CHEMISTRY: W. D. HARKINS 
155 
The above list, giving the order of intensity of the stray electro- 
magnetic fields, will be seen to give the substances in increasing order of 
cohesion. The atoms or molecules giving the lowest cohesion, such as He, 
Ne, A, Kr, Xe, etc., are just those which, according to the valence theory 
of G. N. Lewis (/. Amer. Chem. Soc, 38, 1916, (762-85) ) have complete 
outer shells of eletrons (2 for He, 8 for Ne and A). The substances with 
the highest cohesion, such as carbon, silicon, iron and cobalt, ruthenium, 
and tungsten and osmium, are elements which lie in the periodic system 
exactly half way between the elements of lowest cohesion, so high cohesion 
may be said to result when the outer shell of electrons is half filled or half 
complete. In other words a low stray field is found when according to the 
theory of Lewis the atomic or molecular outer shell of electrons is complete, 
the highest stray field, when the shell is half filled, that is when there is the 
possibility of the greatest number of electronic linkages directly between the 
atoms, without the intermediate formation of molecules of a simple type 
containing only a few atoms. Accompanying a high field the substance is 
found to have a low atomic volume, a high cohesion, melting point, a low 
coefficient of expansion, a low compressibility, etc. When the field is low in 
intensity, the properties are just the opposite. 
In analyzing a solubility problem it is well to consider the attraction be- 
tween the molecules of (A), between those of (B), and also that between (A) 
and (B). Consider octane and water which are mutually insoluble. It has 
sometimes been considered that this insolubility is due to the fact that water 
molecules attract each other more than they do molecules of octane, and that 
octane molecules attract each other more than they do molecules of water. 
Now the work of this laboratory shows that while the molecules of water do 
attract each other much more than those of octane, on the other hand the 
molecules of octane attract those of water very slightly more than they do 
those of octane. The much greater attraction of the water molecules for 
each other is a sufficient cause to produce immiscibility, since it is only neces- 
sary that when a group of water molecules is once formed, the mutual at- 
traction shall be great enough to cause the molecules of water to leave the 
group less often than they enter it, so long as there is an appreciable quantity 
of water in the octane. The octane molecules are thus left in a phase by 
themselves. 
Since an increase in the intensity of the stray field around the molecule is 
accompanied by an increase in molecular attraction, it causes an increase in 
the cohesion of the liquid or of the solid, so in its application to pure liquids 
the above theory gives predictions which are similar to those obtained by that 
of Hildebrand,^ that liquids of like cohesions are miscible, while those whose 
cohesions are very unlike are practically insoluble in each other. However, 
solubility is a molecular scale phenomenon, so there are certain advantages 
of a molecular theory of the phenomenon. Moreover, the theory presented 
