252 
CHEMISTRY: I. LANGMUIR 
Nebraska, discovered by James H. Cook in 1897. After the lapse of the 
Carnegie researches and explorations, the American Museum entered this 
quarry and through five years of continuous exploration (1911-1916) an 
irregular area within a square of about 36 feet yielded nearly complete skulls of 
ten individuals and skeletal parts of seventeen individuals all together. From 
this wonderful material it has been possible to supplement the descriptions of 
Holland and Peterson and to present for the first time the proportions and 
pose, by which we may estimate the habits of this animal. We reach the 
conclusion that the Moropus type was not plains living, but forest living; 
that it was the seclusion of the forests which protected this type and which 
accounts for its great rarity in fossil deposits, for it is characteristic of forest- 
living forms that they are not readily entombed. We form an entirely dif- 
ferent conception of the habits of the animal when we observe the extremely 
long fore limbs, which are not of the digging or fossorial type, and which 
thus belie the apparently fossorial or digging structure of the terminal pha- 
langes. It appears more probable that these terminal claws were used partly 
for purposes of offense and defense, but largely for the pulling down of the 
branches of the trees. The animal was probably forest living like the African 
okapi, with which in its general body and head proportions it has many anal- 
ogies. Like the okapi it survived through retreat to the recesses of the forests. 
THE STRUCTURE OF ATOMS AND THE OCTET THEORY OF 
VALENCE 
By Irving Langmuir 
Research Laboratory, General Electric Company, Schenectady, New York 
Read before the Academy, April 29, 1919 
In a paper soon to be published in the Journal of the American Chemical 
Society, I wiU give a new theory of the structure of atoms and molecules based 
upon chemical data. This theory, which assumes an atom of the Rutherford 
type, and is essentially an extension of Lewis' theory of the 'cubical atom,'^ 
may be most concisely stated in terms of the following postulates. 
1. The electrons in atoms are either stationary or rotate, revolve or oscil- 
late about definite positions in the atom. The electrons in the most stable 
atoms, namely, those of the inert gases, have positions symmetrical with re- 
spect to a plane called the equatorial plane, passing through the nucleus at 
the center of the atom. No electrons lie in the equatorial plane. There is 
an axis of symmetry (polar axis) perpendicular to the equatorial plane through 
which four secondary planes of symmetry pass, forming angles of 45° with 
each other. These atoms thus have the symmetry of a tetragonal crystal. 
2. The electrons in any given atom are distributed through a series of con- 
centric (nearly) spherical shells, all of equal thickness. Thus the mean 
