Heat-Qf-solution and ignition-loss methods have been developed for determining the degree of hydration of magnesia in hydrated dolomitic limes and in hydrated dolomitic lime putties. The rate of hydratiQn Qf magnesia in such limes has been studied when the limes were aged as putties at different temperatures, when subjected to steam hydration at variQus temperatures and pressures, and when eXPQsed to different reJative humidities. In the six commercial hydrates studied, 70 to. 90 percent Qfl the magnesia was still unhydrated after the customary 1 day of soaking. From 2 to. 4 months were required to hydrate 95 percent of the magnesia when the limes were soaked at rOQm temperature, but the rate Qf hydratiQn was much mQre rapid at higher temperatures.

The heat evolved in the solution of MgO in hydrochloric acid was found to decrease as the tcmperaturc at which the MgO was prepared was increased. This is shown in the following eq uations, where the values given are those of !:lH, the increment in the heat content. The negative sign , therefore, indieate~ the evolution of heat. MgO(e) [prepared by dehydrating Mg(OHh for 2.5 h ours at 450° CJ+ 17,630 g of 2.085 !vI HCI = solution; !:lH29S. 1oK =-36.67 ± 0.07 kcal, and MgO (c) [prepared by dehydrating Mg(OH) 2 for 2.5 hours at 1,425° CJ+ 17,630 g of 2.085 !vI HCI =solution; !:lH298.1°K=-35.72 ± 0.03 kcal. The decrease in the heat evolved is ascribed mainly to an increase in the particle size of the M gO as the temperature of ignition was increased. Since X-ray dif-fraction patterns were similar there appeared to be no change in crystalline form. The increase in particle size was indicated by a decrease in the rate of solu tion in the calorimeter and was verified by sed imentation experiments. Heat values have a lso been obtained for the solution of Mg(OHh, CaO, and Ca(OHh in bydroehloric acid. Heat of hydration of MgO and CaO was calculated from beat-of-solution data. The heat of hydration of MgO to Mg(OH)2 dcpends on the characteristic properties of both the MgO and Mg(OHh. Heat of hydration of CaO to Ca(OHh was also determined by direct hydration of CaO in a solu tion saturated with respect to Ca(OHh. CONTENTS Page

IntroductionThe first study of the peculiar structures obtained when two dissolved substances are allowed to react slowly to form a precipitate was made by Rungel in 1865, when he was attempting to add stiffness or rigidity to blotting papers. He conceived the idea of forming precipitates between the fibres of the paper by first saturating the paper with some soluble compound that would later be precipitated by the diffusion of a second soluble compound into the paper. In the course of his investigations he noticed peculiarities in the form and shape of the precipitates, which peculiarities he found to be a function of the rates of diffusion.Ord,* in 1869, accidentally obtained other growth forms when he allowed ammonium oxalate to diffuse into isinglass which was slightly impregnated with calcium chloride. The insoluble calcium oxalate was obtained in some very fantastic forms, quite different from those obtained when the two reacting salts were mixed in solution.While experimenting with diffusion phenomena, Lupton3 noticed that in certain cases layers or rings were produced. However, he merely mentioned their occasional occurrence and did not attempt to investigate them.It remained for Liesegangl in 1896, to report his exhaustive investigations of ring formation in gels. He is the first author to give exact data upon these ring structures, and is among the first to point out t'heir significance in explanation of geological diffusion phenomena. His first preparations were made by placing a drop of silver nitrate solution upon a glass plate that had been coated with gelat,in impregnated with potassium bichromate. This gave a series of concentric rings consisting of insoluble silver chromate, the rings being spaced a t wider intervals apart in proportion as the distance from the center increased.In 1914, E. Kiisters reported and described the rhythmic crystallizations obtained when trisodium orthophosphate, cupric sulfate, ferrous sulfate, potassium ferrocyanide, and ammonium sulfate separated from gelatin solutions on drying at ordinary temperatures. This is thought to be the