CHAPTER V THE FLUIDITY OF SOLUTIONS The fluidity curves of solutions are most logically considered under four types: I. In the simplest case the fluidity of the mixture caa be calculated from the fluidities of the components. There is no volume change on mixing, and it is assumed that the components neither dissociate nor interact with each other on mixing. The method of calculation of the fluidity of the ideal mixture has been the subject of much discussion, and it will be discussed presently. Examples of this simplest type are carbon tetrachloride and benzene, and diethyl ether and benzene. Thorpe and Rodger (1897) found that there was a very slight contraction on mixing carbon tetrachloride and benzene, thus confirming the earlier observation of F. D. Brown. In the case of methyl iodide and carbon disulfide there was a very slight expansion which decreased as the temperature was raised. Ramsay and Aston found that the surface tension of mixtures of carbon tetrachloride and benzene followed the mixture rule. Zawidski furthermore observed that the vapor pressures of these same mixtures showed but a slight deviation from the mixture rule, due, according to Dolazalek, to association of the carbon tetrachloride. This is the sort of parallelism which needs much further investigation because it affords the most nearly indis- putable evidence to aid the investigator in the selection of ideal mixtures. In much of our physico-chemical reasoning, it would beyond any question be a great advantage if we could assume certain mixtures as ideal in the sense defined above. The fluidity-volume concentration curves of this class are nearly but not quite linear, as will be explained. II. There are instances where there is a well-defined expansion on mixing, accompanied with heat absorption, and in such mix- tures we generally find the fluidity greater than calculated. The fluidity-volume concentration is convex upward, i.e., the curva- ture d*