ULTIMATE CONSTITUTION OF MATTER Si 



given space, or what its weight, the gas is always composed 

 of a certain fixed (though not ascertained) number of mole- 

 cules, provided the pressure and temperature are constant. 

 Therefore one gas is heavy and another light, because in the 

 one the molecules are large, in the other small. A gas is a 

 gas (and not a solid or a liquid) because its molecules repel 

 one another. When pressure is put upon a gas its mole- 

 cules are squeezed nearer together, and the amount by 

 which they are approximated varies directly as the amount 

 of pressure. Again, when a gas is heated the mutual re- 

 pulsion of the molecules is increased, and the force of re- 

 pulsion is exactly proportional to the temperature. When 

 air is heated from o C. to i C. it expands by ^ of its 

 volume, and if it be heated from 95 C. to 96 C., its volume 

 is increased by exactly the same amount. But owing to the 

 fact that the atoms of some gases are heavy while those of 

 other gases are light, it takes more heat to raise the tem- 

 perature of the former than of the latter. If it were desired 

 to raise the temperature of two gases i, a spirit lamp 

 would need to be kept just as much longer under a vessel 

 filled with the gas made of heavy atoms than under the gas 

 made of light atoms, as the atoms were heavier in the one 

 case than in the other. In other words, the atomic weight 

 of a gas divided by its specific heat gives a constant number 

 as dividend. All lines of evidence converge to support the 

 modern view that matter, in a gaseous state, consists of 

 separate molecules, or groups of atoms, which are at the 

 same distance apart in all gases under the same conditions 

 of temperature and pressure ; and therefore that the weight 

 of a gas depends, not upon the number of the molecules 

 which it contains, but upon the weight of each molecule. 

 From this it follows that there can be no uncertainty as to 

 the molecular weight of any element if it can be examined 

 in the gaseous state. It is directly proportional, both to its 

 specific gravity and to its specific heat. The molecule of 

 every element when in a gaseous state is a group of two 



