250 The Outline of Science 



or about a thousand times thinner than the paper on which these 

 words are printed; yet the leaf must be several molecules thick. 



The finest gold leaf is, in fact, too thick for our purpose, and 

 we turn with a new interest to that toy of our boyhood the soap- 

 bubble. If you carefully examine one of these delicate films of 

 soapy water, you notice certain dark spots or patches on them. 

 These are their thinnest parts, and by two quite independent 

 methods one using electricity and the other light we have 

 found that at these spots the bubble is less than the three-millionth 

 of an inch thick! But the molecules in the film cling together 

 so firmly that they must be at least twenty or thirty deep in the 

 thinnest part. A molecule, therefore, must be far less than the 

 three-millionth of an inch thick. 



We found next that a film of oil on the surface of water may 

 be even thinner than a soap-bubble. Professor Perrin, the great 

 French authority on atoms, got films of oil down to the fifty- 

 millionth of an inch in thickness ! He poured a measured drop of 

 oil upon water. Then he found the exact limits of the area of 

 the oil-sheet by blowing upon the water a fine powder which 

 spread to the edge of the film and clearly outlined it. The rest is 

 safe and simple calculation, as in the case of the beaten grain of 

 gold. Now this film of oil must have been at least two molecules 

 deep, so a single molecule of oil is considerably less than a 

 hundred-millionth of an inch in diameter. 



Innumerable methods have been tried, and the result is 

 always the same. A single grain of indigo, for instance, will 

 colour a ton of water. This obviously means that the grain con- 

 tains billions of molecules which spread through the water. A 

 grain of musk will scent a room pour molecules into every part 

 of it for several years, yet not lose one-millionth of its mass in 

 a year. There are a hundred ways of showing the minuteness of 

 the ultimate particles of matter, and some of these enable us to 

 give definite figures. On a careful comparison of the best 

 methods we can say that the average molecule of matter is less 



