alt="FIG 6"/>This lamp is generally resorted to by blowpipe analysts, for the purpose of experiments in glass apparatus, as the oily combustibles will coat the glass with soot. Some substances, when exposed to the dark part of the flame, become reduced and, in statu nascendi, evaporated; but by passing through the external part of the flame, they become oxidized again, and impart a color to the flame. The spirit flame is the most efficient one for the examination of substances the nature of which we wish to ascertain through color imparted to the flame, as that of the spirit-lamp being colorless, is, consequently, most easily and thoroughly recognized by the slightest tinge imparted to it.
It is necessary that in operating with such minute quantities of substances as are used in blowpipe analysis, that they should have some appropriate support. In order that no false results may ensue, it is necessary that the supports should be of such a nature that they will not form a chemical combination with the substance while it is exposed to fusion or ignition. Appropriate supports for the different blowpipe experiments are charcoal, platinum instruments, and glass tubes.
(a.) Charcoal.—The value of charcoal as a support may be stated as follows:
1. The charcoal is infusible, and being a poor conductor of heat, a substance can be exposed to a higher degree of heat upon it than upon any other substance.
2. It is very porous, and therefore allows easily fusible substances (such as alkalies and fluxes) to pass into it, while other substances less fusible, such as metals, to remain unabsorbed.
3. It has likewise a great reducing power.
The best kind of charcoal is that of pinewood, linden, willow, or alderwood, or any other soft wood. Coal from the firwood sparkles too freely, while that of the hard woods contains too much iron in its ashes. Smooth pieces, free from bark and knots, should be selected. It should be thoroughly burnt, and the annual rings or growths should be as close together as possible.
If the charcoal is in masses, it should be sawed into pieces about six inches in length by about two inches broad, but so that the year-growths run perpendicular to the broadest side, as the other sides, by their unequal structure, burn unevenly.
That the substance under examination may not be carried off by the blast, small conical concavities should be cut in the broad side of the charcoal, between the year-growths, with a conical tube of tin plate about two or three inches long, and one quarter of an inch at one end, and half an inch at the other. These edges are made sharp with a file. The widest end of this charcoal borer is used for the purpose of making cavities for cupellation.
In places where the proper kind of charcoal is difficult to procure, it is economical to cut common charcoal into pieces about an inch broad, and the third of an inch thick. In each of these little pieces small cavities should be cut with the small end of the borer. When these pieces of charcoal are required for use, they must be fastened to a narrow slip of tin plate, one end of which is bent into the form of a hook, under which the plate of charcoal is pushed.
In general, we use the charcoal support where we wish to reduce metallic oxides, to prevent oxidation, or to test the fusibility of a substance. There is another point to which we would direct the student. Those metals which are volatile in the reduction flame, appear as oxides in the oxidation flame. These oxides make sublimates upon the charcoal close in the vicinity of the substance, or where it rested, and by their peculiar color indicate pretty correctly the species of minerals experimented upon.
(b.) Platinum Supports.—The metal platinum is infusible in the blowpipe flame, and is such a poor conductor of heat that a strip of it may be held close to that portion of it which is red hot without the least inconvenience to the fingers. It is necessary that the student should be cognizant of those substances which would not be appropriate to experiment upon if placed on platinum. Metals should not be treated upon platinum apparatus, nor should the easily reducible oxides, sulphides, nor chlorides, as these substances will combine with the platinum, and thus render it unfit for further use in analysis.
(c.) Platinum Wire.—As the color of the flame cannot be well discerned when the substance is supported upon charcoal, in consequence of the latter furnishing false colors, by its own reflection, to the substances under examination, we use platinum wire for that purpose, when we wish to examine those substances which give indications by the peculiar color which they impart to fluxes. The wire should be about as thick as No. 16 or 18 wire, or about 0.4 millimetre, and cut into pieces about from two and a half to three inches in length. The end of each piece is crooked. In order that these pieces should remain clear of dirt, and ready for use, they should be kept in a glass of water. To use them, we dip the wetted hooked end into the powdered flux (borax or microcosmic salt) some of which will adhere, when we fuse it in the flame of the blowpipe to a bead. This bead hanging in the hook, must be clear and colorless. Should there not adhere a sufficient quantity of the flux in the first trial to form a bead sufficiently large, the hook must be dipped a second time in the flux and again submitted to the blowpipe flame. To fix the substance to be examined to the bead, it is necessary, while the latter is hot, to dip it in the powdered substance. If the hook is cold, we moisten the powder a little, and then dip the hook into it, and then expose it to the oxidation flame, by keeping it exposed to a regular blast until the substance and the flux are fused together, and no further alteration is produced by the flame.
The platinum wire can be used except where reduction to the metallic state is required. Every reduction and oxidation experiment, if the results are to be known by the color of the fluxes, should be effected upon platinum wire. At the termination of the experiment or investigation, if it be one, to, clean the wire, place it in water, which will dissolve the bead.
(d.) Platinum Foil.—For the heating or fusing of a substance, whereby its reduction would be avoided, we use platinum foil as a support. This foil should be of the thickness of good writing paper, and from two and a half to three inches long, by about half an inch broad, and as even and smooth as possible. If it should become injured by long use, cut the injured end off, and if it should prove too short to be held with the fingers, a pair of forceps may be used to grasp it, or it may be placed on a piece of charcoal.
(e.) Platinum Spoon.—When we require to fuse substances with the acid sulphate of potash, or to oxidize them by detonation with nitrate of potash, whereby we wish to preserve the oxide produced, we generally use a little spoon of platinum, about from nine to fifteen millimetres1 in diameter, and shaped as represented in Fig. 7. The handle of this spoon is likewise of platinum, and should fit into a piece of cork, or be held with the forceps.
(f.) Platinum Forceps or Tongs.—We frequently are necessitated to examine small splinters of metals or minerals directly in the blowpipe flame. These pieces of metallic substances are held with the forceps or tongs represented as in Fig. 8, where ac is formed of steel, and aa are platinum bars inserted between the steel plates. At bb are knobs which by pressure so separate the platinum bars aa, that any small substance can be inserted between them.
(g.) Iron Spoons.—For a preliminary examination iron spoons are desirable. They may be made of sheet iron, about one-third of an inch in diameter, and are very useful in many examinations where the use of platinum would not be desirable.
(h.) Glass Tubes.—For the separation and recognition of volatile substances before the blowpipe flame, we use glass tubes. These should be about one-eighth of an inch in diameter, and cut into pieces about five or six inches in length. These tubes should have both ends open.
The French millimetre is about the twenty-fifth part of an English inch.
