Of all objects which are commonly met with, few are such general favourites as the POLYCYSTINÆ, and deservedly so. Their forms are most beautiful, and often peculiar--stars varying in design, others closely resembling crowns; the _Astromma Aristotelis_ like a cross, and many whose shapes no words could describe. The greater part, perhaps, of those which are usually sold, is from the rocky parts of Bermuda; but they are also found in Sicily, some parts of Africa and America. They are usually mounted in balsam, but are equally beautiful mounted “dry” and used with the lieberkuhn. They require as much care in cleaning as the Diatomaceæ, but the process is a different one. Sometimes this is effected by simply washing until they are freed from all extraneous matter, but this is seldom as effectual as it should be. In the _Microscopic Journal_ Mr. Furlong gives the following method of treatment as the best he knew:-- Procure-- A large glass vessel with 3 or 4 quarts of water. New tin saucepan holding 1 pint. 2 thin precipitating glasses holding 10 oz. each. Take 3 oz. of dry “Barbadoes earth” (lumps are best), and break into rather small fragments. Put 3 or 4 oz. of common washing soda into the tin and half fill it with water. Boil strongly, and having thrown in the earth, boil it for half an hour. Pour nine-tenths of this into the large glass vessel, and gently crush the remaining lumps with a soft bristle brush. Add soda and water as before, and boil again; then pour off the liquid into the large vessel, and repeat until nothing of value remains. Stir the large vessel with an ivory spatula, let it stand for three minutes, and pour gently off nine-tenths of the contents, when the shells will be left, partially freed only, like sand. 2ND PROCESS.--Put common washing soda and water into the tin as before, and having placed the shells therein, boil for an hour. Transfer to the large vessel as before, and after allowing it to stand for one minute pour off. Each washing brings off a kind of “flock,” which seems to be skins. 3RD PROCESS.--Put the shells in precipitating glass and drain off the water until not more than ½ oz. remains. Add half a teaspoonful of bicarbonate of soda, dissolve, and then pour in gently 1 oz. of strong sulphuric acid. This liberates the “flock,” &c., and leaves the shells beautifully transparent. Wash well now with water to get rid of all salts and other soluble matter. Some of the large shells are destroyed by this method, but none that are fit for microscopic use. An oblique light shows these objects best. These are sometimes treated in the manner described in Chapter II. where the diatoms are spoken of, but many forms are liable to be injured by this severe process.