THE CENTURY'S PROGRESS IN CHEMISTRY 



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 atmospheric 

 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 had shown, into the two elementary sub- 

 stances hvdrogen and oxvgren. Hence the atom of 



*/ / o 



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 nothing else. Differ- 

 ently mixed together or united, atoms produce different 

 gross substances; but the elementary atoms never change 

 their chemical nature their distinct 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 

 to a single atom of oxygen), then the relative weights 

 of the original masses of hydrogen and of oxygen must 



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