98 FLUIDITY AND PLASTICITY unusually rapidly as the solutions are cooled toward the critical- solution temperature. (2) But this abnormality appears before the critical-solution is reached and continues on and through the critical-solution temperature. (3) In the region where the abnormality appears, it is very difficult to obtain concordant values for the apparent fluidity. It may be added that this is to be expected since according to the theory, the apparent fluidity depends upon the size of the drops. (4) By reflected light the solu- tions in this region appear opalescent: by direct light the liquid shows unequal refraction, the images of objects being distorted. Drapier (1911) studied two mixtures in which water is not a component, viz. hexane and nitrobenzene, and cyclohexane and aniline. The fluidity-temperature curves and the fluidity-weight concentration curves of the latter are shown in Fig. 37. Drapier states that the relations are similar when volume-concentrations are employed. According to his experiments, the contention that fluidities are normally additive in homogeneous mixtures is fully sustained. "II semble done que dans un intervalle assez etendu de varia- tion de temperature on puisse considerer la fluidity comme une fonction lin^aire de la temperature, sauf pour les corps trds associes comme Peau, ...... "Pour les melanges, loin de la temperature critique la variation de la fluidity est encore Iin6aire. Mais plus on approche de la region critique, moins les formules lin£aires sont exactes. Elles ne peuvent m6me plus pr£tendre & un semblant d'exactitude, ainsi que le montrent bien les lignes de fluidity des melanges it concentrations voisines de la concentration critique: elles sont tout & fait courbes et concaves vers Paxe des temperatures. D'aiUeurs, d6j pour des concentrations 61oign£es de la concentra- tion critique, au voisinage de la temperature de demixtion le coeffi- cient de fluidite varie tr&s fort. Mais le changement est plus graduel pr£s de la concentration critique. "Si Ton examine Failure des isothermes de fluidite, on voit que pour les melanges de corps normaux la loi d'additivite: est assez bien satisfaite a des temperatures superieures & la temperature critique de dissolution. J'ai porte en abscisses les d e< C( is