300 SCIENCE IK SHORT CHAPTERS. 



another full of air, and carry them both to the summit of Mont 

 Blanc, or to a similar height in a balloon. We shall then have left 

 nearly half of the atmosphere below, and thus both liquid and gas 

 will be under little more than half of the ordinary pressure. What 

 will happen if we uncork them both ? The liquid will still display 

 its definite surface, and remain in the bottle, but not so the gas. It 

 will overflow upward, downward, or sideways, no matter how the bot- 

 tle is held, and if we had tied an empty bladder over the neck before 

 uncorking, we should find this overflow or expansion of the gas 

 exactly proportionate to the removal of pressure, provided the tem- 

 perature remained unaltered. Thus, at just half the pressure under 

 which a pint bottle was corked, the air would measure exactly one 

 quart, at one eighth of the pressure one gallon, and so on. 



We cannot get high enough for the latter expansion, but can easily 

 imitate the effect of further elevation by means of an air-pump. 

 Thus, we may put one cubic inch of air into a bladder of 100 cubic 

 inches capacity, then place this under the receiver of an air-pump, 

 and reduce the pressure outside the bladder to T ^th of its original 

 amount. With such atmospheric surrounding, the one cubic inch of 

 air will plump out the flaccid bladder, and completely fill it. The 

 pumpability of the air from the receiver shows that it goes on over- 

 flowing from it into the piston of the pump as fast as its own elastic 

 pressure on itself is diminished. 



Numberless other experiments may be made, all proving that all 

 gases are composed of matter which is not merely incohesive, but is 

 energetically self-repulsive ; so much so, that it can only be retained 

 within any bounds whatever by means of some external pressure or 

 constraint. For aught we know experimentally, the gaseous contents 

 of one of Mr. Glaisher's balloons would outstretch itself sufficiently 

 to occupy the whole sphere of space that is spanned by the earth's 

 orbit, provided that space were perfectly vacuous, and the balloon 

 were burst in the midst of it, the temperature of the expanding gas 

 being maintained. 



Here, then, in this self-repulsiveness, instead of self-cohesion, thif 

 absence of self-imposed boundary or dimensions, we have a veiy 

 broad and well-marked distinction between gases and liquids, so 

 broad that there seems no bridge that can possibly cross it. This 

 was believed to be the case until recently. Such a bridge has, how- 

 ever, been built, and rendered visible, by the experimental re^Garches 

 of Dr. Andrews ; but further explanation is required to render this 

 generally intelligible. 



Until quite lately it was customary to divide gases into two 

 classes " permanent gases" and " condensable gus^s" or" vapors." 

 Gaseous water or steam was usually described as typical of the 

 latter ; oxygen, hydrogen, or nitrogen of the former. Earlier than 

 this, many other gases were included in the permanent list ; but 

 Faraday made a serious inroad upon this classification when he 

 liquefied chlorine by cooling and compressing it. Long after this, 

 the gaseous elements of water, and the chief constituents of air, 

 oxygen, hydrogen, and nitrogen, resisted all efforts to conden.se 

 them ; but now they have succumbed to great pressure and .extreme 

 cooling. 



We thus arrive at a very broad generalization viz., that all gases 

 are physically similar to steam (I mean, of course, " dry steam/' /.., 



