oxides, when dissolved in water, enter into combination with a portion of it, producing sodic hydrate and potassic hydrate, two substances which have pre-eminently the properties which we term alkaline, and which exert a strong solvent action upon all forms of animal and vegetable grease. These solutions exercise a wonderful power of cleansing upon the grease-bound particles of dirt which veil our skin, but so strong is their solvent power upon animal membrane, that not only do they dissolve fatty matter, but also the cuticle itself, so that they are manifestly unfitted for removing dirt from a tender skin, and we are forced to look further afield for a grease solvent.
If instead of dissolving our sodic and potassic oxide in water we had left them exposed to ordinary air, we should have found that they gradually attracted from the atmosphere a gas called carbon dioxide, which exists in all air to the extent of four parts in ten thousand, and that by combining with this gas they became converted into sodic and potassic carbonates, bodies which we call salts, and which, although not so violent in their action upon the skin, will retain to a certain extent their solvent action on fatty matters.
The carbonates of sodium and potassium are found in the ashes of many vegetable and animal substances, and in the earliest records which have been discovered we find mention of the cleansing power of wood ashes, the ashes of certain marine plants, seaweed, and "natron," which is an alkaline efflorescence from some kinds of soil; nor has the use of ashes for this purpose entirely died out at the present time.
As early as a. d. 69, however, we find that the elder Pliny mentions another form of cleansing material made from tallow and ashes, the components most recommended being goat's suet and the ash of beech wood; while the ruins of Pompeii were found to contain a fairly perfect soap factory.
Although soap and Christianity date from the same period, it was only at the commencement of this century that the classical researches of Chevreul on the constitution of fats gave the key to the reactions taking place during its formation, while even at the present time we probably only know a true explanation of part of the actions which lead to its cleansing effect upon the skin.
If we take sulphuric acid diluted with water, we find that it has certain well-marked characteristics which leave no room for doubting its acidulous nature; and if we pour a few drops of it into the violet-colored solution obtained by boiling sliced red cabbage in water, the violet solution at once becomes bright red. On repeating this experiment with the violet cabbage solution and a few drops of sodic hydrate solution, we obtain a vivid green