ATOMS AND MOLECULES 55 



coloring matter, which covered only the point of a knife, was 

 barely one grain; dispersed over the great bulk of water which 

 you see here, it nevertheless retained its pink coloration, 

 which must be a proof to you how finely its substance is capa- 

 ble of subdivision without being decomposed. One faintly 

 pink drop of the most dilute solution must contain at least one 

 of the smallest particles of the red fuchsine; and each of these 

 small particles or molecules must be made up of a number of 

 smaller particles or atoms of four elements, one solid and 

 black, the three others colorless gases with which you are 

 undoubtedly familiar, and which make up the important parts 

 of the atmosphere and of water. If these four elements had 

 been merely mixed together, one would separate at once, as 

 happens now when I mix a little carbon dust with air. Hence 

 we conclude that in the red compound the carbon, oxygen, hy- 

 drogen, and nitrogen exist not as we know them, but with the 

 atoms joined intimately together to form a new chemical 

 compound; and it is only when we split this compound into a 

 condition bordering on the fineness of the atoms, that we can 

 be said to decompose it into its constituent parts. The point 

 to which we can reduce the compound before it breaks up is 

 called the molecule, often defined as the smallest portion of 

 the compound which can exist and still retain the properties 

 of that substance. In the dilution just shown, the molecules 

 were very widely separated and were surrounded by molecules 

 of water. 



What, then, is the size of a molecule? This is one of the 

 most interesting problems that has been put before the physi- 

 cist, and it is a great triumph for science that four or five 

 different methods have been discovered for determining this 

 size, and that the results have been found to agree tolerably 

 well with one another. We are accustomed to look with more 

 awe on big things than on little ones; and so, when we are told 



