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artificial cryolites preserve the ratio of the natural mineral and contain only about 5 per cent, silica. The fluorine in cryolite and other insoluble fluorides that may contain silica is estimated as follows : One gramme of the finely ground material is fused with 4grms. of mixed carbonates. The melt is digested with lOOc.c. water and filtered. The filtrate, which contains the fluorides, is evaporated until gummy, then diluted and just neutralised with nitric acid and phenolphthalein. Last traces of silica are removed by adding 20c.c. of Schaffgotsch solution. (This is made by dissolving 250grms. of ammon- ium carbonate in ISOc.c. of ammonia (sp. gr. 0.92) and diluting to one litre, then adding 20grms. of mercuric oxide and shaking till solution is effected.) The filtrate from the Schaffgotsch treatment is boiled with large excess of calcium chloride solution. The precipitate, consisting of calcium carbonate and fluoride, is washed, dried, gently ignited and treated with dilute acetic acid till evolution of carbon dioxide ceases. The liquid is then evaporated to dryness, the residue taken up with hot water (slightly acidified with acetic acid), filtered, dried, gently ignited and weighed as CaF 2 . This may be checked by heating with sulphuric acid and weighing again as CaS0 4 . The method gives results which are 2 to 4 per cent, too low in fluorine. SILICOFLUOBIDES are now sold as by-products in the manufacture of artificial manures. In these, hydrofluo-