornor. For the Author. Royal 8vo. pp. 32, 5s. Two Plates. 1823.

WE are induced to notice this little Brochure, not because it possesses any claim (nor indeed does it advance any,) to literary distinction; but from our wish to make more generally known the design which a very enterprising artist has undertaken, and the singular energy and ardour which he has manifested in conducting it.

Mr. Hornor has been for many years engaged in a branch of drawing, from which we, among others, have derived much pleasure, but which still wants a name. It is a pictural and graphic survey, in which the eye is presented, not with the jagged lines only which form the boundaries of property, but with every bush and tree, and hedge and ditch, which can find its station in a bird's eye perspective. Having practised this mode, as he informs us, extensively in the neighbourhood of London, he had formed a collection of sketches peculiarly applicable to a general view of this district; and he has also constructed an apparatus, by which the most distant and intricate scenery may be transferred to paper with mathematical accuracy. An apparatus which, though it is not so stated, may fairly be supposed to be some modification of the Camera obscura. Thus prepared, he passed the whole of the summer of 1820 in the lantern of St. Paul's, immediately under the ball, and when the view which he had taken from this point was nearly completed, he eagerly availed himself of the opportunity afforded by the scaffolding,