ON PNEUMATIC EQUILIBRIUM. 205 



tube such, that the portion of air must expand to twice its natural bulk, 

 before the mercury acquired a height sufficient to counterpoise it, this 

 height would be 15 inches only. For it appears to be a general law of 

 all elastic fluids, that their pressure on any given surface is diminished 

 exactly in the same proportion as their bulk is increased. If, therefore, the 

 column of mercury in the vacuum of the air pump were 60 inches high, the 

 air would be reduced to half its natural bulk ; and for the same reason, the 

 pressure of a column of 30 inches of mercury in the open air will reduce 

 any portion of air to half its bulk, since the natural pressure of the atmo- 

 sphere, which is equal to that of about 30 inches of mercury, is doubled by 

 the addition of an equal pressure. In the same manner the density of the 

 air in a diving bell is doubled at the depth of 34 feet below the surface of 

 the water, and tripled at the depth of 68 feet. This law was discovered by 

 Dr. Hooke ;* he found, however, that when a very great pressure had been 

 applied, so that the density became many times greater than in the natural 

 state, the elasticity appeared to be somewhat less increased than the density; 

 but this exception to the general law has not been confirmed by later and 

 more accurate experiments, t 



Not only the common air of the atmosphere and other permanently elastic 

 gases, but also steams and vapours of all kinds, appear to be equally subject 

 to this universal law : they must, however, be examined at temperatures 

 sufficient to preserve them in a state of elasticity ; for example, if we wished 

 to determine the force of steam twice as dense as that which is usually pro- 

 duced, we should be obliged to employ a heat 30 or 40 degrees above that of 

 boiling water, we should then find that steam of such a density as to support, 

 when confined in a dry vessel, the pressure of a column of 30 inches of 

 mercury, would be reduced to half its bulk by the pressure of a column 

 of 60 inches. But if we increased the pressure much beyond this, the 

 steam would be converted into water, and the experiment would be at an 

 end. 



That the air which surrounds us is subjected to the power of gravitation, 

 and possesses weight, may be shown by weighing a vessel which has been 

 exhausted by means of the air pump, and then allowing the air to enter, and 

 weighing it a second time. In this manner we may ascertain the specific 

 gravity of the air, even if the exhaustion is only partial, provided that we 

 know the proportion of the air left in the vessel to that which it originally 

 contained. The pressure derived from the weight of the air is also the cause 

 of the ascent of hydrogen gas, or of another portion of air which is rarefied 

 by heat, and carries with it the smoke of a fire ; and the effect is made 

 more conspicuous, when either the hydrogen gas, or the heated air, is con- 

 fined in a balloon. The diminution of the apparent weight of a body by 

 means of the pressure of the surrounding air, is also shown by the destruc- 

 tion of the equilibrium between two bodies of different densities, upon 

 their removal from the open air into the vacuum of an air pump. For 

 this purpose a light hollow bulb of glass may be exactly counterpoised in 

 tty3 air by a much smaller weight of brass, with an index, which shows, 



* Birch's History of the Royal Society, 1678, iii. 384, 387. 



t Rickmann on the Compression of the Air by Ice, Nov. Com. Petr. ii. 162. 



