210 PHYSICAL SCIENCE 



do the properties of matter-in-bulk appear as a 

 consequence of the collaboration of vast numbers 

 of particles essentially different in nature from any 

 lump of matter we can touch or see ? Again, 

 are the particles which make up different kinds 

 of matter different from each other, or has all 

 matter a common constituent ? Are the different 

 elements composed of identical particles of which 

 the number and arrangement form the determining 

 factors of the chemical atoms ? 



Such questions have puzzled mankind from 

 early times, and, until theories began to be 

 founded on facts and tested by experiment, the 

 track of history is strewn with the speculative 

 hypotheses of the metaphysicians and the poets. 

 Here and there a lucky guess or shrewd suggestion 

 chances to agree with the views which represent, 

 temporarily it may be, the conclusions of experi- 

 mental science. It is curious and interesting 

 that, to many highly educated people, the 

 problems connected with the constitution of 

 matter are better known by such triumphant 

 proofs of the sagacity, scientific insight and good 

 luck of some Greek philosopher than from the 

 definite theories, slowly put together by Kelvin, 

 J. J. Thomson, Rutherford, and Bohr, on the firm 

 basis of experimental knowledge. 



The problems at issue could not even be 

 formulated profitably till the work of Dalton and 

 Avogadro had fixed our ideas of atoms and 

 molecules. In the light of present knowledge, 

 we define an atom to be the smallest particle of 

 matter which can take part in chemical action, 

 or enter into the chemical structure of a compound. 

 It is the ultimate chemical unit ; the particles 



