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minutes to a quarter of an hour, without any other inconvenience than the mere lassitude of the lips, caused by compressing the mouthpiece of the instrument.

After having conquered the difficulty of keeping up a continuous blast, the student must learn how to attain the maximum of heat with the least exertion to himself. The chief points to be observed, are neither to blow too fiercely or too gently; in the first case, the force of the blast would carry away heat by the quantity of cold air thrown into the flame, and in the second, a sufficient amount of heat would not be obtained, because a less amount of air would pass into the flame than that required for perfect combustion.

The highest degree of temperature is required in testing the fusibility of many bodies, as also in the reduction of certain oxides, as those of iron, tin, &c. We have yet another class of phenomena to describe, which do not essentially depend on a high temperature; these are the processes of reduction and oxidation. In order to explain and point out the best methods of effecting these two objects, it will be necessary to enter somewhat into the nature of flame; this will be done as briefly as is consistent with perspicuity. The species of flame examined will be that of a candle, as it is with a similar one to that the blow-pipe operator will have to experiment.

On careful examination, it will be found that the flame of a candle or lamp may be divided into four distinct portions; firstly, a deep blue ring at the base; this consists of the vapour of the combustible, which can hardly burn because it has not acquired a sufficient temperature; secondly, a dark cone in the centre; this

is also the vapour, but heated intensely, not, however, in a state of combustion, on account of the absence of air; thirdly, of a very brilliant envelope, which surrounds the dark part just mentioned, this is the partially consumed vapour at a very high temperature; the luminous property it possesses is due to the precipitation and subsequent ignition of particles of solid carbon; and fourthly, of an almost invisible envelope which surrounds the luminous portion; this is the substance of the combustible in full ignition, it here mingles with the atmospheric oxygen, and is consumed. The highest degree of temperature in the whole flame is to be found at the point of contact between the luminous and this part. It must be particularly borne in mind, that the inner portions of the flame have an excess of carbonaceous matters, and the outer an excess of oxygenated matters.

Having premised thus much, we will examine the nature of the flame of a candle when acted on by the blow-pipe blast, and ascertain how far it is altered, and what are the properties of its separate parts in relation to their oxidating and reducing powers. Supposing the lighted lamp or candle be ready and neatly snuffed, place the nozzle of the blow-pipe just in the edge of the flame, and about the sixteenth of an inch above the level of the wick; when things are in this state, blow gently and evenly through the blow-pipe, and a conical jet or dart of flame will be produced, which when formed in a steady atmosphere, free from accidental draughts and currents, will be found to consist of two essential parts, the inner cone, blue, small, and well defined, the outer, brownish and vague. The greatest

intensity of heat is found a little beyond the apex of the blue flame; it is there, also, reduction takes place. It is formed of the combustible matter mixed with air, which, however, does not burn, because it is not sufficiently heated. The outer flame is formed by the complete combustion of the combustible matter of the inner; and it is there and just beyond it, that oxidation takes place.

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Oxidation, as before stated, takes place at the extremity of the outer flame, hence it is termed the oxidating flame; in it all the combustible portions are super-saturated with oxygen. In general, the further the substance to be oxidated can be placed from the extremity of the flame, the better the operation proceeds, provided always that the necessary temperature be maintained. Dull redness is the best suited to oxidation. Reduction. In this operation the jet of the blowpipe must be introduced into the body of the flame, so as only to produce a small dart, and a jet having a smaller hole than that used for oxidation ought to be employed. By operating thus, a more brilliant flame than the last is produced; it is the result of a less perfect combustion, and therefore contains a large amount of carbonaceous matter, fitting it more especially for the purpose of separating oxygen from all metallic bodies.

Berzelius says, "the most important point in blowpipe assays is the power of producing oxidation and reduction at will." Oxidation is so easy, that to do it requires only to read a description of it; but reduction requires some practice and a certain knowledge of producing various kinds of blasts.

One

of the best methods of exercise in this operation is to take a small grain of tin and place it on charcoal, then direct the blow-pipe dart upon it, it will soon fuse, and if the operator has not produced a good reducing flame, it will become covered with a crust of oxide, so that it becomes a witness against him each time this happens. The nature of the flame must be altered until, by observation, the proper kind is produced at will. The longer the button of tin is kept bright, the better and more expert the operator.

AUXILIARY APPARATUS, &c. Supports.-The support is the substance destined to hold the material to be assayed whilst under the influence of heat. From this it will be seen that a solid body must necessarily be employed; it ought also to be exceedingly refractory, so as not to give way under the excessive heat; and lastly, ought to have no chemical action on the substances placed in contact with it. Supports may be divided into combustible and incombustible; the former is charcoal, and the latter, metal, glass, and earthenware, and in some cases, certain minerals have been employed.

Well burnt wood charcoal is the best support in most cases. Berzelius remarks that light woods make the best charcoal for blow-pipe purposes, and recommends that made from the willow. Alder forms an excellent charcoal for blow-pipe experiments.

Hard woods generally give so much ash as to render them unfit, on account of the chemical action of its contents. It is generally very ferruginous. Gahn

supposed that box-wood charcoal would be best; but on trial it was found to crack very much.

It is almost needless to observe that the charcoal must be well made, because that which scintillates, smokes, or burns with flame, is worse than useless. It ought to be cut with a saw into conveniently sized pieces, and a small hole bored in them, so as to receive the substance to be assayed. I have used a very convenient charcoal support, contrived by Griffin, of Glasgow. The following is his description* of its manufacture and properties, the latter of which I most gladly corroborate: "Several of the most important experiments performed with the blowpipe require the assistance of charcoal, upon which the object submitted to examination is supported in the flame. The charcoal employed for this purpose should be of soft wood, well burnt, compact, and free from crevices. Such charcoal is difficult to obtain. I have several times examined a sackful of charcoal without finding above half-a-dozen sticks adapted for these experiments. This circumstance induced me to seek for a substitute, and having found one which seems likely to prove serviceable, I think it possible that other persons accustomed to operate with a blow-pipe, and accustomed also to feel the want of appropriate charcoal, may be willing to learn in what manner they can efficiently replace it. For this reason I have drawn up the following notice:

"The blow-pipe experiments that require the as

* Proceedings of the Philosophical Society of Glasgow, April, 26, 1843.

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