FILLING THE TUBE. 



let the space it occupies at different parts of the tube be measured. 

 If this space be everywhere the same, the bore is uniform ; if not, 

 the tube must be rejected. 



18. The bulb, whether spherical or cylindrical, can be formed 

 upon the end of the tube by the ordinary process of glassblowing. 

 The sensibility of the thermometer requires that the capacity of 

 the bulb should bear a large proportion to the calibre of the tube. 

 If, however, the capacity of the bulb be considerable, the quantity 

 of mercury it contains may be so great that it will not be affected 

 by the temperature of the surrounding medium with sufficient 

 promptitude. 



A cylindrical bulb of the same capacity will be more readily 

 affected by the temperature of the surrounding medium than a 

 spherical bulb, since it will expose a greater surface. 



The glass of which the bulb is formed should be as thin as is 

 compatible with the necessary strength, in order that the heat may 

 pass more freely from the external medium to the mercury. 



19. The tube to be filled is represented in fig. 2, where B A c is 

 a tube, and c D a reservoir formed at the 



top for the purpose of filling it, which is 

 to be afterwards detached. Let the tube 

 be first dried by holding it over the flame 

 of a spirit-lamp, so as to evaporate and 

 expel all moisture which maybe attached 

 to the inner surface of the glass. To fill 

 it, let a quantity of purified mercury be 

 poured into the reservoir c D. This will 

 not fall through the bore, being prevented 

 by the air included in the reservoir A B 

 and in the tube. To expel this, and cause 

 the mercury to take its place, let the tubo 

 be placed in an inclined position over a 

 charcoal fire or the flame of a spirit-lamp, 

 so that the air shall be heated. When 

 heated it will expand, force itself in 

 bubbles through the mercury in c D, and 

 escape into the atmosphere. This will 

 continue until all the air in the bulb A B 

 and in the tube A c has been expelled. 

 The pressure of the atmosphere acting on 

 the mercury in c D will then force it 

 through the tube into the bulb A B, which, 

 as well as the entire length of the tube, 

 it will ultimately fill. If a sufficient quantity of mercury be 

 supplied to the reservoir c D, the bulb A B, the tube A c, and a 



151 



