ARTICLES 59 



Substances like CO2 and NaO, or N, and CO, which have 

 the same number and arrangement of electrons, Langmuir calls 

 " isosteres." If the substances have also equal positive 

 charges, their external fields will be very similar, leading to 

 almost identical physical properties. If the nuclei differ in 

 total charge, one of the isosteres must be electrically charged 

 when the other is not, so that the external fields will be very 

 different and most of their physical properties will also differ. 

 A number of such cases are described in the original paper. 



In conclusion we will consider the structure of hydrofluoric 

 acid as an example of an acid. The octet theory indicates 

 that the hydrogen atom shares its electron with the fluorine 

 atom, thus completing the stable pair in the first shell for the 

 hydrogen atom and forming an octet round the fluorine atom. 

 The hydrogen nucleus is then held by a pair of electrons forming 

 the octet. From its structure the molecule of hydrofluoric 

 acid should have a small stray field. Most of its surface 

 resembles that of neon, except where the hydrogen nucleus 

 is. But lack of symmetry should make the stray field greater 

 than that of hydrogen or neon. We thus expect hydrofluoric 

 acid to be a gas or liquid not greatly different from water in 

 its boiling-point. As a liquid it should be a non-conductor, 

 but, because of its polar character, it should become an electro- 

 lyte when dissolved in water. 



The properties of a few typical elements and compounds 

 have now been considered. To continue the investigation 

 would exceed the limits of a short paper and, incidentally, 

 both the reader's and the editor's patience. For a fuller 

 account reference must be made to the original paper, the 

 study of which may be facilitated in some degree by this 

 preliminary survey. It will there be seen that the theory 

 explains the magnetic properties of the ferromagnetic elements 

 as well as the peculiar characteristics of the metals of the rare 

 earths. In addition, the covalence equation accounts for the 

 existence of the class of substances which Werner regards as 

 second-order compounds, and shows that they may preferably 

 be looked upon as typical primary valence compounds. Un- 

 fortunately, the hypothesis is not easily reconciled with Bohr's 

 theory of the astronomical atom which has had such marked 

 success in explaining spectra, especially those of hydrogen, 

 but Langmuir hopes that later developments may show that 

 the two theories are not inconsistent. Indeed, it seems as if 

 stationary electrons are unnecessary for Langmuir's theory. 

 The resultant force holding two atoms is electrostatic, and 

 may be fairly constant in direction, while the shared electrons 

 continue to revolve in conformity with the electric field. 



