In order to use the blow-pipe, the breath impelled through it is to be directed across the flame of a lamp or candle, applying the orifice from which the air issues a little above the upper end of the wick ; a jet of flame is thus formed, as represented at fig. 8. This jet is made to fall on the body to be heated. The operation may be continued for a considerable length of time ; an uninterrupted blast is kept up by the muscular action of the cheeks, whilst the ordi nary respiration goes on through the nose : a little practice is sufficient to enable the operator to suc ceed. The jet of flame is conoidal, internally blue, and externally-yellow, by more or less immersion in this jet of flame. The subject of operation receives a greater or less degree of heat, and becomes oxidated in a greater or less degree. If a bead of borax, con , taining oxide of manganese, be kept fused for some time in the inner flame, the bead becomes colourless ; when it is afterwards kept fused in the outer flame, the manganese acquires more oxygen, and the bead becomes of a violet colour. This violet colour may be made to appear more speedily, by adding a particle of nitre.
The first who applied the blow-pipe to the analysis 'of minerals was Swab, Counsellor of the College of Mines in Sweden, in 1738. Its application to the science of Mineralogy was afterwards farther im proved by Cronsted, Rinman, Gahn, Scheele, and Bergman, and by other men of science since their time.
The blowpipe is useful to the mineralogist and chemist, as alfording a ready method of knowing what the component parts of bodies are. Trials with the blow-pipe are generally made by the che mist in order to know the nature of the consti tuent parts, before he proceeds to the other steps of dry or humid analysis, which are requisite for as certaining the quantities of the constituent pans. Then recourse is had to other means than the blow pipe ; for, in order to come at a knowledge of the proportions of the constituent parts, it is necessary that the quantity of each constituent part be large enough to be weighed in a balance, and, for this purpose, the quantity of the substance employed must be larger than what can be managed with the blow-pipe.
In experimental mineralogy, with the blow-pipe, the small fragment of the body, subjected to trial, should not exceed the size ,of half a peppercorn ; if larger, it cannot be sufficiently heated. It is placed in a lenticular cavity, made with a knife, in a piece of well burnt charcoal of wood, free from cracks, and not too porous, and of the length of four or five inches, so as to be held-conveniently in the left hand. Someblow-pipes have been made with a stand, to which they are connected by a ball and socket joint; the stand is fixed to the table by a clamp ; this con struction leaves the right hand at liberty. In re ducing fragments of metallic ores by the blow pipe, charcoal should be used as a support, as the charcoal attracts the oxygen from the metal lic oxide, and reduces it to a metallic form ; and when thus reduced, the metal may be kept fused on the charcoal, which prevents or retards its again at tracting oxygen. The charcoal support has like
wise the advantage of increasing the heat by its in candescence. For both these reasons, to prevent oxidation, and to increase and reverberate the heat proceeding from the jet of flame, the goldsmith who solders his small work by the blow-pipe, attaches his work to a piece of charcoal, by means of wires, in the process of soldering.
When it is required that the fragment of a mine ral should be heated, without tke contact of char coal, the fragment is exposed to the flame in a small spoon of platina, with a wooden handle; the cavity of the spoon is a hemisphere of three-tenths of an inch in diameter; or on a thin lamina of platina two or three inches long, and half an inch broad, and of the thickness of common writing paper ; or it is held by a forceps, three inches long, made of thin platina. Bergman, who published his treatise on the blow-pipe in 1780, before the working of plating had come in to use, employed a small gold spoon, as metal has the quality of remaining pure and unoontami rutted, whilst in contact with many of the chemical agents; but platina is preferable, for, besides possess ing the quality of resisting the action of many che mical agents, it-has likewise the advantage of diffi cult fusibility. It has now, for a good many years, been wrought into various instruments both in London and Paris ; when wrought, it is sold at the price of about a guinea the ounce, which is one quarter the price of gold, Some platina workers, as Jeanette of Paris, who was one of the first, form the crude granular platina into masses, by melting it with arsenic, and subsequent heating and forging ; others dissolve the crude platina in nitro•muriatic acid, and reduce the nitro-muriate of plating to a metallic state by heat. Platina, however, although infusible alone by the heat of the common blow-pipe, will be dissolved and melted, if heated along with some of the metals. Platina supports, therefore, should not be used where they are liable to be in contact with a fused metal. These effects are notable in the case of tin ; when tin is melted in contact with a vessel of platina, the tin enters into a combination with the platina, corrodes and renders it brittle, so that pieces of the platina vessel come off on the ap plication of a small force, and the vessel is thus ren dered useless. Platina vessels also become unser . vieeable by frequent and continued exposure to great heat. Platina crucibles that are much used be come brittle, and crack at the edges; and care should be taken to cool these vessels gradually, that they may last as long as possible. A platina vessel, in which sulphuric acid was boiled, for a Wag time, at last became perforated and unserviceable.