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Deposits of coal dust collected on filter paper from airborne clouds have been studied in connexion with their light screening power. This depends on the nature of the paper as well as the size and concentration of the particles. Nine kinds of paper were investigated, two of them in detail. Some idea has been obtained of the qualities necessary in a filter paper in order that the dust deposits will have maximum optical density. Theoretical expressions are derived for light screening according to how the particles are dispersed in the paper. Maximum screening would result if each particle shadowed individually, but this is only possible in very sparse deposits; otherwise, overlapping of the particles has to be allowed for. Diffraction of light by the particles and the effects of diffusion of light in the paper are discussed.

The Paper is the fourth of a series dealing with the subject of Cohesion. The aim of the present Paper is to consider the value of molecular force as indicated by Van der Waals gas formula (particularly at the critical state where the liquid and gaseous states merge), and to relate the results to the previous enquiry.

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To re-state and add further evidence in favour of an electrical theory of cohesion. To provide tentative empirical formulae for the expression of inter-molecular forces. The author defines cohesion as the net attraction (i.e., balance of attraction over repulsion) between molecules which are relatively chemicaly saturated, at distances not greatly exceeding the molecular diameters, and the following formula is proposed for this attraction: t2 = Gm2/a(2 + 4d0/d) where G is the Newtonian constant of gravitation, m the molecular mass, d the molecular interval (centre to centre), and d0 is the molecular diameter.

The theory of Davies and Aylward(1) for interpreting the light-screening power of deposits of coal-dust on a uniform filter paper is extended, and the modified theory is tested using data from samples taken in a dust tunnel.