VISIBLE MOLECULES, CORPUSCLES 

 AND IONS 1 



WHEN, a hundred years ago, John Dalton gave its modern 

 shape to the atomic theory, which may be traced back to 

 ancient philosophy and to Democritus, nobody expected that 

 scientists would some day isolate, or at least render visible, 

 the single atom and molecule. The kinetic theory of gases 

 ascribed the gas pressure to the bombardment of the walls 

 of the confining envelope by the gas molecules. It taught 

 us how to count and to measure the molecules. But it did 

 not bring the probability of our ever seeing them any 

 nearer ; that looked hopeless with 3 X 10 19 molecules in a 

 cubic centimeter of a gas, and 640 trillion of atoms in 

 a milligram of hydrogen. The recent discovery of par- 

 ticles one-seventeen hundredth of the size of a hydrogen 

 atom offers problems even more difficult than those of an 

 atom. Yet it is claimed that the visibility of the smallest 

 particles has been demonstrated in various ways. It may 

 be opportune to examine some of the experiments on which 

 such claims are based. 



The existence of particles smaller than the atom would 

 not in reality contradict the atomic theory. The atomic 

 theory does not assert that the atom is the smallest particle 

 capable of existence. The name ''atom/' indeed, suggests 

 something that cannot further be cut or divided. But the 

 essence of the theory is that an elementary substance con- 

 sists of particles or atoms peculiar to that element, and that 

 the single atom is the smallest particle which can enter into 

 combination. The molecule is the combination product of 

 atoms. The atom need not necessarily be the smallest ulti- 

 mate particle, and many considerations induce modern 

 ^Engineering, April 8, 1910. 

 279 



