229 



BLOCKING-COURSE. 



BLOW-PIPE. 



220 



be incessantly ploughed up by the fire of artillery directed into it by 

 the enemy from the surrounding heights. Here then the blockhouse 

 may with propriety be constructed as an independent work ; its plan 

 may have re-entering angles, or be in the form of a cross, in order to 

 allow the faces to be defended by flanking fires of musketry from 

 within ; and the walls may be thick enough to resist the shot from light 

 or mountain artillery. For this purpose they must be made by planting 

 parallel to each other, at the distance of three or four feet, two rows 

 >ng piles, those in each row being close together, and the interval 

 between the rows being filled with earth up to the height of the loop- 

 holes, which should now be immediately under the roof of the 

 building. The roof must be made shell-proof as before ; but it has 

 been recommended, when the work is not overlooked by the enemy, 

 and when its breadth will permit, to have the piles forming the side- 

 walls long enough to rise above the roof, and, either alone, or with a 

 mass of earth behind them, to serve as a parapet. 



To prevent the enemy from setting fire to the blockhouse, it should 

 be surrounded by a ditch ; part of the earth obtained from thence 

 should be raised against the building as high as the loop-holes, in order 

 to strengthen it exteriorly, and the rest may be thrown beyond the 

 counterscarp to form there what is called a reverse glacis. 



Any area inclosed for the purposes of defence by piles or palisades, 

 or by logs of timber horizontally disposed, but without a roof, and 

 defended by loop-holes and machicolations, is sometimes called a 

 blockhouse : more generally, however, such constructions are called 

 stockades, under which word they will be described. 



(See Bousmard, Eai Gfnfral de Fortification ; Dufour, Memorial 

 ! ' Travattx de Guerre; Macauley, Captain J. S., Treatise on Field 

 Fortification.) 



BLOCKING-COURSE. [EXTABLATT/RE.] 



BLOW-PIPE. The instrument to which this name has been applied 

 was originally employed by jewellers and others in the soldering of 

 metals on the small scale, whence it derives its name in the German 

 language, " Lothrohr," from the two words " lothen," to solder, and 

 " rohr," a tube or pipe. When used for such purposes it is constructed 

 of a simple metallic tube seven or eight inches in length, the bore of 

 which at the larger extremity is about one-fourth of an inch in dia- 

 meter, and gradually contracts as it approaches the other, where it 

 terminates in an almost capillary orifice ; and the instrument is formed 

 by simply bending this tube at a right angle at an inch or an inch and 

 a half from its finer extremity. In this form it is used by the work- 

 man to direct the flame of a lamp on the portion of solder to be 

 employed, by which he is enabled to bring it readily and without loss 

 of time into a state of fusion ; the solder is placed on a fragment of 

 charcoal, which he holds in his left hand, and upon which the flame is 

 made to play by blowing a gentle current of air against it by means of 

 the pipe. 



Such was its sole use until the year 1 738, when, as we are informed 

 1 'V Bergman, Antony Swab, a Swedish bergrath, or counsellor of mines, 

 and a man of very considerable knowledge for his time, introduced it 

 notice of the scientific world, by employing it in determining 

 the nature of the metals in the various ores and minerals which came 

 under his notice. Swab however wrote no work on the subject, nor 

 does it appear to have received any particular attention from any one 

 until Cronstedt proposed his system of mineralogy, in which the 

 arrangement is dependent on the chemical composition of the minerals. 

 It thus became to him of vital importance for the general adoption of 

 his system we may almost say for its very existence to possess some 

 reedy and Dimple means of determining the constituents of mineral 

 bodies, as it was evident that those offered by the slow and laborious 

 operations of chemical analysis could not be generally employed by 

 mineralogist*. This he found in the blow-pipe ; and by the employ- 

 ment of fluxes in the experiments performed with this instrument, he 

 may be considered as the founder of a new department of the chemical 

 science. His results are to be found in his ' System of Mineralogy,' 

 the first edition of which was published in 1758, and was translated 

 into English by Von Engestrom in 1765 ; also in an essay by the latter, 

 published in London in 177'1 under the title of ' An Essay towards a 

 System of Mineralogy,' by Cronstedt, translated from the Swedish by 

 Von Kngestrom, revised and corrected by Mendez da Costa, London, 

 1770. 



The employment .of the blow-pipe in detecting the constituents of 

 minerals being thus brought into notice, excited the attention of the 

 chemists and mineralogists to the cultivation of this branch of chemistry, 

 Mid its application to chemical analysis and to the determination of the 

 mineralogical species. In Sweden however it still appears to have been 

 studied with the greatest success ; and it is to the chemists and mine- 

 ralogists of Sweden that we are indebted for the greater portion of the 

 information which has been received on this subject, and more par- 

 ticularly to Bergman, Gahn, and Berzelius. Bergman, after extending 

 its limits by a series of original researches, in which he investigated the 

 properties of most of the then known species of minerals, and by a 

 more general application to chemical analysis, published the results of 

 his observations in a treatise written in the Latin language, which 

 appeared at Vienn in 177S, under the following title : ' De Tubo 

 Ferruminatorio, ejundemque usu in explorandis Oirporibus, presertim 

 Minoralibus.' A translation of the above into English will be found in 

 oml volume of Bergman's ' Chemical and Physical Essays,' by 



Dr. Cullen, London, 1788. Gahn, though indefatigable in his observa- 

 tions and experiments with the blow-pipe, and though far exceeding 

 any of his predecessors both in the conception and execution of his 

 experiments, has however left no work on the subject. As an instance 

 of his power of detecting the presence of metallic bodies, we are told 

 by Berzelius that he has often seen him extract from the ashes of a 

 quarter of a sheet of paper distinct portions of copper, and that too 

 before the knowledge of the occurrence of this metal in vegetables was 

 known, and therefore before he could have been led from this circum- 

 stance to suspect its presence in paper. 



Although we cannot but feel regret at having received no work from 

 a man so eminently qualified to instruct on this subject as Gahn, still 

 we must consider ourselves most happy that under such circumstances 

 the loss of the knowledge and experience of so long and laborious a life 

 is not also to be lamented. Fortunately for science, accident, as it 

 were, made Berzelius the medium through which this information was 

 to be communicated to the world ; and while his good fortune in thus 

 having it in his power to add another to the many benefits he has 

 bestowed on mankind cannot but be envied, it must be universally 

 felt and acknowledged that if he was favoured by fortune he proved 

 himself one of the most worthy of her favour by the manner in 

 which he fulfilled the task assigned to him. The assiduity of Gahn 

 in this study, together with the circumstances to which we are 

 indebted for the preservation of his labours, cannot be better told than 

 in the words of Berzelius himself. " Gahn," says he, " was never with- 

 out his blow-pipe, not even during his shortest journeys. Every new 

 substance, or any thing with which he was not previously acquainted, 

 was immediately submitted to an examination before the blow-pipe ; 

 and it was indeed an interesting sight to observe with what astonishing 

 rapidity and certainty he was thus enabled to determine the nature of 

 a body, which from its appearance and exterior properties could not 

 have been recognised. Through this constant habit of using the blow- 

 pipe he was led to invent many improvements, and to make many 

 conveniences, which he could have at hand whether at home or abroad. 

 He examined the action of a number of reagents, for the purpose of 

 finding new methods of recognising bodies ; and this he did in such 

 detail, and conducted his operations with such accuracy, that all his 

 results may be relied upon with the greatest confidence. Nevertheless 

 it never occurred to him to give a written description of his new or 

 improved methods : he gave himself however all passible trouble to 

 instruct all who were willing to learn, and many foreign men of science, 

 who passed some time with him, have made known his great dexterity 

 in this subject ; but no one has communicated a perfect knowledge of 

 his methods. 



" I had the good fortune, during the last ten years of the life of this 

 in many respects most remarkable man, to enjoy his most intimate 

 acquaintance. He spared himself no trouble to communicate to me 

 all the results of his experience, and I have consequently held it as a 

 sacred duty to allow nothing of this experience and of hia labours to 

 be lost." 



Such then is the origin of Berzelius's treatise, a work which must be 

 considered as a very high authority on this subject ; and as there are 

 translations in the English, French, and German languages, we cannot 

 too highly recommend it to the study of those desirous of obtaining a 

 more intimate acquaintance with the uses to which the blow-pipe may 

 be applied. The use of the blow-pipe has also been of late years 

 much extended and simplified by Professor Plattner, whose work on 

 this instrument is now doubtless the best extant. An English 

 translation by Muspratt has recently placed this excellent treatise 

 within the reach of all students. 



As our limits will not allow of our entering into a very minute 

 description of the phenomena presented by the different chemical 

 elements and minerals when experimented on by the blow-pipe, we 

 must confine ourselves chiefly to a general description of the nature 

 of the experiments performed by this instrument, and the conclusions 

 to which it leads in determining the chemical constitution of a mineral, 

 and consequently in recognising to what species it belongs. For this 

 purpose it may be convenient to class the experiments under four 

 heads : 



1. The characteristic changes produced on bodies when exposed to a 

 high temperature. 



2. The deoxidising effect of the flame, and the redxiction of metals 

 from their ores. 



3. The oxidising effect, or the changes produced by the oxygen of 

 the air on the body. 



4. The action produced by the application of fluxes or reagents. 

 The first three classes are dependent on the unaided .action of the 



blow-pipe flame ; and as the total effect is produced by properties pecu- 

 liar to particular parts of the flame even in the cases where fluxes are 

 employed, it becomes a matter of great importance to possess a good 

 knowledge of the flame itself, a description of which will therefore be 

 first given. If a burning lamp or candle be carefully observed, it will 

 be found that the flame may be divided into four parts, which may 

 readily be distinguished from each other. First, on the lower 

 extremity of the tiame, where it is in contact with the wick, will be 

 seen a blue portion, which extends from the wick and terminates at 

 the points c, ftj. 1 , where the boundaries of the flame assume a vertical 

 direction. The second most striking part of the flame is the brigh 



