The “platinum foil” is employed as a support during fusions; pieces about an inch and a half long, by half an inch in width, are generally used. A small platinum spoon is sometimes adopted when fusing substances with acid, potassium, sulphate, or nitre.
Minerals may be tested to see whether, in the ordinary blowpipe flame, they are fusible or not. To do this, a fragment of the substance to be tested is held in the flame by means of the “platinum-pointed forceps.” If the mineral is found to be fusible, then its “degree of fusibility” may be ascertained according to the following table. The “degrees of fusibility” are six in number:—
1. Fusible in ordinary gasflame, even in large fragments. Example: Stibnite, or grey antimony.
2. Fusible in fine, thin pieces, in the ordinary gasflame, and in larger fragments in the blowpipe-flame. Example: Natrolite, a hydrous silicate of alumina and soda.
3. If very thin splinters be used, fusible without difficulty with the blowpipe-flame. Example: Almandite, or iron-alumina-garnet.
4. In thin splinters fusible to a globule. Example: Actinolite, a non-aluminous variety of hornblende.
5. Thin edges may be fused and rounded without great difficulty. Example: Orthoclase felspar—already described.
6. Fusible with great difficulty on the finest edges. Example: Bronzite, one of the augite group of minerals.
Now, it is highly probable that many of our readers will not understand, or be able to recognise the six minerals above enumerated; and we recommend those who may be sufficiently interested, to purchase them from a mineral dealer—such as Damon, of Weymouth, or Russell, or Gregory, or Henson, or Butler, in London. A set, comprising the six, should cost from two to three shillings. With these, as a standard for comparison, the operator readily grasps the method of assigning a fusible mineral to its proper degree in the scale.
Another object of examination in the forceps is to see what colour (if any) is imparted to the flame by the divers minerals experimented upon. It is a good rule not to permit the specimen, when being fused, to touch the forceps in the neighbourhood of the actual part fused. For a mineral containing antimony or arsenic would tend to form a fusible alloy with the platinum points, and so ruin the forceps.
The pieces of “charcoal” alluded to in our inventory, are used for placing the mineral substance upon in certain parts of the blowpipe operation, which may be briefly described. Essentially the charcoal forms a support to the substance during fusion; but the glowing carbon has also a kind of reducing effect. Taking a long prism of charcoal, such as that described, page 63 ante, the mineral to be dealt with should be placed near one end of a flat surface and the prism so held that the flame from the blowpipe, will sweep down its full length. The object of so doing is to give a chance to any volatile substance (derived by the operation from the mineral) to deposit on the comparatively cool surface, which deposit is often indicative of the chemical nature of the mineral. To carry this point home, the following experiments may be conducted by the student. Taking a piece of stibnite (sulphide of antimony), which, as we have just learnt, is a most fusible mineral, we place it on the charcoal in the manner indicated. Whilst melting, and the blowpipe flame be continued to be directed upon it after it has become fused, it will be noticed that a yellowish-white deposit is taking place on the length of charcoal; this is called a sublimate.
Mineral substances may also be assisted in fusing on the charcoal by using the reagents described in our list of chemicals, &c., included in a blowpipe set.
In regard to the use of the “glass tubes,” it may be remarked that they are used principally for the examination of minerals which yield a volatile substance on being heated therein, and to detect the presence of water and the like. It is important to make a distinction between the closed and the open tubes. When a mineral fragment is placed in a tube, closed at one end, whatever takes place will be in presence of very little air, or oxygen; on the other hand, when the tube is open at both ends, and is inclined during the experiment, a constant stream of oxygen passes through the tube, and the mineral is being dealt with in presence of that. The employment of this oxygen makes a great deal of difference in the results obtained, as a few elementary experiments will show. If we place a piece of sulphur in a tube, closed at one end, and heat it gently, we notice that a yellow coating takes place inside the tube; but if we now employ a tube open at both ends and heat it very slowly indeed, we notice that the sulphur goes off as an invisible gas, and if the experiment has been properly conducted, there should hardly be a trace of the sulphur left on the glass. A number of experiments of a similar nature might be quoted, but enough has been said for the present to show the utility of the tubes.
The “chemical reagents” alluded to have already been sufficiently described to render any further discussion on them unnecessary for our immediate purpose.
In regard to the “miscellaneous articles” mentioned, it may be remarked that the test papers are employed in the detection of certain acids and bases; whilst a strip of brazil-wood paper is for the detection of fluorine. The hammer and anvil are for breaking the substance to be tested into small fragments; the magnet for withdrawing particles of iron from the pulverised material; the three-cornered file for assisting in determining the relative hardness of minerals, &c., &c.