PRESIDENTIAL ADDRESS. / 



be made visible to the eye, as in the Spinthariscope invented 

 by the late Sir William Orookes. 



It is then at last known positively that atoms do exist. 

 Mattel 1 is not continuous in structure. While elementary 

 matter lias a structure of atoms, the atoms themselves 

 have a structure which while not quite understood, involves 

 the existence of electrons. "Atoms" are not the smallest 

 things obtainable. The electron is now the smallest thing 

 obtainable. It is a unit of negative electricity, and changes 

 in the properties of the atom are caused by the entrance 

 or expulsion of an electron or by the expulsion of a larger 

 positively charged particle. 



Structure of the Atom. 

 The exact number of electrons in any atom is not known 

 but it is probably rather more than half the number repre- 

 senting the atomic weight, or rather more than the "atomic 

 number'" (see below). There is now considered to be in 

 the atom of every element a positive nucleus and one or 

 more surrounding shells in which the electrons revolve. 

 Not only then is the structure of matter discontinuous in 

 that it contains discrete particles (atoms), but the atoms 

 themselves contain smaller discrete units — electrons. 



Motion within the Atom. 

 According to Jeans (4) "The problem of determining how 

 the constituents are arranged and move inside the atom is 

 still far from solution. . . Some knowledge of a general kind 

 can be obtained (from the spectrum), — in particular the 

 laws of motion are necessarily discontinuous, for continu- 

 ous laws of motion would lead to a continuous (atomic) 

 spectrum. . . Planck's spectral formula (for the continuous 

 spectrum of a solid) could not possibly be arrived at except 

 from a system of laws which involved discontinuities of 

 some kind." 



