Cooling isomorphous

The prism faces bearing Cl(2)—Cl(5) and Cl(2)—Cl(3) contract rapidly along the direction parallel to the c-axis. Thus, in the hexagonal UCls structure type, GdCls represents the smallest unit cell (mol. vol) and no change in the structure type was observed [178) on cooling, in contrast to the isomorphism observed in the trifluorides near this region. 

Fergusson et al. were the first to report the existence of binary compounds with a general formula Se Sg in these melts. They carried out an extensive investigation by X-ray powder diffraction and by absorption spectroscopy in the infrared, visible, and ultraviolet regions over the whole composition range of molten mixtures of sulfur and selenium cooled down to 20 °C. They also examined phases obtained by recrystallization of the cooled melts from carbon disulfide. All phases were isomorphic with one of the allotropes of Sg and SCg indicating that the structures also consist of cyclic eight-membered molecules ... 

NaC1.2H20, is formed when a sat. soln. of salt is cooled below —5°. The monoclinic crystals are isomorphous with the corresponding bromide, NaBr.2H20. Hydrates of the other alkali chlorides have not been prepared. 

The tetrahydrate separates on cooling concentrated aqueous solutions of arsenic acid and is the product obtained in the industrial preparation of arsenic acid (see p. 183). It may be obtained2 by concentrating the aqueous solution until it boils at 1502 C., a small portion is then inoculated with a crystal of the isomorphous tetrahydrate of phosphorus pentoxide3 and the crystals which separate used to inoculate the main solution. The crystals are rhombic prisms or plates which melt 4 at 36-14 — 0-05 C. 

The discussion of crystals and ion geometry leads to a question. If the mineral crystals in the igneous rocks can largely be accounted for by Si-O tetrahedra plus the major elements, how do the trace elements occur The answer lies in the natural impurity of the minerals. As magma cools and the minerals appropriate to a particular fluid composition and temperature - pressure combination form, then other elements in the magma are incorporated into the growing crystals, but this is not a haphazard affair and involves isomorphous substitution, ie, substitution by an ion of similar size. 

The crystals obtained in this way are red in colour, and isomorphous with the corresponding chlor-platinate. Density 2-738. On heating they turn black, but regain their red colour on cooling. They are readily reduced on warming in a current of hydrogen gas, wdiilst ignition converts them first into the chlor-palladite and finally into potassium chloride and metallic palladium. 

Potassium osmocyanide is easily soluble in hot water, and the boiling saturated solution deposits, on cooling, the salt as a fine yellow powder. It is almost insoluble in alcohol, and quite insoluble in ether. On slow evaporation of its aqueous solution, the salt yields crystalline leaflets isomorphous with the ferro- and rutheno-cyanides of potassium.3 It is a useful reagent for detecting iron, giving with ferrous salts a violet precipitate, and with ferric salts a dark violet-blue precipitate.4... 

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