An atom is the smallest particle of a chemical element. Two or more atoms come together to form a molecule: thus molecule* form 

 the mass of matter. A molecule of water is made up of two atoms of hydrogen and one atom of oxygen. Molecules of different substances. 

 therefore, are of different sizes according to the number and kind of the particular atoms of which they are composed. A starch molecule 

 contains no less than 25,000 atoms. 



Molecules, of course, are invisible. The above diagram illustrates the comparative sizes of molecules. 



INCONCEIVABLE NUMBERS AND INCONCEIVABLY 

 SMALL PARTICLES 



The molecules, which are inconceivably small, are, on the 

 other hand, so numerous that if one was able to place, end to 

 end, all those contained in. for example, a cubic centimetre of 

 gas (less than a fifteenth of a cubic inch), one would obtain a 

 line capable of passing two hundred times round the earth 



WHAT IS A MILLION? 



In dealing with the infinitely small, it is difficult to ap- 

 prehend the vast figures with which scientists confront us. 

 A million is one thousand thousand. We may realise 

 what this implies if we consider that a clock, beating 

 seconds, takes approximately 278 hours (i.e. one week 

 four days fourteen hours) to tick one million time*. A 

 billion is one million million. To tick a billion the clock 

 would tick for over 3I-73S years. 



(In France and America a thousand millions is called 

 a billion.) 



