376 SCIENCE IN SHORT CHAPTERS. 



mained unaltered. Thus, at just half the pressure under 

 which a pint bottle was corked, the air would measure ex- 

 actly one quart, at one-eighth of the pressure one gallon, 

 a'nd 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 j^-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 overflowing 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 baloons 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 baloon were burst in the midst of it, the tempera- 

 ture of the expanding gas being maintained. 



Here, then, in this self-repulsiveness, instead of self -co- 

 hesion, this absence of self-imposed boundary or dimen- 

 sions, we have a very 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, however, been 

 built, and rendered visible, by the experimental researches 

 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 gases," 

 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 in- 



