APPARATUS 



I I 



than the quarter of an atmosphere, the pressure within them must 

 have been essentially the same as that applied without. Some ex- 

 perience and art were needed in order to prevent these bulbs from 

 being so thin in places as to be fractured by the buoyant pressure of the 

 mercury ; and a number of exasperating accidents oc- 

 curred from this cause. It is perfectly possible, however, 

 to make a glass bulb, containing several cubic centimeters, 

 which will change under pressure by five per cent, of its 

 volume and yet be strong enough to endure immersion in 

 mercury. Our experiments were made with such bulbs. 

 The diagram (Fig. i) represents one of them. Their flat- 

 tened sides were best produced by well directed heating 

 after the cylindrical shape had been first attained. 



For the purpose of filling, the neck of the bulb was at 

 first drawn down stoutly in the fashion indicated by the 

 dotted lines in Fig. i . After having been filled by means 

 of a capillary funnel tube, the bulb was packed in ice and 

 water. When the liquid within had contracted so much as 

 to leave the narrowed part far above the meniscus, this 

 narrow portion was drawn out to a very fine point, the bulb 

 itself being shielded from the heat by asbestos. Upon 

 warming the bulb through a degree or two this capillary 

 point was at once filled with liquid, and was then sealed by fusion, 

 usually without enclosing a visible trace of air, and always without 

 enclosing a measurable trace. The weight of the glass in the bulb 

 was always determined, either by subtracting the drawn-off tip from 

 the total original weight, or else by weighing the glass fragments after 

 the experiment. The weight of the enclosed liquid was obtained by 

 weighing the sealed bulb, and subtracting from this the weight of the 

 glass. 



The bulb having been filled at two or three degrees above zero the 

 expansion of the liquid within caused the walls of the vessel to swell 

 outward at 20 ; and thus the possibility of compression of the bulb at 

 ordinary temperatures was greatly increased. In the calculations the 

 slight compressibility of the glass of this bulb was taken into account. 



It is now necessary to describe the apparatus containing mercury by 

 means of which the decrease in volume of the bulb was found. This 

 apparatus, pictured in Fig. 2, consisted simply of a wide short test 

 tube, with a very well ground hollow stopper terminating above in 

 a fine funnel tube provided with a downward pointing platinum wire. 

 For the sake of clearness this apparatus will always be called the glass 



Fig. 1. 



