MOLECULES. 363 



But we must now go on to molecules. Molecule is a modern word. It does 

 not occur in Johnson's Dictionary. The ideas it embodies are those belonging 

 to modern chemistry. 



A drop of water, to return to our former example, may be divided into 

 a certain number, and no more, of portions similar to each other. Each of 

 these the modern chemist calls a molecule of water. But it is by no means 

 an atom, for it contains two different substances, oxygen and hydrogen, and by 

 a certain process the molecule may be actually divided into two parts, one 

 consisting of oxygen and the other of hydrogen. According to the received 

 doctrine, in each molecule of water there are two molecules of hydrogen and 

 one of oxygen. Whether these are or are not ultimate atoms I shall not 

 attempt to decide. 



We now see what a molecule is, as distinguished from an atom. 



A molecule of a substance is a small body such that if, on the one hand, a 

 number of similar molecules were assembled together they would form a mass 

 of that substance, while on the other hand, if any portion of this molecule 

 were removed, it would no longer be able, along with an assemblage of other 

 molecules similarly treated, to make up a mass of the original substance. 



Every substance, simple or compound, has its own molecule. If this molecule 

 be divided, its parts are molecules of a different substance or substances from 

 that of which the whole is a molecule. An atom, if there is such a thing, 

 must be a molecule of an elementary substance. Since, therefore, every molecule 

 is not an atom, but every atom is a molecule, I shall use the word molecule 

 as the more general term. 



I have no intention of taking up your time by expounding the doctrines 

 of modern chemistry with respect to the molecules of different substances. It 

 is not the special but the universal interest of molecular science which encourages 

 me to address you. It is not because we happen to be chemists or physicists 

 or specialists of any kind that we are attracted towards this centre of all 

 material existence, but because we all belong to a race endowed with faculties 

 which urge us on to search deep and ever deeper into the nature of things. 



We find that now, as in the days of the earliest physical speculations, 

 all physical researches appear to converge towards the same point, and every 

 inquirer, as he looks forward into the dim region towards which the path of 

 discovery is leading him, sees, each according to his sight, the vision of the 

 same Quest. 



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