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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. We have not, I suppose, mixed the acid and the alkali in the exact proportions that are required for the formation of the salt, otherwise the mixture would have been almost immediately changed to a solid mass; but, in order to obtain it in crystals, as you see it in this bottle, it would be necessary first to dilute it with water, and afterwards to evaporate the water, during which operation the salt would gradually crystallise. CAROLINE. But of what use is the addition of water, if it is afterwards to be evaporated? MRS. B. When suspended in water, the acid and the alkali are more at liberty to act on each other, their union is more complete, and the salt assumes the regular form of crystals during the slow evaporation of its solvent. Sulphat of soda liquefies by heat, and effloresces in the air. EMILY. Pray what is the meaning of the word _effloresces_? I do not recollect your having mentioned it before. MRS. B. A salt is said to effloresce when it loses its water of crystallisation on being exposed to the atmosphere, and is thus gradually converted into a dry powder: you may observe that these crystals of sulphat of soda are far from possessing the tra
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