CHEMISTRY SINCE TIME OF DALTON 



vapor exists in the atmosphere as an independent gas. 

 But since two bodies cannot occupy the same space at 

 the same time, this implies that the various atmos- 

 pheric gases are really composed of discrete particles. 

 These ultimate particles are so small that we cannot 

 see them cannot, indeed, more than vaguely imagine 

 them yet each particle of vapor, for example, is just 

 as much a portion of water as if it were a drop out of 

 the ocean, or, for that matter, the ocean itself. But, 

 again, water is a compound substance, for it may be 

 separated, as Cavendish has shown, into the two ele- 

 mentary substances hydrogen and oxygen. Hence 

 the atom of water must be composed of two lesser 

 atoms joined together. Imagine an atom of hydrogen 

 and one of oxygen. Unite them, and we have an atom 

 of water ; sever them, and the water no longer exists ; 

 but whether united or separate the atoms of hydrogen 

 and of oxygen remain hydrogen and oxygen and noth- 

 ing else. Differently mixed together or united, atoms 

 produce different gross substances ; but the elementary 

 atoms never change their chemical nature their dis- 

 tinct personality. 



It was about the year 1803 that Dalton first gained a 

 full grasp of the conception of the chemical atom. At 

 once he saw that the hypothesis, if true, furnished a 

 marvellous key to secrets of matter hitherto insoluble- 

 questions relating to the relative proportions of the 

 atoms themselves. It is known, for example, that a 

 certain bulk of hydrogen gas unites with a certain bulk 

 of oxygen gas to form water. If it be true that this 

 combination consists essentially of the union of atoms 

 one with another (each single atom of hydrogen united 

 VOL. iv. 4 39 



