io6 MOLECULAR ASSOCIATION molecules. Water and hydroxyl-containing molecules are sup- posed to form aggregates through oxygen acting as connecting link (see chap. VIII.); whilst for the sulphur molecule, one of the three following formulae is supposed to represent it 25°c' according as the sulphur atom is assumed to be di-, tetra-, or hexavalent \ S s \\ \\ s M The best way of illustrating the relation between association and chemical combination is, perhaps, to consider the properties of the components from which molecular compounds are formed. For this purpose the following list, containing, in the main, sub- stances which are associated, has been drawn up. Each pair is only a type of many similar ones. Constituents. Hydrogen iodide : Water-51 Nitric acid : Water 2Ba Potassium hydroxide : Water 253 Ammonia: Water254 Lithium iodide : Water 2nr> Lithium iodide : Me. alcohol25'5 Lithium iodide : Et. alcohol2n(i Lithium iodide : Pr. alcohol250 Lithium iodide : iso-Am. alcohol2<™ Hydrogen bromide : Pr. alcohol257 Lithium iodide : Acetic acid25(5 Hydrogen chloride: Formamide358 Sodium iodide : Acetamide 2S