This remarkable experiment is very easy to carry out, and sodium is now easily procured at any shop where chemicals are sold. The combustion of sodium in water can be explained in a very simple manner. Water, as we know, is composed of hydrogen and oxygen. Sodium, by reason of its great affinity for the latter gas, combines with it, and forms a very soluble oxide; the hydrogen is released and thrown off, as we shall perceive by placing a lighted match in the jar, when the combustible gas ignites. Oxide of sodium has a great affinity for water; it combines with it, and absorbs it in great quantities. It is a solid, white substance, which burns and cauterizes the skin; it is also _alkaline_, and brings back the blue colour to litmus paper that has been reddened by acids. Sodium combines easily also with chlorine. If plunged into a jar containing this gas it is transformed into a substance, which is sea salt. If the chlorine is in excess a part of the gas remains free, for simple substances do not mingle in undetermined ratios; they combine, on the contrary, in very definite proportions, and 35·5 gr. of dry chlorine always unites with the same quantity of soda equal to 23 grams. A gram of kitchen salt is formed, therefore, of 0·606 gr. of chlorine, and 0·394 gr. of sodium. Besides sea salt, there are a number of different salts which may be made the object of curious experiments. We know that caustic soda, or oxide of sodium, is an alkaline product possessing very powerful properties; it burns the skin, and destroys organic substances. Sulphuric acid is endowed with no less powerful properties; if a little is dropped on the hand it produces great pain and a sense of burning; a piece of wood plunged into this acid is almost immediately carbonized. If we mix forty-nine grams of sulphuric acid and thirty-one grams of caustic soda a very intense reaction is produced, accompanied by a considerable elevation of temperature; after it has cooled we have a substance which can be handled with impunity; the acid and alkali have combined, and their properties have been reciprocally destroyed. They have now originated a _salt_ which is _sulphate of soda_. This substance exercises no influence on litmus paper, and resembles in no way the substances from which it originated. There are an infinite number of salts which result in like manner from the combination of an acid with an alkali or _base_. Some, such as sulphate of copper, or chromate of potash, are coloured; others, like sulphate of soda, are colourless. The last-mentioned salt, with a number of others, will take a crystalline form; if dissolved in boiling water, and the solution left to stand, we shall perceive a deposit of transparent prisms of very remarkable appearance. This was discovered by Glauber, and was formerly called _Glauber’s salts_. [Illustration: Fig. 306.—Preparation of a solution saturated with sulphate of soda.]