WATER 225 



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 am- 

 monia, 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 with- 

 out 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 at- 

 tract 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 cal- 

 cium 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 funda- 

 mental 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 : 



VOL. lxxxiv. — 16. 



