184 NINTH ANNUAL REPORT OF 



The bulk of gas is increased by heat, which must not be looked upon 

 as increasing the size of the atoms, but only as increasing the repulsive 

 force between them. The repulsive or expansive t()rce is measured by 

 the weight which it can support. In the atmosphere, the expansive 

 force of a portion of air, or its elasticity, is exactly balanced by the 

 gravity of all the air above it. The weight of the atoms, from the top 

 to the bottom of the atmosphere, amounts, as we have said before, to 

 about fifteen pounds on each square inch; the elastic or repulsive force 

 which keeps the atoms apart is exactly equal to this amount. Gravity 

 and elasticity are so equally poised, and the atoms move so freely 

 amongst each other, that the air is in a slate of the most delicate 

 balance that can be imagined. 



The expansive property of gases is a remarkable ])henomenon in 

 physics. We have no means of ascertaining its limits, but we know if 

 the whole air was exhausted from I his r(X)m, a single cubic inch of 

 either oxygen or nitrogen would, if admitted into so large a vacuum, 

 instantly occupy every part of it, and still press, though with dimin- 

 ished force, against the walls for further expansion. The repuls^'e 

 force which exists among the atoms, though greatly weakened, would 

 not be exliausted. 



The law which regulates the density and elasticity of gases was dis- 

 covered about half a century after Toricelli invented the barometer. 

 Mariotte found, by experiment, "That the density and elasticity of at- 

 mospheric air are directly, but the space it occupies inversely, as the 

 force of compression." Tliat is to say, if you exhausted the air from the 

 receiver of an air-pump until the barometer stood at fifteen inches, the 

 pressure or elasticity of the air would only be half of what it was before 

 the experiment. It would take two cubic inches of air in this state of 

 rarity to weigh as mucli as ont^ did when the barometer was at thirty 

 inches; or, in other words, one half of the atoms being removed, the 

 remaining half are fiirther apart, since they still occupy the same space. 

 The number of atoms being reduced one half, if tlie temperature is the 

 same, their repulsive force is also reduced in the same ratio, and, there- 

 fore, the repulsion of the particles of any gas increases as the cube 

 root of the distance between them diminishes. The repulsion be- 

 tween the atom at tlie ver}' top of the atmosphere and those below it is 

 so much weakened by separation that it is precisely equal to the weight. 



From the simple fact that the repulsion of the atoms of gases varies 

 as the number of atoms contained in a given space, it follows that the 

 elasticity and density of a gas are as the pressure directly, that the 

 volume is as the pressure inversely, and that, consequently, the one 

 can be deduced from the other by the simple rule of proportion. This 

 law holds true in regard to the most minute additions of weight, and 

 we have the full assurance of reason, t()unded on experiment, bther 

 things being equal, that the distance which separates ever}' atom of 

 air from the lop to the bottom of the atmosphere decreases as w^e de- 

 scend ; in short, each atom is nearer the atom immediately below it 

 than the one above. The various strata of the atmospiiere thus in 

 some measure resembles fleeces of wool or loose balls of cotton piled 

 upon each other. The wool or cotton is more compressed, and there- 

 fore more dense, in proportion to the weight it bears ; it is most so 



