Sulphur is susceptible of a third combination with oxygen, in which the proportion of the latter is too small to render the sulphur acid. It acquires this slight oxygenation by mere exposure to the atmosphere, without any elevation of temperature: in this case, the sulphur does not change its natural form, but is only discoloured, being changed to red or brown; and in this state it is an oxyd of sulphur. Before we take leave of the sulphuric acid, we shall say a few words of its principal combinations. It unites with all the alkalies, alkaline earths and metals, to form compound salts. CAROLINE. Pray, give me leave to interrupt you for a moment: you have never mentioned any other salts than the compound or neutral salts; is there no other kind? MRS. B. The term _salt_ has been used, from time immemorial, as a kind of general name for any substance that has savour, odour, is soluble in water, and crystallisable, whether it be of an acid, an alkaline, or compound nature; but the compound salts alone retain that appellation in modern chemistry. The most important of the salts, formed by the combinations of the sulphuric acid, are, first, _sulphat of potash_, formerly called _sal polychrest_: this is a very bitter salt, much used in medicine; it is found in the ashes of most vegetables, but it may be prepared artificially by the immediate combination of sulphuric acid and potash. This salt is easily soluble in boiling water. Solubility is, indeed, a property common to all salts; and they always produce cold in melting. EMILY. That must be owing to the caloric which they absorb in passing from a solid to a fluid form. MRS. B. That is, certainly, the most probable explanation. _Sulphat of soda_, commonly called Glauber’s salt, is another medicinal salt, which is still more bitter than the preceding. We must prepare some of these compounds, that you may observe the phenomena which take place during their formation. We need only pour some sulphuric acid over the soda which I have put into this glass. CAROLINE. What an amazing heat is disengaged! --I thought you said that cold was produced by the melting of salts? MRS. B. But you must observe that we are now _making_, not _melting_ a salt. Heat is disengaged during the formation of compound salts, and a faint light is also emitted, which may sometimes be perceived in the dark. EMILY. And is this heat and light produced by the union of the opposite electricities of the alkali and the acid? MRS. B. No doubt it is, if that theory be true. CAROLINE. The union of an acid and an alkali is then an actual combustion? MRS. B. Not precisely, though there is certainly much analogy in these processes. CAROLINE. Will this sulphat of soda become solid? MRS. B.