Borax (borate of soda) is used along with the frag ment of mineral in many cases. When exposed to the flame, it becomes opaque, swells and ramifies much, in consequence of parting with its water of crystalli zation; afterwards it fuses into a colourless and trans parent bead. It is convenient to use calcined borax, which is borax deprived of its water of crystalliza tion by heat in a crucible ; this melts into a bead on the charcoal at once. The solubility of a mineral in borax, with effervescence, or without effervescence and the colour that the mineral communicates to the borax, are the chief distinctive characters obtained by a mineral with that substance. Phoi te of ammonia is also sometimes used as a flux in the same manner as borax, and carbonate of soda ; but both these, especially the latter, have the incon venience of sinking into the charcoal, which borax is free from.
Mention may be made here of a few of the most prominent phenomena, characteristic of different mi neral substances when treated by the blow-pipe. Some minerals are fusible alone, such as garnet and felspar ; this last is rather difficult to fuse. Some are infusible and change colour ; bituminous shale loses its black colour, and becomes white, green, and dark; coloured steatite become white. Some dissolve in borax without effervescence, as agate, quartz, felspar, amiantus, garnet. Some dissolve in borax with ef fervescence ; this is the case with carbonate of lime ; it forma with borax a globule transparent whilst in fusion, but in cooling it, the globule becomes opaque, the lime being no longer held in solution by the bo rax ; in like manner, as the watery solutions of cer tain salts, saturated when hot, deposit a part of the salt on cooling. Some of the metals communicate peculiar colours to borax. Copper, in certain pro portions, and at a certain degree of oxidation, gives a brown colour to borax when heated by the blow-pipe. Cobalt gives a deep blue tinge. Manganese a violent colour. Iron tinges borax brown, and if in greater quantity black. These colours are produced by the metals in a state of oxide. The smell emitted by some minerals, when heated by the blow-pipe, is another character serving to distinguish them. That of minerals containing sulphur, is the peculiar suffocating smell of sulphureous gas ; minerals that contain arsenic, emit, when heated, a smell like that of garlic. The nature of some minerals is recognised by the parti cular form of crystallization which they assume in cooling ; this is the case with phosphate of lead, which, after being fused, cools on the charcoal into • an opaque white spheroidal polyhedron. Some ores are reduced to a metallic globule, with great ease, on the charcoal ; thus the native sulphuret of lead, called Galtena, being heated by the blow-pipe, the sulphur is driven off, and the lead remains in its me tallic state. A small particle of silver may be melt ed by the blow-pipe, likewise gold, copper, and, Bergman says, cast-iron. Metallic zinc, when ex- .
posed to the flame of the blow-pipe on the charcoal, melts and burns with a bluish green flame, and be comes covered with oxide, which flies off and floats in the air in light white flocks. Metallic antimony becomes red hot, and melts on the charcoal ; and if the operator ceases to blow, a white fume rises, and oxide of antimony forms upon the globule, in whitish crystalline spicuhe : if the globule, in a state of fusion, be thrown upon a brick floor, it runs along for a con siderable way, rebounding several times, and leaving a trace of white oxide of antimony.
Some substances communicate colour to the flame of the blow-pipe. Muriate of copper, whose crystals are green, communicate a vivid green to the flame ; sulphate and nitrate of copper, whose cry stals are blue, likewise impart a green colour to the flame when they are exposed to its action. Some of the salts of strontian give a purple tinge to the flame.
What precedes relates to the blow-pipe worked by the breath. When it is required to continue the use of the blow-pipe, so long as would be fatiguing if the breath merely were employed, the glass-blow era' table, fig. 9. Plate XXXIV.is used. It consists of a double bellows, so fixed as to be worked by the foot, and to impel a current of air through a tin blow-pipe, , against the flame of a lamp fixed on the table. For the sake of durability, the blow-pipe is sometimes of brass, on which is screwed a nozle of platina. The blow-pipe may have a stop-cock, as in fig. 9. serving to regulate the blast. The lamp has a cot ton wick of nearly an inch in thickness ; the wick is kept together by a tin wick-holder, whicg is sol dered to the lamp ; melted tallow fills the lamp, and feeds the wick with fuel. In order to get rid of the smoke, which is in considerable quantity, there may be placed, at a convenient distance above the flame, a tin funnel ending in a tube, which conveys the smoke out of the room. A convenient method of carrying away the smoke from the glass-blowers' lamp, is represented in fig. 18. It consists of a coyer of thin sheet copper, which is placed on the table, covering the lamp and nozle. The fore-part of this cover is open, so as to allow the jet of flame to pass freely. From the upper part of the .cover two tubes go upwards for the exit of the smoke ; betweep these two tubes the glass-blower has a view of the object he is at work upon, whilst his eyes are screen ed from the light of the flame. The two tubes join above in one short tube. Over the open end of this short tube, at a small distance above, is a tube sus pended from the ceiling by wires, which conveys the smoke into the chimney -of the room. By a handle attached to the cover, the cover with its tubes is removed when it is necessary to trim the wick. The flame of gas from pit-coal may be used instead of a lamp, with a bellows of this kind.