coal rank. This will form the subject of a further communication. It may be mentioned here, however, that at room temperature the degree of extraction of low-rank coals rapidly approaches a limit (about 10 per cent) as the solvent/coal ratio is increased ; if the sol is then removed by filtration and fresh solvent is added, a further substantial amount of extract (about 5 per cent) is obtained. This can be repeated many times before the amount of extract becomes negligible.It is now well recognized that coal has a colloidal structure 3 • The type of extraction phenomenon outlined in the last paragraph is characteristic of colloidal systems, although the present case appears to have some unusual features. Swelling in solvents is a normal property of colloidal substances, including polymers. In the case of coal, a thermodynamic treatment must take into account the distribution of colloidal material in appreciable concentration between both phases.
It is possible, in principle, to calculate the heat of immersion of a solid in a liquid from adsorption data. Thus, for reversible adsorption of an insoluble vapour, the free-energy lowering AG of the adsorbent is given by the integrated form of the Gibbs-Duhem equation,where PI, p 2 are the initial and final pressures of the vapour, m is the quantity in grams of vapour adsorbed at a pressure p (or fugacity f), and M is the molecular weight of the adsorbate. This equation holds whatever the mechanism of interaction : e.g. whether the adsorbed phase Bangham, Nature, 1944, 154, 837.
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