323. =Relation of Latent Heat to the Forms of Substances.=--Whether a substance shall be in the form of a solid, liquid, or gas depends upon the amount of heat which is latent in it. If you take a piece of ice and melt it in a vessel, the ice and the water in the vessel that comes from the melting of the ice are both at 32° until the ice is all melted. But all this time heat is being communicated to the ice and water. What becomes of it? It is all taken up by the ice as it changes from its solid to its fluid state, and becomes latent in it. In fact _every particle of ice must have just so much of latent heat in order to become fluid_. So, also, if water be heated to the boiling point, 212°, and be kept boiling, the water will remain at that point till it is all vaporized. All this time the water is receiving heat, which, instead of raising its temperature, is becoming latent in the particles as they change from their liquid to their vaporous state. As I said of the change from the solid to the liquid state, so here, _every particle of the liquid must have just so much of latent heat in order to become aeriform_. Whenever therefore any solid substance becomes liquid, or liquid becomes aeriform, heat is absorbed and becomes latent. So, on the other hand, whenever any aeriform substance becomes liquid, or liquid becomes solid, latent heat is given out, and becomes free and sensible. The freezing of water, then, is a source of warmth to the air in its neighborhood--a fact which is practically made use of when tubs or pails of water are placed in conservatories to keep plants from freezing; and the thawing of snow and ice is a source of cold, as is exemplified by the chilliness of the air occasioned by this process.