~Calcium Hydrate~ or ~"Lime Water."~--This is used in testing for carbon dioxide and in estimating the amount of that gas present in air. It may be made by slaking quicklime and digesting the slaked lime with water. One hundred c.c. of water at 15° C. dissolves 0.1368 grams of the hydrate (CaH_{2}O_{2}), and hot water dissolves still less. "_Milk of lime_" is slaked lime suspended in water. ~Cobalt Nitrate~ (Co(NO_{3})_{2}.6H_{2}O) is used in a 10 per cent. solution for the detection of oxides of zinc, aluminium, &c.; on ignition with which it forms characteristically coloured compounds. ~Copper~, Cu.--Pure copper, as obtained by electrolysis, can be purchased. This only should be used. ~Copper Oxide~, CuO.--It occurs as a black, heavy, and gritty power, and is used for the oxidation of carbon and hydrogen in organic substances. It should be ignited and cooled out of contact with air just before using, since it is hygroscopic. Oxide of copper which has been used may be again utilised after calcination. ~Copper Sulphate~ (CuSO_{4}.5H_{2}O) contains 25.4 per cent. of copper. It is used in the outer cell of a Daniell-battery. The commercial salt is used for this purpose. The re-crystallised and pure salt is used for preparing the anhydrous sulphate, which is used for detecting moisture in gases. For this purpose it is dried at 200° C. till no trace of green or blue colour remains. It must be prepared when wanted. It may be conveniently used in the form of pumice-stone, saturated with a solution of the salt and dried. Traces of moisture develop a green colour. ~Ferric Chloride~, Fe_{2}Cl_{6}. (When crystallised, Fe_{2}Cl_{6}.6H_{2}O.)--The solution is prepared as described under iron. The commercial salt contains arsenic, and, since the chief use of ferric chloride is for the determination of this substance, it must be purified (_see_ under ARSENIC). ~Ferric Sulphate~ (Fe_{2}(SO_{4})_{3}) is a yellowish white deliquescent salt. It is used as an indicator in volumetric silver assaying, and for the separation of iodine from bromine. It may be purchased as iron alum, Am_{2}Fe_{2}(SO_{4})_{4}.24H_{2}O. But it is best prepared by adding strong sulphuric acid to ferric hydrate in equivalent proportions. Use it as a solution containing 2 or 3 per cent. of iron. ~Ferrous Sulphate~, FeSO_{4}.7H_{2}O.--The granulated form is best, and can be purchased pure. It is used for standardising. It keeps better in crystals than in solution. It is readily soluble in water, but the solution is best made with the help of a little free acid. As a re-agent use a 10 per cent. solution. The crystals should be clear bluish-green; if their colour is dark green, brown, or blue, they should be rejected. ~Ferrous Sulphide~ (FeS) is used for the preparation of sulphuretted hydrogen. It may be purchased and broken in small lumps, nut-size, for use.