Page:The fairy tales of science.djvu/194

 continuance of the same heat for five hours more to boil it off entirely. Yet liquids do not become hotter after they begin to boil, however long or with whatever violence the boiling is continued. This fact is of importance in domestic economy, particularly in cookery, and attention to it would save much fuel. Soups made to boil in a gentle way by the application of a moderate heat, are just as hot as when they are made to boil on a strong fire with the greatest violence. Again, when water in a copper is once brought to the boiling point, the fire may be reduced, as having no further effect in raising its temperature.

If a thermometer be plunged into the steam that fills the upper part of the kettle, it will indicate 212. The steam is thus found to be no hotter than the water itself. What then becomes of all the heat that passes into the kettle, since it is neither discovered in the water nor in the steam? It becomes latent—that is to say, it enters into the water and converts it into steam without raising its temperature. As much heat disappears in the vaporization of a single pint of water as would suffice to raise the temperature of 1000 pints by one degree! But the reader will be able to form a more adequate conception of the latent heat of steam, from the fact that one gallon of water converted into steam will, by condensation, raise five gallons and a half of ice-cold water to the boiling point!