A quantitative theory is presented which describes the kinetics of coagulation of colloidal systems containing more than one dispersed species. A general expression has been derived to describe the potential energy of interaction between dissimilar spherical colloidal particles, using the linear (Debye-Huckel) approximation for low surface potentials. An overall stability ratio has been defined which takes into account the possibility of interactions between like, as well as unlike, particles in the system. The errors introduced by the use of the linear approximation have been assessed in terms of their effects on the stability ratio, and found to be quite small. The theory has been used to describe the behaviour of a hypothetical system under various conditions.

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Mixing in powders generally

Opening ParagraphRecent studies of African pastoralism have come more and more to concentrate on its political economy and to note the increasing social and economic differentiation occurring within pastoral societies. As Swift and Maliki write of West Africa: ‘Since the 1973 drought, there has been an increasing process of proletarianization in the countryside which has particularly affected herders, who are in many places being transformed from independent rural producers into cowboys herding other people's animals on land they no longer control’ (1984: 2). In Kenya this process has become increasingly apparent since independence, as pastoralism has become dominated by a town-based elite (see Dahl, 1979a; Little, 1983, 1985a and b; Ensminger, 1984). In this article I trace the origins of a new kind of pastoralism in northern Kenya, and argue that poverty and dependence is becoming a permanent way of life to many pastoralists.

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