« PreviousContinue »
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 Auxes, 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, we 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.
6 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 borar, 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 pare specimens to operate upon. He mentions the names of Haüy, 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 bur
rounding 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, 58. 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 wbich 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, then raised for the removal of the ball and cross, to exalt his aerial studio about sixty feet higher. For this purpose, he obtained permission to erect an observatory, supported by a platform several feet above the top of the highest part of the present cross, and in this watch tower he commenced a new series of sketches on a very enlarged scale.
" To effect this, it was found necesary, from time to time, to adopt various contrivances to meet the numerous obstacles which opposed the progress of the work. In weather partially cloudy, portions of the scene would be in bright sunshine, and others in total obscurity, producing an incessant alternation of light and shade: it therefore became requisite to alter and modify the previous arrangements, that advantage might instantly be taken of the clear light, in any particular part of the entire circle of the View, and that an immediate transition might be made from one sketch to another. Trilling as this difficulty may at first appear, it gave rise to more trouble and anxiety than any other part of the undertaking, since the time necessarily occupied in selecting the particular sketch, independently of the requisite adjustment of the apparatus, frequently exceeded the transient period during which the object continued visible. The difficulty, also, of connecting the detached parts thus seized at the most favourable moment was so great at times, as almost to preclude the hope of completing the performance. After a variety of attempts, the obstacle was at length removed by the construction of a comprehensive keysketch, which served to indicate the precise relation of any particular portion to the general View. The remaining difficulties were in a great measure obviated by placing the sketches (about 300 in number) in a rotatory frame, in such order that any particular one might be referred to at the moment it was required." P. 15.
To this lofty station Mr. Hornor repaired every morning during the summer of 1821, at the early hour of three o'clock, in order that his operations might commence before the ascending smoke should impede bis view.
« On entering the Cathedral at three in the morning, the still. ness which then prevailed in the streets of this populous city, contrasted with their mid-day bustle, was only surpassed by the more solemn and sepulchral stillness of the Cathedral itself. But not less impressive was the developement, at that early hour of the immense scene from its lofty summit, whence was frequently be. held "the Forest of London," without any indication of animated existence. It was interesting to mark the gradual symptoms of returning life, until the rising sun vivified the whole into activity, bustle, and business. On one occasion the night was passed in the observatory, for the purpose of meeting the first glimpse of day; but the cold was so intense, as to preclude any wish to repeat the experiment.
" In proceeding with the work, every assistance was readily afforded by the gentlemen connected with the Cathedral; and through their kind attention, all possible precautions were taken for the prevention of accidents to be apprehended in such an exposed situation. But the weather was frequently so boisterous during the stormy summer of 1821, as to frustrate the most judicious contrivances for security. Indeed scarcely a day passed without derangement of some part of the scaffolding, or machinery connected with it ; and so strong became the sense of danger arising from these repeated casualties, that notwithstanding the powerful inducement of increased remuneration, it was difficult on these emergencies to obtain the services of efficient workmen. This will not appear surprising, when it is known that during high winds, it was impossible for a person to stand on the scaffolding without clinging for support to the frame-work; the creaking and whistling of the timbers, at such times, resembled those of a ship labouring in a storm, and the situation of the artist was not unlike that of a mariner at the mast-head. During a squall more than usually severe, a great part of the circular frame-work of heavy planks, erected above the gallery for the prevention of accidents, was carried over the house tops to a considerable distance. At this moment a similar fate had nearly befallen the observatory, which was torn from its fastenings, turned partly over the edge of the platform, and its various contents thrown into utter confusion. The fury of the wind rendered the door impassable; and after a short interval of suspense, an outlet was obtained by forcing a passage on the opposite side. By this misfortune, independently of personal inconvenience, considerable delay and expense were occasioned ere the work could be resumed; and it became necessary to provide against similar misfortunes, by securing the observatory to a cross-beam, and constructing a rope-fence, as seen in the lower part of the vignette. Thus fortified, the work was proceeded in without
any other accidents of a nature worthy to be noticed, until all the sketches which could be taken from the observatory were completed. These sketches, comprising 280 sheets of drawing paper, extend over a surface of 1680 square feet ; a space which will not appear surprising, when considered as including a portion of almost every public building and dwelling-house in the metropolis, with all the villages, fields, roads, villas, rivers, canals, &c. visible from the summit of the Cathedral.” P. 19.
After a general acquaintance with his subject had been thus acquired, several weeks were employed in visiting many of the particular and principal points, and in collating them on the spot with the distant drawings, for the purpose of correction. A reduced drawing was then made from the original sketches, divided into four parts, and diminished to one-tepth of their first size. From these Mr. Hornor now proposes to publish four engravings. Two, comprising the