That is when a liter sample of the mixing basin effluent is set by the side of a laboratory sample in which the same amount of alum has been added they will settle at the same rate and filter to the same degree of clarity. [...] Loeb (9) says: The colloidal state is defined by colloid chemists as that state of matter in which the ultimate units in solution are no longer isolated molecules or ions but aggregates of molecules for which Naegeli has introduced the term micella." It has been the belief of many workers that the unit of the colloid was not the molcule or ion of the solute and that the stoichiometrical laws [...] It may be so far on the acid side in some cases that the most economical method of precipitation when present in large amounts will be to make the water the desired pH for best precipittion of the colloid allow it to go through a period of sedimentation and after most of the coagulated matter has settled out add the proper amount of alkali for precipitation of the alumina This has actuall [...] The broken line gives the ratio of the cost of acid to that of alum which when the tests were made was 0.6 times the cost of alum. [...] It further appears that the slowest factor in this reaction is the rate of diffiusion of the dissolved oxygen through the stationary film of water on the iron surface and hence oxygen concentration and velocity of flow are the two most important variables temperature alone being important though less so than in the other ranges while the effect of the hydrogen-ion codoentration and the nature
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