630 SCIENCE PROGRESS 



the energy-quantum theory, which is really a new system of 

 statistical mechanics. Planck has applied it in the first in- 

 stance to radiation phenomena. According to the quantum 

 idea, emission of radiant energy takes place discontinuously, 

 not continuously as on the classical electro-magnetic theory. 

 In other words, emission of radiation takes place in terms of 

 units, called quanta, and a substance is only capable of emit- 

 ting a whole number of such quanta, each quantum being 

 itself indivisible. This view, although utterly heterodox, has 

 proved itself of so much value not only as regards the quan- 

 titative treatment of radiation, but also in the hands of Ein- 

 stein, Debye, and others in connection with molecular heats 

 and other problems, that it must be taken into account in all 

 processes which involve energy exchanges, and therefore in 

 problems of atomic structure. What is particularly important 

 for our present purpose is that Bohr(io) has recently suc- 

 ceeded in demonstrating that the Rutherford atom-model, 

 though unstable on the basis of classical mechanics, is not 

 necessarily so on the basis of the quantum theory ; and, in 

 fact, Bohr has shown, that by introducing the quantum idea 

 into the Rutherford atom, a number of very important rela- 

 tions in spectroscopy, notably the Balmer Law of series, can 

 be satisfactorily deduced. It would take us too far to discuss 

 further the Rutherford-Bohr atom-model. There can be no 

 question, however, but that it represents the most successful 

 attempt hitherto made to solve, even partially, the question 

 of the structure of the atom. The reader who is interested 

 should consult the papers of Bohr and of Rutherford already 

 referred to. 



Chemical Reaction 



As already mentioned, a chemical change in its simplest 

 significance means a change in the nature of a molecule. The 

 molecule may lose one or more of its atoms, and these may be 

 replaced by others. Thus two substances A and B may react 

 with one another to give quite new substances C and D, the 

 differences consisting essentially in a different molecular struc- 

 ture. Let us take a simple case and try to visualise it in terms 

 of the considerations dealt with in the preceding section. 



A molecule of hydrogen contains two atoms of hydrogen. 

 According to Bohr, the hydrogen molecule consists of two 



