233 



BLOW-PIPE. 



BLOW-PIPE. 



234 



operation should first be attempted on very small fragments, as the 

 difficulty increases with the size of the tin grains. 



We now come to speak of the experiments in which fluxes are 

 employed, the most important of which and their uses will be briefly 

 described. They are, carbonate of soda, borate of soda, the duuMe 

 phosphate of soda and ammonia, saltpetre, boracic acid, bisulphate of 

 potash, 'gypsum, fluor-spar, nitrate of cobalt, tin, iron, lead. Of these the 

 first three only are of general use, while the others are employed to 

 test the presence of particular bodies ; we shall confine our attention 

 therefore to the former, as to touch upon the particular cases in which 

 the others may be advantageous would not only lead us too much into 

 detail, but belongs more particularly to the chemical description of the 

 properties of these bodies. 



Care should be taken that the carbonate of soda employed for these 

 experiments be free from any impurities, particularly from the sxil- 

 phate. The purest which can be purchased is the bicarbonate of 

 commerce; if this cannot be obtained, a saturated solution of the 

 ordinary carbonate should be taken, through which a current of car- 

 bonic acid must be transmitted, when the bicarbonate will be precipi- 

 tated in the form of fine grains, which must be washed with cold 

 water and then dried. It may be tested for sulphuric acid by means 

 of the blow-pipe itself in the following manner : Let a glass be formed 

 by fusing a portion of the carbonate of soda with a small quantity of 

 pure silica, and let the resulting glass be well acted on by the deoxi- 

 dising flame. If on cooling it retains its colourless condition, the soda 

 may be considered free from sulphuric acid, the presence of which would 

 be indicated by the glass assuming a yellow passing into a hyacinth-red 

 colour, owing to the presence of the liver of sulphur. The application 

 of carbonate of soda answers two purposes ; to determine whether the 

 body is fusible in it as a flux, and to assist in the [reduction of metals. 

 The carbonate of soda is best applied by mixing it in powder with the 

 substance to be examined, which should also be in powder ; the mixture 

 U formed into a paste by the addition of a little water, a small portion 

 of which must then be placed on the charcoal, where, after drying, it 

 must be brought into a state of fusion. It ia usual for the carbonate 

 of soda, as soon as it is fused, to be entirely absorbed by the charcoal, 

 but it is not on that account less active ; a continued effervescence is 

 observed on the substance under examination, and its fusibility is indi- 

 cated by the formation of a glass globule. 



But the greatest use of carbonate of soda is decidedly in promoting 

 the reduction of metals, which it does in a most unaccountable manner. 

 If a small quantity of the oxide of tin be placed on the charcoal, a 

 dexterous blower, at some expense of time and trouble, will be able 

 to obtain from it a small globule of metallic tin. If, however, a little 

 carbonate of soda be added to the oxide of tin, the reduction is effected 

 with ease and rapidity. 



The influence of the carbonate of soda in this operation is not under- 

 stood, but its action in constant ; and Uahn has given the following pro- 

 cess, by which the metals platinum, gold, silver, molybdenum, tungsten, 

 antimony, tellurium, bismuth, tin, lead, copper, nickel, cobalt, and iron 

 may be obtained, and consequently their presence detected, whenever 

 they occur in any ore : 



The assay is reduced to powder, and formed as before into a paste 

 with the moistened carbonate of soda ; this must then be placed on the 

 charcoal, and submitted to the action of a good reducing flame. After 

 'ime ;in additional quantity of soda salt must be added, and the 

 blast must be again renewed, and this process must be repeated until 

 the whole of the assay is absorbed by the charcoal. When this is 

 entirely effected, those portions of the charcoal which have thus become 

 ;<aturated with carbonate of soda, must be moistened by a few drops of 

 water ; and they must then be carefully removed with a knife and 

 reduced to jnwder in an agate mortar. This must then be washed, by 

 which the fine and light particles of charcoal may be readily removed 

 from the metallic particles, which, if any be present, will be found in a 

 pure metallic form in the mortar. The form in which the rnetal will 

 be found depends on its fusibility and malleability ; should it possess 

 these properties, it will be formed into small thin leaves ; if not, it will 

 be found as a metallic powder. By this process the operator should 

 be aware that the metals antimony, bismuth, and tellurium may have 

 escaped his observation, from having been volatilised as soon as re- 

 duced, which is also always the case with selenium, arsenic, cadmium, 

 zinc, and mercury, which can only be obtained by sublimation. 



The borate of soda of commerce is never sufficiently pure for these 

 purposes, but it may readily be obtained fit for use by solution in pure 

 water and re-crystallisation. It may be employed either in the form 

 of small grains, or of powder, or it may be first fused to free it from 

 its water of crystallisation. The advantages of its use in the blow-pipe 

 are dependent on its forming a most powerful flux, by which a number 

 of otherwise refractory substances may readily be brought into a state 

 of fusion. It is usual, in the first place, to endeavour to fuse a small 

 fragment of the assay ; as, if this process be successful, we are able to 

 observe the phenomena taking place during the fusion better than 

 when it is applied in the form of a powder ; and what is the most 

 important, we see whether the assay is partially or entirely fusible in 

 this flux. The principal facts to be observed are, whether the fusion 

 is accompanied with effervescence, or whether it takes place tranquilly; 

 nuine the colour of the glass when obtained, and the changes it 

 undergoes according as it is acted on by the oxidising or reducing 



flame, and also to observe whether any change takes place either in 

 the colour or transparency of the glass as it cools. 



The microcosmie salt, to use the term by which it is usually desig- 

 nated in works on this subject, is a double salt of phosphoric acid, 

 ammonia, and soda. It is best prepared, according to Berzelius, by 

 adding to a solution of 16 parts of chloride of ammonium in a small 

 quantity of boiling water 100 parts of crystallised phosphate of soda : 

 this latter must then be brought also to a state of solution over the 

 fire, after which the solution must be immediately filtered, and then be 

 allowed to cool slowly, when the double salt will be deposited as crystals. 

 It may be considered as pure if the crystals when fused give a glass 

 which does not become opaque on cooling. The object of this salt is 

 to enable us to try the action of a free and strong acid on the assay, 

 which is best obtained by this means, as on heating the ammonia is 

 driven off, and the acid with which it was combined is then at liberty 

 to exercise its influence on the body tested. It is therefore a powerful 

 agent in proving the presence of the metallic oxides, with which it 

 frequently forms characteristic coloured salts ; and it is also a good 

 test for determining the presence of silica in minerals, the phosphoric 

 acid depriving it of the bases with which it was combined, and pre- 

 senting it in the form of a gelatinous substance. 



In subjecting metallic oxides to qualitative examination before the 

 blow-pipe with the above-mentioned reagents, which fuse at the tem- 

 perature employed, a stout platinum wire is bent in the form of an ear, 

 of about one-sixth of an inch in diameter, around the tapering nozzle 

 of the blow-pipe. Such a loop is then heated red hot, and plunged 

 into the powdered flux which is to be used. The particles of flux 

 which thus become attached to the wire, are again heated and made to 

 take up some more of the flux, until enough of the latter is attached to 

 form a bead filling the entire ear. Borax, microcosmie salt, and salt- 

 petre give in this way transparent beads ; carbonate of soda, and a 

 mixture of this with nitre give a milky enamel ; in all cases the bead so 

 formed must be colourless. 



The following table taken from Plattner's elaborate treatise on the 

 blow-pipe, gives the reactions of the principal metallic oxides when 

 heated in the oxidising and reducing blow-pipe flames, respectively with 

 borax and microcosmie salt : 



The alkalies, when heated by themselves in either of the blow-pipe 

 flames, impart u, colour to the flame which is probably the colour of 



