A System of Instruction in the Practical Use of the Blowpipe | Page 2

a degree of perfection, that we have a right to term its use, "Analysis in the dry way," in contradistinction to analysis "in the wet way." The manipulations are so simple and expeditious, and the results so clear and characteristic, that the Blowpipe analysis not only verifies and completes the results of analysis in the wet way, but it gives in many cases direct evidences of the presence or absence of many substances, which would not be otherwise detected, but through a troublesome and tedious process, involving both prolixity and time; for instance, the detection of manganese in minerals.
Many substances have to go through Blowpipe manipulations before they can be submitted to an analysis in the wet way. The apparatus and reagents employed are compendious and small in number, so that they can be carried easily while on scientific excursions, a considerable advantage for mineralogists and metallurgists.
The principal operations with the Blowpipe may be explained briefly as follows:
(_a._) By Ignition is meant the exposure of a substance to such a degree of heat, that it glows or emits light, or becomes red-hot. Its greatest value is in the separation of a volatile substance from one less volatile, or one which is entirely fixed at the temperature of the flame. In this case we only take cognizance of the latter or fixed substance, although in many instances we make use of ignition for the purpose of changing the conditions of a substance, for example, the sesquioxide of chromium (Cr^{2}O^{3}) in its insoluble modification; and as a preliminary examination for the purpose of ascertaining whether the subject of inquiry be a combination of an organic or inorganic nature.
The apparatus used for this purpose are crucibles of platinum or silver, platinum foil, a platinum spoon, platinum wire or tongs, charcoal, glass tubes, and iron spoons.
(_b._) Sublimation is that process by which we convert a solid substance into vapor by means of a strong heat. These vapors are condensed by refrigeration into the solid form. It may be termed a distillation of a solid substance. Sublimation is of great consequence in the detection of many substances; for instance, arsenic, antimony, mercury, etc.
The apparatus used for the purposes of sublimation consist of glass tubes closed at one end.
(_c._) _Fusion._--Many substances when exposed to a certain degree of heat lose their solid form, and are converted into a liquid. Those substances which do not become converted into the liquid state by heat, are said to be infusible. It is a convenient classification to arrange substances into those which are fusible with difficulty, and those which are easily fusible. Very often we resort to fusion for the purpose of decomposing a substance, or to cause it to enter into other combinations, by which means it is the more readily detected. If insoluble substances are fused with others more fusible (reagents) for the purpose of causing a combination which is soluble in water and acids, the operation is termed unclosing. These substances are particularly the silicates and the sulphates of the alkaline earths. The usual reagents resorted to for this purpose are carbonate of soda (NaO, CO^{2}), carbonate of potash (KO, CO^{2}), or still better, a mixture of the two in equal parts. In some cases we use the hydrate of barytes (BaO, HO) and the bisulphate of potash (KO, 2SO^{3}). The platinum spoon is generally used for this manipulation.
Substances are exposed to fusion for the purpose of getting a new combination which has such distinctive characteristics that we can class it under a certain group; or for the purpose of ascertaining at once what the substance may be. The reagents used for this purpose are borax (NaO, 2BrO^{3}) and the microcosmic salt (NaO, NH^{4}O, PO^{5}, HO). Charcoal and the platinum wire are used as supports for this kind of operation.
(_d._) _Oxidation._--The chemical combination of any substance with oxygen is termed oxidation, and the products are termed oxides. As these oxides have qualities differing from those which are non-oxidized, it therefore frequently becomes necessary to convert substances into oxides; or, if they are such, of a lower degree, to convert them into a higher degree of oxidation. These different states of oxidation frequently present characteristic marks of identity sufficient to enable us to draw conclusions in relation to the substance under examination. For instance, the oxidation of manganese, of arsenic, etc. The conditions necessary for oxidation, are high temperature and the free admission of air to the substance.
If the oxidation is effected through the addition of a substance containing oxygen (for instance, the nitrate or chlorate of potash) and the heating is accompanied by a lively deflagration and crackling noise, it is termed detonation. By this process we frequently effect the oxidation of a substance, and thus we prove the presence or the absence of a certain class