102 



Laboratory Arts 



through 1 80, so that the hot glass falls back again. Similarly, 

 even during the period of transference from flame to mouth, the 

 glass may bend over, in which case it must be inverted before 

 blowing, which must take place at the instant the hot portion comes 

 into line, and at this same instant rotation may be resumed, in 

 order to prevent any further falling out of line. 



The tube is next heated for the second cm. of its length, and a 

 second bulb blown on the top of the first in order to provide more 

 glass, the two bulbs being subsequently fused into one before a 

 somewhat larger flame, and blown into a single bulb as before. 

 Blow strongly at first, and more gently as the bulb swells, in 

 order not to reduce its thickness too much. 



A 2-cm. diameter bulb is the smallest one should be satisfied 

 with, and if a 2o-cm. length of 4-mm. bore tubing were taken origin- 



FIG. 91. Stages in the construction of a thermometer bulb. 



ally, the resulting bulb should be sufficiently strong to withstand the 

 shock of dropping on the bench, bulb downwards, from a height of 

 two feet. It should be a perfect sphere, the centre of which rests 

 upon the produced axis of the tube, and it should have no extra 

 thickness visible at X. Should this, however, be the case, it may be 

 removed by heating this thickened portion with a fairly small flame, 

 blowing out gently, and reheating and reblowing the whole bulb. 



Success at this exercise is much more easily attained than at the 

 previous one, and nothing short of perfection should be accepted 

 having attained which an attempt should be made using thermo- 

 meter tubing (i-mm. bore). The only difficulty here is due to the 

 liability of the glass to collapse and so close the bore. To avoid 

 this, after closing by drawing off a small portion, heat the tube at the 



