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CHEMISTRY: W. D. HARKINS 
that the forces around the oxygen or nitrogen atoms of organic compounds or 
of water, or such atoms as are commonly called ^ polar,' extend to a greater 
distance, although their intensity is less at the ordinary atomic distances. 
However, the following list may be considered to give something of this 
order of increasing intensity for a limited number of substances. This list 
has been obtained for the most part from a consideration of the surface tension 
relations of substances, and is as follows, beginning with those substances 
around whose molecules the stray field is weakest: helium, neon, hydrogen 
(molecular, not atomic), argon, krypton, xenon, nitrogen, oxygen, methane, 
carbon monoxide, and the following organic compounds — saturated aliphatic 
hydrocarbons, aromatic hydrocarbons, sulphides, mercaptans, halogen deriva- 
tives (methyl chloride, carbon tetrachloride, chloroform, and ethylene chloride, 
with rapidly increasing fields), unsaturated hydrocarbons, ethers, esters, nitro 
compounds, nitriles, aldehydes, ketones, alcohols, amines, acids, and unsat- 
urated acids. Following these are water, molten salts, heavy metals, boron, 
and carbon. The list of organic substances is arranged for derivatives with 
short hydrocarbon chains. A lengthening of the chain causes a displacement 
in the direction of lower intensity for polar derivatives, but probably toward 
higher intensity in the case of the hydrocarbons themselves. It will be seen 
that in general the greater the distance between the substances in this list, the 
less their solubility in each other, the closer together, the more soluble. For 
organic substances, though the present list is much more extensive, it is in 
agreement with that found by Rothmund from solubility data.^ It is well 
known that metals in general give concentrated solutions only with metals, 
carbon, and other similar substances; molten salts dissolve salts or water; 
pairs of organic liquids are miscible unless the members of the pair lie at the 
very opposite extreme of the list of organic substances; water dissolves salts 
or organic substances which are close to it in the list. An interesting illustra- 
tion of this relation is given by data on the organic halogen derivatives listed 
above. The solubility of carbon tetrachloride per 100 grams of water is 0.80 
grams, while that of chloroform, which lies closer to water, is 0.822 grams; 
and methylene chloride, approaching water still more closely, has a solubility 
of 2.00 grams. This is also the order of increasing hydrogen content of the 
molecule, but that this is not the determining factor is indicated by the fact 
that methyl chloride and methane, similar compounds containing still more 
hydrogen, are much less soluble in water. In organic compounds the in- 
tensity of the stray field is much higher adjacent to what are commonly called 
double bonds, than it is near single bonds, and this intensity grows much 
larger still if triple bonds are introduced. Corresponding to this the solu- 
bility of ethane, with its single bond between two carbon atoms, is 0.0507 
volumes of gas per volume of liquid; that of ethylene with its double bond is 
0.1311, or more than twice as great; while acetylene with its triple bond has a 
solubility of 1.105, or about 22 times more than that of the single bonded 
compound. 
