Sodium and potassium are very remarkable metals. They are only a little harder than putty, and can easily be cut with a knife or moulded between the fingers. When exposed to the air, they rust or oxidize very rapidly, so much so that they have to be preserved in some mineral oil or in airtight tins. They are lighter than water, which they decompose with the liberation of hydrogen, and under favourable circumstances the hydrogen takes fire so that the metals appear to burn on the surface of the water. After the reaction is over and the sodium or potassium has disappeared, a clear colourless liquid remains which has a strongly alkaline reaction, and when this is evaporated until the residue solidifies on cooling, caustic soda or potash is obtained. For very special purposes, the caustic alkalis are sometimes made by the action of the metals on water, but for production on a large scale, less expensive methods are adopted. Caustic Alkali is obtained from the corresponding mild alkali in the following way. The substance—washing soda, for example—is dissolved in water and the solution is warmed. Lime is stirred into this solution, and from time to time a small test portion of the _clear_ supernatant liquid is removed and mixed with a dilute mineral acid. When this ceases to cause effervescence, the change is complete. The clear liquid is now separated from the solid matter (excess of lime together with calcium carbonate) and evaporated in a metal dish. Since the caustic alkalis are extremely soluble in water, they do not crystallize as do most of the compounds previously described. Evaporation is, therefore, carried on until the liquid which remains solidifies when cold. Caustic Soda. To describe the process by which caustic soda is manufactured, we must return to the making of black ash. The mixture from which black ash is made contains limestone. It is heated to 1000° C., which is a sufficiently high temperature to convert limestone into lime. When the black ash is subsequently treated with water, the lime which is present converts some of the mild alkali to caustic; consequently, black ash liquor always contains both alkalis. When the manufacturer intends to make caustic soda and not soda crystals, the composition of the black ash mixture is varied by adding a larger proportion of limestone, so that there may be an excess of lime in the black ash produced. The treatment with water is carried out as described under washing soda, and then more lime is added to convert the mild soda into caustic soda. After the excess of lime and other suspended matter has settled down, the clear caustic liquor is evaporated in iron kettles until it becomes molten caustic, which will solidify on being allowed to cool.