BLOCKADE. See MILITARY TACTICS.
in chemistry, mineralogy, and the arts, is an extremely useful instrument, employed to raise an intense heat by the flame of a lamp or candle. It operates by throwing a rapid current of air through the flame, and by this ::scans urging it violently against the object to be heated, which must necessarily be of small size. The blow-pipe is capable of throwing such a heat on a small object as would be difficult to obtain for a larger quantity of the same substance in the most powerful furnaces, and with this advan tage, that the process is all the time under the in spection of the operator, whereas he can only con jecture what passes in the centre of a furnace. The stream of air for blow-pipes is usually raised by a klast from the mouth ; in some instances by the va pour of boiling alcohol ; and in others from bellows, 1 or other pneumatic machines. We shall begin with ' blow-pipes of the first kind, as being the most simple and convenient. The common blow-pipe in use among artificers, consists of a conical metal tube, re gularly tapering from the size convenient to be held. in the mouth to the size of a small pin : The srr. 11 end is bent with a regular curvature, so as to be nearly at right angles to the main tube. This pipe being held in the mouth, and a. regular stream of air. discharged through it into the flame of a candle, the flame is projected sidewise into a long conical spi racle of fire, which is of a. blue colour at its root, or. the part where it joins the flame ; farther on it is of a yellow cast, growing more and more faint towards.
the extreme point. The object to be heated is held so that the flame strikes upon it ; or, if it is large, it should be placed Lion a piece of charcoal, which re verberates the flame forcibly on all sides of the ob ject, and at the same timeimaintains the heat by its own combustion. instrument is very ef fective in the hands of a dextrous operator ; but the principal objection to it is, that, after using it a few minutes, the moisture of the air blown from the lungs is condensed, and accumulates in the tube till a drop is formed, which, by means of the current of air, is thrown so forcibly through the flame upon the ob ject to be heated, as to cool it, and spoil the expe riment. To remedy this defect, and render the in
strument more convenient, many different forms have been proposed.
In consequence of an application to Mr Accum of Compton Street, London, that ingenious chemist has favoured us with a sight of all the blow-pipes which he keeps for sale, and we have gladly availed our selves of his permission to represent several of them in Plate LXII.
A very common blow-pipe, for chemical and mine ralogical experiments, known by the name of Cron stad's Blow-pipe, is represented in Fig 1.. It is the same as the common blowpipe, above mentioned, ex cept in having a globular ball at A. This unscrews in its largest diameter, to remove the moisture which may collect in it. The small pipe a, passing away from the ball, enters into it, and projects nearly into the centre, as is shewn by the dotted lines, by which means the globe will hold more water than can ever be collected in it in the course of one experiment, without any danger of getting away at a. The mouth piece B is made of ivory, and the rest of the pipe of brass ; the nose b, or aperture through which the air issues, may be'removed to screw on others of different sized holes. The pipe has generally three of these sizes ; the smallest but just large enough to admit a bristle, and the largest only the size of a small pin.
The blow-pipe represented hi Fig. 2. is attributed to Dr Black. It is simply a conical tin tube, of a convenient size to be held in the mouth at a, and en larged to an inch diameter at the other end. The jet 6, fixed to it at one side, is .a short pipe soldered into thd tube, and projecting inwards almost to its centre on the outer end. It is made conical, and a small jet b is stuck upon it. If well fitted, the fric tion will be quite. sufficient to fasten it ; and any number of jets may be adapted to fit on. This is a very good kind of blow-pipe, as the large internal surface of the conical tube effectually condenses the vapour of the breath, and affords a lodgment for it in the bottom of the tube. When it collects into a quantity so as to be troublesome, it may be poured. out at the end a.