The molecular dimensions of organic compounds. Part III.-The viscosity of vapours: Thiophen and α - Methylthiophen, pyridine and thiazole

The measurements described below were made with the purpose of determining the effect of replacing two methenyl-groups in the benzene ring by an atom of sulphur. This investigation differed from those described in Part II, in that the substances were no longer available in indefinitely large quantities, and that in consequence of this difficulty of supply a large number of their physical properties were still undetermined. Thus, in addition to the viscosities of the vapours, which were the only measurements that it was necessary to make in the case of benzene, toluene and cyclohexane, the experiments on thiophen included measurements of density and of vapour-pressure, and those on methylthiophen included measurements of the melting point of the solid, and of the boiling point, critical temperature, vapour pressure, viscosity and surface tension of the liquid. Moreover, even in the measurements of vapour viscosity, certain modifications were introduced as a result of the altered conditions of working. In particular, trials were made to find out how the transpiration-apparatus would work when the “condensation pressure” was reduced to zero, since this method of working would make it possible to determine the viscosity of a vapour without determining also the vapour-pressure curve, a point of special importance when the substance can only be obtained in small quantities. (a)Thiophen .—The thiophen was fractionally distilled over sodium, and a middle fraction was used which boiled within one-tenth of a degree at 84·3° under 760 mm. pressure at 30·3°.