90 Laboratory Arts 



perfectly cold before reheating for the final bend, otherwise the 

 residual heat in each bend (different in each case) will be added at 

 these points to that given to the straight portion of the tube, with the 

 consequence that the old bends will soften first, and all the subse- 

 quent bending will take place there instead of along the tube. 



The closing of the end of the tube is dealt with in following 

 pages. 



Spirals are made by winding hot glass tube round a cylinder 

 of carbon, or, if small, by direct bending. A hollow and collapsible 

 cylinder of copper may be substituted for the carbon cylinder, the 

 method of working being as follows : 



A sheet of copper is cut, the length of the finished spiral and 

 three times its internal diameter. This is bent into a cylinder 

 round a mandrel of wood, mounted in its turn by means of two 

 nails driven axially into the wood and fitting into two V-shaped 

 grooves cut in two upright wooden supports. A length of glass 

 tubing is placed above this cylinder, and at the suitable angle, the 



fflt 



FIG, 8r. 



blowpipe flame is allowed to play upon the tube until it bends 

 round the cylinder, to which it is then wired with copper wire. 

 The blowpipe is then used to soften the next piece of glass, and 

 the cylinder gently rotated as the glass softens, winding the tubing 

 round it into a spiral. When completed, the whole is lifted from 

 the support, and the mandrel withdrawn. The copper sheet may 

 be made of smaller diameter by overlapping its edges and likewise 

 withdrawn. See Fig. 81. 



Students able to perform these preliminary exercises may 

 proceed to Part IV. 



IV. EXERCISES IN GLASS BLOWING. 

 Preliminary Notes. 



The blowpipe flame will require some little investigation 

 before the commencement of actual work, and students should 

 make themselves thoroughly familiar with the various flames 

 and the quickest methods of obtaining them. 



