The measurement of humidity is of far wider importance than the mere forecasting of local weather conditions.  The close relation between humidity and health has led many institutions, such as hospitals, schools, and factories, to regulate the humidity of the atmosphere as carefully as they do the temperature.  Too great humidity is enervating, and not conducive to either mental or physical exertion; on the other hand, too dry air is equally harmful.  In summer the humidity conditions cannot be well regulated, but in winter, when houses are artificially heated, the humidity of a room can be increased by placing pans of water near the registers or on radiators.

30.  Heat Needed to Melt Substances.  If a spoon is placed in a vessel of hot water for a few seconds and then removed, it will be warmer than before it was placed in the hot water.  If a lump of melting ice is placed in the vessel of hot water and then removed, the ice will not be warmer than before, but there will be less of it.  The heat of the water has been used in melting the ice, not in changing its temperature.

If, on a bitter cold day, a pail of snow is brought into a warm room and a thermometer is placed in the snow, the temperature rises gradually until 32 deg.  F. is reached, when it becomes stationary, and the snow begins to melt.  If the pail is put on the fire, the temperature still remains 32 deg.F., but the snow melts more rapidly.  As soon as all the snow is completely melted, however, the temperature begins to rise and rises steadily until the water boils, when it again becomes stationary and remains so during the passage of water into vapor.

We see that heat must be supplied to ice at 0 deg.  C. or 32 deg.  F. in order to change it into water, and further, that the temperature of the mixture does not rise so long as any ice is present, no matter how much heat is supplied.  The amount of heat necessary to melt 1 gram of ice is easily calculated. (See Laboratory Manual.)

Heat must be supplied to ice to melt it.  On the other hand, water, in freezing, loses heat, and the amount of heat lost by freezing water is exactly equal to the amount of heat absorbed by melting ice.

The number of units of heat required to melt a unit mass of ice is called the heat of fusion of water.

31.  Climate.  Water, in freezing, loses heat, even though its temperature remains at 0 deg.  C. Because water loses heat when it freezes, the presence of large streams of water greatly influences the climate of a region.  In winter the heat from the freezing water keeps the temperature of the surrounding higher than it would naturally be, and consequently the cold weather is less severe.  In summer water evaporates, heat is taken from the air, and consequently the warm weather is less intense.

32.  Molding of Glass and Forging of Iron.  The fire which is hot enough to melt a lump of ice may not be hot enough to melt an iron poker; on the other hand, it may be sufficiently hot to melt a tin spoon.  Different substances melt, or liquefy, at different temperatures; for example, ice melts at 0 deg.  C., and tin at 233 deg.  C., while iron requires the relatively high temperature of 1200 deg.  C. Most substances have a definite melting or freezing point which never changes so long as the surrounding conditions remain the same.