88 Laboratory Arts 



bending ; three or four seconds will usually suffice. Should the tube 

 not go over quite to the desired angle, it may be assisted by a 

 very gentle pressure, but this must be done with caution, and 

 before the tube has become hard again. All bending should be 

 done out of the flame, and at one operation. Reheating is trouble- 

 some, and usually the second bending follows a different plane 

 from the first, spoiling the appearance of the bend. 



The test of a bend is that no sharp angle appears, and that the 

 diameter is the same throughout the whole bend, in every direction, 

 i.e. that the tube does not become elliptical in section at the bend, 

 see Fig. 76 (2). 



A Bunsen burner cannot satisfactorily be used for bending, as 



FiG. 76. Sharply bent tubes: (i) produced with too small a flame; 

 (2) bent when insufficiently heated ; (3) correctly bent. 



the flame is hollow, and too narrow, but one fitted with a 

 Ramsay head is suitable for long bends. 



EXERCISE 2. Having bent a i5-cm. tube at right angles, 

 bend similar pieces at 30 and 60, noting the increasing tendency 

 to kink at the angle as this becomes smaller. This tendency 

 may be minimized by pulling or rather pressing the two portions 

 of the tube apart when past the right angle, so producing a more 

 curved corner. 



EXERCISE 3. It is more difficult to bend a piece of glass 

 tubing exactly at a predetermined spot than to take a larger piece 

 of tube, bend it, and cut out the required piece. Exercises will 

 follow, however, where this second method will not be possible, and 

 it is well therefore to practise bending a tube, say 5 cm. from one 

 end, or to allow a straight piece 5 cm. long to remain after bending. 

 This will, of course, be attained by using as small a flame as will 



