1. The thesis falls into three main sections. In the first the theoretical aspects of the problem are considered, and a rational basis for analysis of experimental results derived. The second section describes the development of a new technique for measuring the progress of mixing, and its application to mixing machines. Findings are reviewed in the final section. The theoretical ideas developed on the assessment of mixing offer a background, to include less comprehensive but more experimentally convenient measures of this state of mixing. A concentration distribution spectrum is proposed, as giving fundamental meaning to the ideas of "clumping" of the components which make up a mixture. Analysis of the influence, on sample composition measures, of sample size, emphasises its association with scale effects and the progress of mixing, and this whole topic is discussed in some detail. All available evidence on rates of mixing is shown to indicate that, over a substantial range, a first order kinetic mechanism applies. The influence of sample size on rate determinations is shown to be small, a finding of value in interpreting experimental results and in scaling up from laboratory trials. Possible measures of efficiency are considered, and though the fundamental criteria are elusive, practical parameters are suggested and developed. An experimental method of following the progress of mixing of flour doughs was evolved, and is described in detail. Analysis is carried out by photographing the pattern produced by colour differentiated components. The photographic plates are then scanned, using a photometric technique, to give a measure of the mixing. This measure is shown to be reproducible, self consistent, and in agreement with such theoretical predictions as can be made. The method is first applied to investigate the theoretical concepts, and then to assess the performance characteristics of two types of laboratory mixers. This produces evidence that the "sigmoid arm" kneading machines, extensively used in practice, are less efficient than other types. Finally, it is shown that the method can he extended readily to a commercial scale mixing machine
