Mild Soda and Potash. Until the middle of the eighteenth century, it was thought that the soluble matter extracted from the ashes of all plants was the same. In 1752 it was shown that the substance obtained in this way from plants which grew in or near the sea differed from that from land vegetation by producing a golden yellow colour when introduced into the non-luminous flame of a spirit lamp, while that from land plants gave to the flame a pale lilac tinge. The former substance is now known as mild soda, and the latter as mild potash. At this point it is well to make it clear to the reader that there are two bodies commonly called soda, and two called potash. One of each pair is caustic and one mild. By a simple chemical test it is easy to distinguish a mild from a caustic alkali. When a little dilute acid is added to the former, there is a vigorous effervescence caused by the escape of carbon dioxide, but no gas is given off when a caustic alkali is treated in the same way. The liberation of carbon dioxide on the addition of acids shows that the mild alkalis are carbonates. Washing Soda is so well known, that very little description of its external characteristics is necessary. It is a crystalline substance, easily soluble in water. The crystals, when freshly prepared, are semi-transparent; but after exposure to air for some time, they are found to lose their transparency and to become coated with an opaque white solid which crumbles easily. This change in appearance is accompanied by a loss in weight. Crystals of soda melt very easily on the application of heat and, on continued heating, the liquid seems to boil. When this operation is carried out in a vessel attached to a condenser, the vapour that is given off from the melted soda condenses to a clear colourless liquid which, on examination, proves to be water. When no more water collects in the receiver, the vessel contains a dry, white solid, which by any chemical test that may be applied is shown to be the same as washing soda, but it contains no water of crystallization and has a different crystalline form. This substance is anhydrous sodium carbonate, or soda ash as it is called in commerce. When soda ash is mixed with water, it combines with about twice its own weight of that liquid, forming soda crystals again. Washing soda, then, contains nearly two-thirds of its weight of water. Some of this water is given off spontaneously when the soda is exposed to air; the water may even be said to evaporate. This accounts for the loss of weight observed and also for the formation of the white layer of partially dehydrated soda over the surface of the crystal. The property of losing water in this way is common to most crystals containing a high percentage of water of crystallization. The phenomenon is known as “efflorescence.” It may here be observed that crystals of washing soda which have become coated over in this way contain relatively more soda than those which are transparent.