IOO 



Laboratory Arts 



EXERCISE 5. Blowing a bulb in the middle of a piece of 

 tubing. 



For this purpose close up one end of a 2ocm. length of tubing ; 

 heat at the point where the bulb is desired, rotating gently, and 

 holding the tube at the blowing end only, giving support to the 

 other by permitting the drawn-out end to rest in the fork of the 

 thumb and forefinger of the left hand. When the flame is coloured 

 yellow, blow gently (still maintaining the rotation until cold), and 

 return to the flame for the purpose of producing another similar 

 bulb by the side of the first, the two bulbs being subsequently 

 blown into one by means of a larger flame, pressing each portion of 

 the tube inwards towards the bulb during the act of blowing (Fig. 89). 

 By this means more glass is melted into the bulb, which is therefore 



FIG. 89. Stages in the blowing of a bulb in the centre of a length of tubing. 



stronger than if blown at once, while, secondly, the pressing 

 inwards produces a more spherical bulb. 



Considerable practice will be necessary before a spherical bulb 

 will be produced, and many reheatings may be required, but 

 an apparent failure should not be immediately discarded ; patience 

 and continued endeavour will frequently produce the desired result 

 from an apparently hopeless piece of work. 



The bulb frequently appears on one side of the tube more than 

 on the other, and this is due to uneven thickness of the glass, and 

 consequent dissimilar softening caused by unequal heating. The 

 remedy is to heat the smaller side of the bulb only, leaving the 

 larger and thinner side untouched, to blow out the bulb until each 

 side is of equal dimensions, then to reheat the whole, take down 

 as far as possible, gently puffing air in from time to time in order 

 to prevent total collapse (and consequent adhesion of glass across 



