310 



♦ KNO^^^LEDGE ♦ 



[Oct. 9, 1885. 



base that will prove as useful as any. It consists simply 

 of a piece of board, say tbree-quarters of an inch thick and 

 one to two inches larger than the instrument. The board 

 may be square or round, and in it cut, or have cut by some 

 friend in possession of a lathe, a circular groove a quarter 

 Cif an inch deep and about the same width. The groove 

 should be of such a diameter that the glass may stand in 

 it without touching either side of it. iLaving placed the 

 glass in position, pour into the groove some liquefied 

 paraflan wax until the level of the board is reached. 

 This is simply to lightly cement the wood and glass 

 together to permit of the instrument being carried about, 

 and to exclude moisture, etc. Feet may be provided by 



'^ 



screwing pieces of wood of equal thickness on to the 

 under side of the piece of board. The thickness need 

 only be sufficient to allow the fingers to pass underneath 

 the base for lifting purposes. 



For my own part I rarely use a base, but, before com- 

 mencing to work my electroscope, lay it carefully on its 

 side before the fire, to dry the glass and expel as much 

 of the moist air as iiossible. Something of the kind 

 should be done before fixing the glass on to the base, or 

 the instrument is likely to prove of little use on account 

 of the confined moisture. 



Ex. ex. — Another plan, which I have frequently and 

 satisfactorily resorted to, is to provide the stopper (D, 

 Fig. 61) with two holes, as shown in section in Fig. 65, 

 where A B C D is the stopper and W W the brass rod or 

 wire. The second hole is bored parallel to it and of such 

 a width, say a quarter of an inch, as to allow a piece of 

 glass tubing, G G', to fit tightly. The tubing, it will be 

 seen, is sealed at one end. The object is to enable one to 

 drive out the damp air by gently warming the instrament 

 when in the ordinary vertical jjosition, the piece of tubing 

 being withdrawn to permit the escape. When the warm- 

 ing is finished the tubing may be restored, and a further 

 supply of moist air, which would otherwise find its way 

 in, excluded. Where the cemented base is used the piece 

 of tubing may be also cemented in (after the drying 

 operation) by means of shellac varnish, &c., as it will 

 rarely require to be withdrawn. 



Ex. CXI. — To seal up the end of a piece of glass- 

 tubing is a matter of little diflSculty. All that is neces- 

 sary is to hold a piece of ordinary glass-tubing over a gas 



Fig. 6C 

 or spirit flame, placing one hand 



, J „^ uuc uituu at A (Fig. 66) and the 



~^^.. at B, and revolving the glass gently in the flame, 

 'hich we will suppose to be a't or under C. When the | 



flame turns decidedly yellow 

 regarded as having been mor 

 then be withdrawn from the 

 both hands. The diamater of 

 be diminished, and may be 

 As we at present only reqi 

 pulling should be done as quickly 

 required to produce a long tapei 



uldur, the glass maybe 

 l.ss snftrmMl. It should 

 r .iimI |iiilK(l apart with 

 lulii' « ill in consequence 

 e as .small as we wish, 

 a short, taper point, the 

 possible. If it were 

 vtvy fine hair-like 



tube, the pulling should be gradual, but equably n 

 tained. A great deal depends upon the shape of the 

 flame employed. When a short taper is required, only a 

 narrow flame, such as that from a small spirit-lamp, 

 should be used. But for most purposes, such as bending, 

 long tapers, &c., a good batswing gas flame answers as 

 well as anything, the glass being placed in the plane of 

 the flame, that is, so that the greatest possible length of 

 glass is heated. The reasons for this are too apparent to 

 need detailing. It should be borne in mind that the 

 glass should be thoroughly dried before being placed in 

 the flame, as the smallest drop of water falling on the 

 heated part is sufiicient to fracture it. The glass should 

 also be kept revolving during the whole of the time it is 

 over the flame, or it will be unequably heated, and will, 

 therefore, only extend badly, possibly it will break. 



Condensing Electroscopes. 

 Ex. CXII. Fig. 67 illustrates a useful addition which 

 may be applied to our electroscope. It is known as a 

 " condensing " plate, and consists primarily of a piece of 

 sheet metal provided with an insulating handle, and 

 having its under face coated more or less perfectly with 

 an insulating material. The plate AB may be of tinned 



iron or any other metal, but brass looks best and costs 

 but very little. It should be of the same size as the 

 plate or cover of the electroscojie (A, Fig. 61), and may be 

 of the same thickness. In the centre of it a small piece of 

 half-inch brass tubing C D should be soldered (Ex. XI. 

 to XIV). The length of the tubing need not be more 

 than half an inch. Then procure a piece of glass tubing, 

 G G', five or six inches long, and about three-eighths of 

 an inch in diameter ; the end G of the glass should be 

 sealed by revolving it in a good strong flame, 

 holding it in a slightly inclined position, so 

 that the glass will have a tendency to run down. If 

 need be, the sealing may be assisted by using a piece of 

 glass in the other hand and working the end G there- 

 with. When the sealing operation is completed the end 

 G' of the tubing should be placed in the soldered cup C D 

 and cemented in with a little plaster of Paris. When 

 the latter is dry the handle G G' should be coated with a 

 layer of shellac varnish, whereby a good insulating handle 

 is insured. If the sealing cannot be accomplished by 



