220 WEIGHT OF AIR. 



If the bladder contains 1,000^ of water, its weigl 

 increases by l,000 gr , but at the same time its volui 

 increases also by 1,000 CC , and the loss of weight in wal 

 amounts, in consequence of the greater volume, exact] 

 to l,000 gr more than before; hence the bladder appeal 

 equally heavy in both cases. Gases transmit pressui 

 and exert pressure upon the bottom of vessels in the 

 same manner as liquids, and the principle of Archi- 

 medes, which is a consequence of fluid pressure, must 

 therefore also be applicable to gases. Accordingly a 

 body in air loses a part of its weight equal to the 

 weight of the displaced air. When the bladder is filled 

 with air, its weight and its volume are increased at the 

 same time; but the bladder now displaces more air 

 than before, and consequently loses an additional part 

 of its weight, and this loss is exactly equal to the 

 increase in the absolute weight; hence the apparent 

 weight remains the same. 



To show the weight of air, a vessel with rigid walls, 

 and hence not capable of altering its volume, must be 

 weighed empty, and again when filled with air. A 

 glass flask with thin walls, of about 1 litre capacity, 

 that is, having, at least, a diameter of 13 cm at the 

 widest part, is provided with a well-fitting cork, glass- 

 tube, india-rubber tube, and pinch-cock, and the air is 

 expelled from it in the manner explained in art. 4 

 (page 18). The bottle is then suspended by a thread, 

 tied round its neck, to the short scale-pan of the 

 balance which was used in the hydrostatic experiments, 

 and exactly counterpoised by weights in the other 

 scale-pan. The pinch-cock is now opened, the air is 

 heard to enter the flask with a distinct sound, and the 



