APPENDIX. 



CHAPTER XXXIX. 

 PRACTICAL NOTES ON MANIPULATION. 



206. Manipulation of Glass Tubing. Most laboratories contain 

 a glass-blower's table ; in its absence the mouth gas blowpipe must be 

 used. The difficulty of keeping up a continuous blast of air with this 

 instrument can be readily overcome by practice, provided that the orifice 

 is not too wide. The blowpipe flame (fig. 325) consists of two parts, an 

 inner blue cone (a) which is the deoxidizing or reducing flame, and an 

 outer envelope (6) which surrounds it. The hottest part of the flame 

 is a very little in front of the tip of the blue cone. The reducing flame 

 is so called because the unburnt gasses present in it have at that high 

 temperature a great tendency to take oxygen from any substance con- 

 taining it. In the outer envelope, on the contrary, the supply of oxy- 

 gen is abundant ; it is therefore called the oxidizing flame. Ordinary 

 English glass tubing contains oxide of lead : when it is heated in the 

 reducing flame, black stains of metallic lead form on its surface. To 

 avoid this, it should always be heated in the extremity of the outer flame. 

 German glass is free from lead, and much less fusible than English glass, 

 and is generally preferable to it. Tubes of German glass may be dis- 

 tinguished from English by looking through them lengthwise ; the for- 

 mer has a greenish color, while the latter looks dark. In drawing out 

 a glass tube so as to form a pipette (nee fig. 326), care must be taken to 

 soften the part to be drawn completely and equally, and to remove it 

 from the flame before extending it. If this precaution is neglected, the 

 drawn-out part will collapse and close, \7hen heating a tube for the 

 purpose of bending, it is important to use as low a temperature as is 

 sufficient to soften it, and not to begin to bend until a considerable ex- 

 tent of the part to be bent is equally softened. For this reason, it is best 

 to use a large flame (that from a gas jet being preferable to a Bunsen's 

 lamp or blowpipe), in which the tube must be moved up and down 

 until the object is attained. Before bending, it must of course be re- 

 moved from the flame. In bending a thin tube, especially, if it be 

 heated too strongly, it is difficult to avoid its becoming wrinkled at the 

 bend. To avoid this, it is a good plan to close one end air-tight 

 and blow in gently at the other during flexion. Large tubes are bent 

 more easily by filling them with clean dry sand and heating them over 

 incandescent charcoal, supported on wire netting. To seal a tube, it 

 must be thoroughly softened at a short distance from its end, and drawn 

 put quickly to a thread. The capillary part of a tube already drawn out 

 is sealed instantaneously by directing the point of a small blowpipe 

 flame upon it and extending' the heated part (fig. 327). To close a tube 

 at its end, another piece of the same kind of glass must be joined to it 



