NEW CONCEPTIONS IN SCIENCE 



a molecule of water or sugar or cotton or salt is no 

 longer any of these familiar substances, but an utter- 

 ly different thing oxygen, hydrogen, carbon, chlo- 

 rine, sodium. We may convert ice into water, and 

 water into steam ; it is the same chemical substance. 

 But make the steam hot enough and it will be dis- 

 sociated into its constituent atoms. These may be 

 in turn brought back to a liquid ; we can now even 

 solidify both oxygen and hydrogen. They are no 

 longer water or ice ; they behave just as differently, 

 for example, as do water and sulphuric acid, or ice 

 and lunar caustic. Every substance is made of 

 molecules, and these are nature's physical units. 

 They are finite, measurable facts. 1 Matter is not 

 infinitely divisible, but granular in structure. These 

 grains of matter can be measured as accurately as 

 the distance of the earth from the sun. 



Chemistry has taken one step further. The 

 most elementary facts of chemical combination 

 force the idea that the molecules are usually com- 

 pound. We know that the molecules of water and 

 salt and sugar are, for they may be broken up. 

 The molecules of the elements 2 are supposed gen- 

 erally to contain at least two atoms. That is the 

 simplest way to account for the facts. There are 

 other facts, however, which make other and more 



1 See " What this World Is Made of," p. 103. 

 8 See " The Search for Primal Matter," p. 145. 



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