316. =Gideon's Fleece.=--If you lay a fleece of wool upon the ground, it is so poor a radiator of heat that no dew will be deposited upon it, although there may be an abundance of it on the grass and leaves in its neighborhood. But this was reversed in the case of Gideon's fleece. The laws of nature were set aside, and the fleece was wet with dew while all around was dry. 317. =Dew-Point.=--What is called the dew-point of the air is that degree of temperature to which any substance must be brought down in order to have dew deposited upon it. This depends upon the amount of water there is in the atmosphere. The more there is the higher is the dew-point. When water condenses on a cold tumbler in a hot day there is much more water in the air, and the dew-point is higher, than when no moisture is condensed upon the tumbler. So after a very hot clear day the earth needs not to be much cooled to produce a deposit of dew, because the air has become so highly charged with moisture through the evaporation of the earth under the hot sun. We can very readily at any time ascertain the dew-point. Take a glass of water, and, having a thermometer in it, drop into it some pieces of ice, and watch the outside of the glass. As soon as it begins to be dimmed with moisture look at the thermometer, and you have the dew-point. 318. =Freezing Mercury.=--Mercury can be frozen by radiation when the cold is excessively severe, although the thermometer may indicate a temperature considerably above -39°, the degree at which mercury freezes. Suppose that in a clear, still night the temperature of the air is at -20°. In order to freeze the mercury it must be cooled down 19 degrees below this. Now this can be done by surrounding the mercury with some good non-conductor, as charcoal. This cuts off the supply of heat to the mercury, while it is all the while giving off heat into space by radiation. In like manner can ice be formed in an atmosphere that is above the freezing point, and this is often done in warm climates. 319. =Latent Heat.=--You have seen, § 270, that our sensations do not inform us accurately of the amount of heat in any substance. The same is also true of the thermometer. This only indicates the _sensible_ or free heat. There may be a great deal of heat locked up, as we may say, in the substance, which can be brought out or made free by some change in the substance. This heat thus locked up is called _latent_ heat. [Illustration: Fig. 219.]