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On various Uses of the Septaria.
The septaria, ludus Helmontii, or loam-stone, is usually found accompanying the London clay, in nodules of different forms and sizes, and is of a pale yellow colour. It is procured on the banks of the Thames at low water, in the island of Shepey, and in various other places; but the best, I believe, is found on the estate of Lord Mulgrave, near Whitby, and at a distance from the sea shore, where it has not been impregnated with the salt water. This is employed for Roman cement, after being calcined to an olive-brown colour, and ground to powder: it does not effloresce, as is too commonly the case with such as has been obtained from the sea shore; and if it be mixed up for use with water containing a dilute solution of sulphate of iron, it will become very considerably harder, and more durable. This, I believe, is nearly the only use it has hitherto been put to in this country; but it is capable of being employed in a variety of other ways, with very considerable advantage; and, particularly in fine casting, of gold, silver, or molu, &c. where it forms a most excellent facing, after being slightly calcined till it is of a red colour and then finely powdered; in this state it lines the moulds when it is sifted over them, and the impression of the patterns or models is again taken upon it with an exceedingly smooth coating: it never burns through by the heat of the melted metal, and gives an extraordinary degree of sharpness to the casts. Indeed, I have no doubt that by the use of it minute casts in iron may be made from cameos, &c. equally perfect with those lately brought into this country from Germany, and which are so deservedly admired. It will also be found useful in taking impressions of gems in glass, in the manner of Tassie, when used instead of Tripoli; which earth it also very much resembles in its property of polishing gold, silver, ivory, varnished wares, &c.; after being calcined to an olive-brown colour and levigated, and then either finely sifted, or, which is better, washed over, letting it remain suspended in the water for half a minute; when it will be found to give a beautiful black lustre to articles of gold or silver, by rubbing them with a little of it, applied upon the palm of the hand, or the smooth side of leather; and indeed it forms an excellent plate powder for domestic use.
These are only a few of the many uses the septaria has been put to; but enough, I should presume, are pointed out, to call the attention of your readers to this valuable production, and to its more extensive employment in the arts of this country.
On the Blow-pipe: from a Treatise on the Blow-pipe by Assessor Gahn, of Fahlun.
THE substance to be submitted to the action of the blow-pipe must be placed on a piece of charcoal, or in a small spoon of platina, gold, or silver; or, according to Saussure, a plate of cyanite may sometimes be used. Charcoal from the pine is to be preferred, which should be well ignited and dried, that it may not crack. The sides, and not the ends, of the fibres must be used; otherwise the substance to be fused spreads about, and a round bead will not be formed. A small hole is to be made in the charcoal, which is best done by a slip of plate iron bent longitudinally. Into this hole the substance to be examined must be put in very small quantity; if a very intense heat is to be used, it should not exceed the size of half a peppercorn.
The metallic spoons are used when the substance to be examined is intended to be exposed to the action of heat only, and might undergo some change by immediate contact with the charcoal. When the spoon is used, the flame of the blow-pipe should be directed to that part of it which contains the substance under examination, and not be immediately applied to the substance itself. The handle of the spoon may be inserted into a piece of charcoal; and if a very intense heat is required, the bowl of the spoon may be adapted to a hole in the charcoal. Small portions may be taken up by platina forceps. Salts and volatile substances are to be heated in a glass tube closed at one end, and enlarged according to circumstances, so as to form a small matrass.
When the alteration which the substance undergoes by the mere action of heat has been observed, it will be necessary to examine what further change takes place when it is melted with various fluxes, and how far it is capable of reduction to the metallic state."
These fluxes arę,
1. Microcosmic salt; a compound of phosphoric acid, soda, and ammonia.
2. Subcarbonate of soda, which must be free from all impurity, and especially from sulphuric acid, as this will be decomposed, and sulphuret of soda will be formed, which will dissolve the metals we wish to reduce, and produce a bead of coloured glass with substances that would otherwise give a colourless one. 3. Borax, which should be first freed from its water of crystallization.
These are kept powdered in small flasks; and when used a sufficient quantity may be taken up by the moistened point of a
knife: the moisture causes the particles to cohere, and prevents their being blown away when placed on the charcoal. The flux must then be melted to a clear bead, and the substance to be examined placed upon it. It is then to be submitted to the action, first of the exterior, and afterwards of the interior flame, and the following circumstances to be carefully observed;
1. Whether the substance is dissolved; and, if so,
2. Whether with or without effervescence, which would be occasioned by the liberation of carbonic acid, sulphurous acid, oxygen, gaseous oxide of carbon, &c.
3. The transparency and colour of the glass while cooling. 4. The same circumstances after cooling.
5. The nature of the glass formed by the exterior flame, and 6. By the interior flame.
7. The various relations to each of the fluxes.
It must be observed that soda will not form a bead on charcoal, but with a certain degree of heat will be absorbed. When, therefore, a substance is to be fused with soda, this flux must be added in very small quantities, and a very moderate heat used at first, by which means a combination will take place, and the soda will not be absorbed. If too large a quantity of soda has been added at first, and it has consequently been absorbed, a more intense heat will cause it to return to the surface of the charcoal, and it will then enter into combination.
Some minerals combine readily with only very small portions of soda, but melt with difficulty if more be added, and are absolutely infusible with a larger quantity: and when the substance has no affinity for this flux, it is absorbed by the charcoal, and no combination ensues.
When the mineral or the soda contains sulphur or sulphuric acid, the glass acquires a deep yellow colour, which by the light of a lamp appears red, and as if produced by copper.
If the glass bead becomes opaque as it cools, so as to render the colour indistinct, it should be broken, and a part of it mixed with more of the flux, until the colour becomes more pure and distinct. To render the colour more perceptible, the bead may be either compressed before it cools, or drawn out to a thread.
When it is intended to oxidate more highly a metallic oxide contained in a vitrified compound with any of the fluxes, the glass is first heated by a strong flame, and when melted is to be gradually withdrawn from the point of the blue flame. This operation may be repeated several times, permitting the glass sometimes to cool, and using a jet of large aperture with the blow-pipe.
The reduction of metals is effected in the following manner: The glass bead, formed after the manner already pointed out, is to be kept in a state of fusion on the charcoal as long as it remains on the surface, and is not absorbed, that the metallic particles may collect themselves into a globule. It is then to be
fused with an additional quantity of soda, which will be absorbed by the charcoal, and the spot where the absorption has taken place is to be strongly ignited by a tube with a small aperture. By continuing this ignition, the portion of metal which was not previously reduced will now be brought to a metallic state; and the process may be assisted by placing the bead in a smoky flame, so as to cover it with soot that is not easily blown off.
The greatest part of the beads which contain metals are frequently covered with a metallic splendour, which is most easily produced by a gentle, fluttering, smoky flame, when the more intense heat has ceased. With a moderate heat the metallic surface remains; and by a little practice it may generally be known whether the substance under examination contains a metal or not. But it must be observed that the glass of borax sometimes assumes externally a metallic splendour.
When the charcoal is cold, that part impregnated with the fused mass should be taken out with a knife, and ground with distilled water in a crystal, or, what is much better, an agate mortar. The soda will be dissolved; the charcoal will float, and may be poured off; and the metallic particles will remain in the water, and may be examined. In this manner most of the metals may be reduced.
Relations of the Earths and Metallic Oxides before the Blow-pipe.
1. THE EARTHS.
Barytes, when containing water, melts and spreads on the charcoal. Combined with sulphuric acid, it is converted, in the interior flame, into a sulphuret, and is absorbed by the charcoal, with effervescence, which continues as long as it is exposed to the action of the instrument.
Strontites. If combined with carbonic acid, and held in small thin plates with platina forceps in the interior flame, the carbonic acid is driven off; and on the side of the plate farthest from the lamp a red flame is seen sometimes edged with green, and scarcely perceptible but by the flame of a lamp. Sulphate of strontites is reduced in the interior flame to a sulphuret. Dissolve this in a drop of muriatic acid, add a drop of alcohol, and dip a small bit of stick in the solution; it will burn with a fine red flame.
Lime. The carbonate is easily rendered caustic by heat; it evolves heat on being moistened, and is afterwards infusible before the blow-pipe. The sulphate is easily reduced to sulphuret, and possesses, besides, the property of combining with fluor at a moderate heat, forming a clear glass. The fluor should be rather in excess.
Magnesia produces, like the strontites, an intense brightness in the flame of the blow-pipe. A drop of solution of cobalt being added to it, and it being then dried and strongly ignited, a faint
reddish colour like flesh is produced, which, however, is scarcely visible by the light of a lamp. And magnesia may by this process be detected in compound bodies, if they do not contain much metallic matter, or a proportion of alumina exceeding the magnesia. Some inference as to the quantity of the magnesia may be drawn from the intensity of the colour produced.
All these alkaline earths, when pure, are readily fusible in combination with the fluxes into a clear, colourless glass, without effervescence; but on adding a further quantity of the earth, the glass becomes opaque.
Alumina combines more slowly with the fluxes than the preceding earths do, and forms a clear glass, which does not become opaque. But the most striking character of alumina is the bright blue colour it acquires from the addition of a drop of nitrate of cobalt, after having been dried and ignited for some time. And its presence may be detected in this manner in compound minerals where the metallic substances are not in great proportion, or the quantity of magnesia large. Alumina may be thus detected in the agalmatolite.
II. THE METALLIC OXIDES.
Arsenic flies off accompanied by its characteristic smell, resembling garlic. When larger pieces of white arsenic are heated on a piece of ignited charcoal, no smell is perceived. To produce this effect the white oxide must be reduced, by being mixed with powdered charcoal. If arsenic is held in solution, it may be discovered by dipping into the solution a piece of pure and well-burned charcoal, which is afterwards to be dried and ignited.
Chrome. Its green oxide, the form in which it most commonly occurs, and to which it is reduced by heating in the common air, exhibits the following properties: it is fusible with microsmic salt, in the interior flame, into a glass which at the instant of its removal from the flame is of a violet hue, approaching more to the dark blue or red, according to the proportion of chrome. After cooling the glass is bluish green, but less blue than the copper glass. In the exterior flame the colour becomes brighter, and less blue, than the former. With borax it forms a bright yellowish or yellow-red glass in the exterior flame; and in the interior flame this becomes darker and greener, or bluishgreen. The reduction with soda has not been examined.
Molybdic Acid melts by itself upon the charcoal with ebullition, and is absorbed. In a platina spoon it emits white fumes, and is reduced in the interior flame to molybdous acid, which is blue; but in the exterior flame it is again oxidated, and becomes white. With microcosmic salt, in the exterior flame, a small proportion of the acid gives a green glass, which by gradual additions of the acid passes through yellow-green to reddish,