Page:Popular Science Monthly Volume 84.djvu/229

Rh 1° C. Now we see why there is always a "warming up" just before a snowstorm.

When gases are allowed to expand suddenly, they cool themselves, taking heat from all surrounding objects. Also if a substance, like ammonia, which at ordinary temperatures is a gas, can be condensed by cooling and pressure to a liquid, and the pressure is removed, it will immediately begin to evaporate rapidly, and in so doing absorb a large amount of heat from everything around. Such a principle is used in the preparation of artificial ice.

Ice is often seen to have much dirt in it. If the water were stirred while freezing so that the crystals which separate are small, they would also be very nearly pure.

So much for solid, liquid and gaseous water. There are still one or two interesting things in connection with water, however, which do not bear directly on any one of these three heads.

Certain compounds have the power to crystallize with a greater or less amount of water—"water of crystallization," as it is called. Most of them can lose this water (or part of it) by heating them, and without detriment to the substances themselves. Examples of such are copper sulphate, sodium sulphate, alum, calcium chloride, etc. Some of these, like calcium chloride, if allowed to stand in the air, will attract moisture and become wet. They are said to be deliquescent. Others like sodium sulphate tend to lose their water of crystallization on standing open to the air. They are called efflorescent. There are still other compounds, called anhydrides, which take up water readily from the atmosphere, not as water of crystallization, but by so doing form a different compound, an acid. Phosphoric anhydride (phosphorus pentoxide) is an example of this kind, and it is the finest substance known for desiccating purposes. Dehydrated copper sulphate and calcium chloride likewise are extensively used.

Sugar, oxalic acid and a number of other substances lose water when being heated, but here the loss is quite a different one from that above. The compounds themselves are completely changed, showing that the water was in direct combination with them and that it was the fundamental part of them.

Many people know that water forms a large part of the human body and of the nourishment of the same, but few know what an enormous percentage of the whole this is. A human body weighing 150 pounds contains about 113 pounds of water (75 per cent., as was stated above), and requires daily for its sustenance, either as a liquid or combined with food, about 5.5 pounds of water. This equals more than half a gallon.

One can see from the following table from what source a large part of this water is derived: