1921-22.] Importance of Prof. Whittaker’s Atomic Model. 223 
XV. — The Importance of Professor Whittaker’s Atomic Model 
and of other Atomic Models.* By Professor W. Peddie. 
(Read May 8, 1922. MS. received May 29, 1922.) 
The atom, even in its simplest form, must possess a most complicated 
structure. The aim of physical science is to find a hypothetical structure, 
and mode of its functioning, which will give a good account of atomic 
phenomena. When the phenomena are complicated, and only very 
partially known, hypothetical investigations are most uncertain and 
provisional, and make little attempt to work otherwise than by analogy 
or illustration. In such cases help is often obtained through the discovery 
of a general law which unifies a wide range of phenomena, and which 
may be followed by many diverse structures in their activities. Well- 
known examples are found in the law of conservation of energy, the law 
of least action, and, recently, the law of quantum action. 
Such a law being found, it may be possible to employ a simple 
mechanism which, in subjection to that law, works according to the 
manner of nature throughout a wide but limited range. For application 
in still wider ranges increased complexity in the model is required ; but 
great initial complexity may be avoided in consequence of the discovery 
of a new general law which, with a little increase in complexity of the 
model, covers the wider field. Thus, in atomic theory, the smooth, hard, 
spherical, perfectly elastic atom, with Newton’s laws of motion super- 
added, accounted for much. And the slightly more complicated hydrogen 
atom of Bohr, consisting of a single electron circulating around a nucleus 
of positive electricity, when postulated to be subject to the new quantum 
laws, gives a wonderful revelation of applicability to the extraordinarily 
complicated phenomena of atomic radiation. 
But these additional laws are mere promissory notes which work in 
lieu of the real valid coinage lying behind them. The quantum postulates 
are analogous to the motional laws of clock hands which, in co-operation 
with the simple two-hand structure, enable us to dispense with actual 
knowledge of the internal mechanical system. But that complicated 
mechanism exists ; and every little advancement in knowledge of the 
mechanism enables us to pass from the “ how ” of an unexplained law to 
* Containing the substance of remarks made as Chairman of the Meeting at which 
Professor Whittaker’s paper (pp. 129-142) was discussed. 
