BLOWPIPE 405 



the breath, is useful in long operations, but may generally be dispensed with. In 

 selecting the blowpipe, tho small aperture should be chosen perfectly round and 

 smooth, otherwise it will not command a good flame. 



A common candle, such as the miner employs under ground, answers very well for 

 the flame. 



To support tho subject of assay, or ' the assay,' as it has been happily denomi- 

 nated by Mr. Children, two different materials are requisite, according as we wish to 

 calcine or reduce it For tho latter purpose, nothing is so good as charcoal ; but that 

 from oak is less eligible, both from its inferior combustibility and from its containing 

 iron, than that from alder, willow, or other light woods. 



For calcination, a very convenient support, where platinum wire is difficult to pro- 

 cure, is white-baked pipe-clay or china-clay, selecting such as will not fuse nor become 

 coloured by roasting with borax. 



The supports are conveniently formed by a process of Mr. Tennant. Tho clay 

 is to be beaten to a smooth stiff body ; then a thin cake of it being placed between a 

 fold of writing-paper, it is to be beaten out with a mallet to the thickness of a wafer, 

 and cut, paper and all, into squares of fths inch diameter, or triangles about the same 

 size. These are to be put in the bowl of a tobacco-pipe, and heated gently till dry, 

 then baked till the paper, is burnt away, and the clay left perfectly white. They 

 should bo baked in a clear fire, to keep out coal dust and smoke as much as possible, 

 as either of these adhering to the clay plates would colour the borax in roasting. A 

 small fragment of the bowl of a new tobacco-pipe will serve instead in the absence of 

 a more convenient material. 



A simple pair of forceps (fig. 145), to move and to take up the hot assay, may be made 

 of a slip of stiff tin plate, 8 inches long. -J- inch wide in the middle, and -^th inch at the 

 ends. The tin being rubbed off the points on a rough whetstone, the slip is to be 

 bent until they approach each other within an inch and the two sides are parallel ; 

 thus there will be spring enough in the forceps to open and let go the assay when 

 not compressed by the finger and thumb. 



145 146 



A magnetic needle, very desirable to ascertain the presence of iron, is easily made 

 of the requisite delicacy where a magnet is accessible. A bit of thin steel wire, or a 

 long fine stocking-needle, having inch cut of at the point, is to be heated in tho 

 middle that it may be slightly bent there (fig. 146). While hot, a bit of sealing-wax 

 is to bo attached to the centre, and the point which had been cut off, being heated at 

 the thick end, is to be fixed in the sealing-wax, so that the sharp end may serve as a 

 pivot, descending about ^th inch below the centre, taking care that the ends of the 

 needle fall enough below the pivot, to prevent it overturning. It must be magnetised, 

 by sliding one end of a magnet half a dozen or more times from the centre to one end 

 of the needle, and the other end a similar number of times from the centre of the 

 needle to its other end. A small brass thimble (not capped with iron) will do for the 

 support, the point of the pivot being placed in one of the indentations near the 

 centre of the tap, when, if well balanced, it will turn until it settles north and south. 

 If one side preponderate, it must be nipped until the balance be restored. 



A black gun-flint is also occasionally used to rub the metallic globules (first attached, 

 whilst warm, to a bit of sealing-wax), and ascertain the colour of tho streak which 

 they give. Thus minute particles of gold, copper, silver, &c., are readily discriminated. 

 A little refined borax and carbonate of soda, both in powder, will complete the 

 requisites. 



Having collected these materials, the next object for the operator is to acquire the 

 faculty of keeping up an unintermittent blast through the pipe whilst breathing freely 

 through the nose. 



A very sensitive and, for most purposes, sufficiently delicate balance (fig. 147, p. 

 406) was also devised by Mr. Prideaux, of which tho following is a description. 



The common marsh-reed, growing generally in damp places throughout the king- 

 dom, will yield straight joints, from 8 to 12, or more, inches long; an 8-inch joint 

 will serve, but the longer the better. The joint is to be split down its whole length, 

 so as to form a trough, say inch wide in the middle, narrowed away to Jrd inch at 

 the ends. A narrow slip of writing-paper, the thinner tho better (bank-post is very 



