The engraving represents a very good filtering apparatus. The best material for the box would be soapstone; the next best material, iron. Mott's cast-iron tank-plates come of a convenient size-18 x 18 inches and 9 x 18 inches. These may be galvanized or coated with slate-paint.

Freezing-Powders. - (1) Four pounds sulphate of soda, 2-1/2 pounds each of muriate of ammonia and nitrate of potash; when about to use, add double the weight of all the ingredients in water. (2) Equal parts of nitrate of potash and muriate of ammonia; when required for use, add more than double the weight of water. (3) Nitrate of ammonia and water in equal proportions. (4) Carbonate of soda and nitrate of ammonia equal parts, and 1 equivalent of water.