The formation of crystals in gels

Abstract

1. 1. Good crystals of silver dichromate were obtained by allowing the slow diffusion of solutions of silver nitrate and potassium dichromate through flowers of sulphur. Barium sulphate powder served as well, and alundum was even better. This coil-firms Dreaper's opinion that diffusion through capillary spaces other than those in a gel may be efficient ill aiding crystal formation. 2. 2. An experiment was devised to prove that in many instances the amorphous or fine-grained precipitate formed when two solutions meet acts as a. nlembrane or network of capillaries. retarding further diffusion and favoring the formation of more of the precipitate in larger crystalline particles. 3. 3. Silicic acid gels of alkaline reaction were made by mixing sodium silicate with less than enough acid for neutralization. These basic gels made possible the production of crystalline salts not possible in a gel of acid reaction. 4. 4. Cold in a beautifully crystalline condition was best prepared by mixing equal volumes of a water-glass, 1.06 density, and 3 N sulphuric acid, adding 1 c.c. of 1 per cent. chloride to 25 c.c. of the mixture and, after solidification of the gel, covering with a solution of 8 per cent. oxalic acid. Startling rainbow bands of red, blue, and green colloidal gold. mixed with scattered crystals, resulted when the water-glass was 1.16 density. Both methods worked readily and gave better results than any yet recorded. 5. 5. Perfect tetrahedrons of copper were secured by using 1 per cent. hydroxylamine hydrochloride to reduce dilute copper sulphate in a silicic acid gel. Peculiar aggregates of tetrahedrons similar to those found in copper deposits in Xature were also observed. All the steps of the reduction of cupric hydroxide to yellow and red forms of cuprous oxide were shown simultaneously in a series of many bands, using glucose as the reducing agent in a silicic acid gel of alkaline reaction. With 1 per cent. hydroxylamine the same reduction was carried through the cupric hydroxide and cuprous oxide to tetrahedrons of metallic copper. This suggests a method of controlling the different steps of other reactions for study. 6. 6. Lead iodide in perfect hexagonal plates, 5 mm. wide, and golden, fern-like growths, 8 cm. long, were formed in silicic acid gels. A basic lead iodide in yellow-white needles, 2 cm. long, formed by the action of excess lead acetate on potassium iodide in a slightly acid gel and even better in a slightly basic gel. Iodine was released on the surface of crystals of lead iodide, as shown by the presence of a little starch mixed with the silicic acid. Some reduction of lead iodide must have taken place, possibly caused by the starch. 7. 7. Red mercuric iodide in 2 cm. needles or, in smaller crystals, banding in as many as forty regular bands in a distance of 8 cm. was obtained by the diffusion of a mercuric chloride solution into a silicic acid gel containing potassium iodide. With certain concentration conditions yellow crystals formed in front of the red needles and were finally changed into the red mercuric iodide. Soluble chlorides exerted a striking influence on the size, form, and arrangement of the mercuric chloride crystals. 8. 8. Magnificent red-black crystals of a basic mercuric chloride resulted when a saturated solution of mercuric chloride diffused into a silicic acid gel of slightly basic reaction. In a very slightly basic gel these crystals were grouped in a remarkable series of bands. The presence of a little glucose exerted a marked influence, changing the color and developing over one hundred compact bands in a distance of 8 cm. 9. 9. Silver sulphate crystals appeared in less than one hour when N silver nitrate diffused into a silicic acid gel made by mixing equal volumes of 1.06 density water-glass and 3 N sulphurie acid. In a few days orthorhombic slabs. 3 cm. long, developed If enough potassium dichromate to make the whole gel O.1 N molar with respect to this salt only was present, the long slabs were a beautiful clear red. In the presence of excess silver nitrate near the surface of the gel silver chromate was formed, and, since silver chromate and silver sulphate are isomorphous, red mixed crystals resulted. The color of the crystals was made any depth of red by varying the concentration of the potassium dichromate in the gel. Farther below the surface of the gel the silver nitrate became less concentrated, and triclinic crystals of the red-black silver dichromate appeared. In time they grew to a size 5 × 5 × 1 mm. 10. 10. Silver acetate grew in gleaming white sheets, 2 cm. long, when N silver nitrate diffused into a silicic acid gel made from water-glass and acetic acid. These crystals appeared in a few minutes. Such protective colloids as gum tragacanth, when present in very small amounts, twisted the straight crystals of silver acetate into fantastic shapes. 11. 11. Basic lead chromate was made in crystalline bands by diffusion of lead acetate solution into a silicic acid gel of alkaline reaction containing potassium chromate; urea nitrate in large crystals, by the diffusion of nitric acid into a silieic acid gel containing urea; perfect crystals, 5 mm. wide, of acid potassium tartrate and perfect crystals of copper tartrate in a silicic acid gel made with tartaric acid: and monosodium phosphate in large, colorless rhombohedrons by mixing disodium phosphate in a sodium silicate-acetic acid gel. 12. 12. Dry batteries discharging for short intervals developed colorless slabs, several centimetres in length, of ZnC 2.2NH 3 next the zinc wall of the container. The gelatinous paste in this part of the cell caused slow diffusion of the zinc chloride, and ammonia formed by the discharge of the cell.