The periodic opacity of certain colloids in progressively increasing concentrations of electrolytes

In the course of an investigation of the complex interaction of an electrolyte, an emulsoid colloid and a suspensoid colloid (1921 (1), 1922 (2)), it was discovered that, as the concentration of the electrolyte present was progressively increased, the opacity of the liquid containing the three reagents increased and decreased in a remarkable periodic manner. Considering the extreme complexity of the heterogeneous system in which the phenomenon was first observed, it was obvious at the outset that a very great deal of work would have to be done, and that the best method of approaching the problem would be by steadily simplifying the conditions of the experiments, and so by a process of elimination retain only those which were concerned in producing the phenomenon. Fortunately a suitable apparatus (referred to below) was available, with which it was possible to make a very wide range of experiments with a minimum amount of material.II. The General Method of investigation. The method of measuring the opacity of the liquids studied was that described in an article recently published by the author (1921 (3)). It was early found that the purity of the water used for making solutions and dilutions was of great importance, since the reactions were so delicate that even the traces of salts found in tap water affected the results. A suitable and very efficient form of still was found to be that designed by Bourdillon (1913 (4)). The “conductivity” water from this was used as fresh as possible and stored, when necessary, in Jena glass flasks, with the exclusion of carbon dioxide and bacteria from the air. Again, to prevent, as far as possible, any reaction from taking place between the water and the glass vessels, only Jena glass test-tubes and flasks were used. The cleaning of the test-tubes was a source of trouble and error in earlier experiments, but later this was obviated by means of the following technique: Immediately after the completion of each series of experiments, the test-tubes were cleaned by washing, first with dilute acid or alkali, followed by several rinsings with “conductivity” water, and then steamed in an apparatus designed by the author. The particular advantages of this steaming apparatus lay in the facts that the steam used contained no traces of calcium,and that, after the preliminary washing with acid and water, there was a minimum of handling of the tubes. The test-tubes were placed in a series of racks, which were dropped into slots over the steamer. When sufficiently steamed the racks of tubes were removed to an electric drying oven and dropped into other slots. It may be mentioned here that it was found convenient to provide the Jena glass test-tubes with a lip, in order to facilitate pouring from the test-tube into the small opacimeter tube.