Not so much as you think. There are certain chemical mixtures which produce a rapid change from the solid to the fluid state, or the reverse, in the substances combined, in consequence of which change latent heat is either extricated or absorbed. EMILY. I do not quite understand you. MRS. B. This snow and salt, which you see me mix together, are melting rapidly; heat, therefore, must be absorbed by the mixture, and cold produced. CAROLINE. It feels even colder than ice, and yet the snow is melted. This is very extraordinary. MRS. B. The cause of the intense cold of the mixture is to be attributed to the change from a solid to a fluid state. The union of the snow and salt produces a new arrangement of their particles, in consequence of which they become liquid; and the quantity of caloric, required to effect this change, is seized upon by the mixture wherever it can be obtained. This eagerness of the mixture for caloric, during its liquefaction, is such, that it converts part of its own free caloric into latent heat, and it is thus that its temperature is lowered. EMILY. Whatever you put in this mixture, therefore, would freeze? MRS. B. Yes; at least any fluid that is susceptible of freezing at that temperature. I have prepared this mixture of salt and snow for the purpose of freezing the water from which you are desirous of seeing the latent heat escape. I have put a thermometer in the glass of water that is to be frozen, in order that you may see how it cools. CAROLINE. The thermometer descends, but the heat which the water is now losing, is its _free_, not its _latent_ heat. MRS. B. Certainly; it does not part with its latent heat till it changes its state and is converted into ice. EMILY. But here is a very extraordinary circumstance! The thermometer is fallen below the freezing point, and yet the water is not frozen. MRS. B. That is always the case previous to the freezing of water when it is in a state of rest. Now it begins to congeal, and you may observe that the thermometer again rises to the freezing point. CAROLINE. It appears to me very strange that the thermometer should rise the very moment that the water freezes; for it seems to imply that the water was colder before it froze than when in the act of freezing. MRS. B. It is so; and after our long dissertation on this circumstance, I did not think it would appear so surprising to you. Reflect a little, and I think you will discover the reason of it. CAROLINE. It must be, no doubt, the extrications of latent heat, at the instant the water freezes, that raises the temperature. MRS. B. Certainly; and if you now examine the thermometer, you will find that its rise was but temporary, and lasted only during the disengagement of the latent heat--now that all the water is frozen it falls again, and will continue to fall till the ice and mixture are of an equal temperature. EMILY.