Dr Wilson stated, that the object of his communication was to supplement a paper read to the Royal Society, April 6, 1846, on the solubility of fluoride of calcium in water. He was not then able to state the extent of its solubility in that liquid, but he has since ascertained the weight of the salt in question, which water at 60° can dissolve. The solutions referred to in the following statements were prepared from native well-crystallized fluor-spar. This was first digested in warm dilute aqua regia, to remove any trace of metallic oxides, soluble lime-salts, or other foreign matters, and afterwards carefully washed till it exhibited no acid reaction. Distilled water was then boiled on the purified fluor till it became saturated; the solution filtered whilst warm, and left at rest for some days in stoppered bottles, at a temperature of about 60°, till it deposited the excess of salt soluble above that temperature. It was then filtered a second time. A certain volume of this solution, measured at 60°, was evaporated to dryness on the vapour bath, in a counterpoised platina basin.