WATER 221 



heat of condensation). It is easy to see, then, why it takes so long to 

 boil away a large quantity of water. The amount of heat absorbed 

 which is necessary to raise the water to its boiling point and keep it 

 there is simply enormous. It may be said here that after the water once 

 reaches the temperature 100° C, it remains there until the whole of the 

 liquid boils away, even though the amount of heat applied is somewhat 

 in excess of that required to keep it at the desired temperature. 



Liquid Water. — Very much more interesting and important than any 

 other form of water is liquid or "wet water." In this form it is the 

 most fascinating of all chemical substances, besides being the most 

 useful. In the first place it forms 75 per cent, of the human body and 

 without it nothing could live. It covers about two thirds of the earth's 

 surface to an average depth of about 12,500 feet. It is the best solvent 

 known ; as will be shown later, it is an essential to almost all chemical 

 action. Here again life as well as nearly all branches of science would 

 be at a standstill if it did not exist. It occurs as rain, fog, dew, river 

 and ocean water, spring water, etc. 



When the vapor of the atmosphere condenses around small particles 

 of dust in the air, clouds are formed, or, if down near the surface, a fog. 

 Whenever these small particles run together they produce drops which 

 fall as rain. Dew is nothing more than water which has condensed out 

 of the atmosphere on to cold objects. Only so much moisture can be 

 held in the air at a given temperature if this is lowered, as would 

 happen after the sun goes down, the dew separates out. If pure, water 

 is an odorless, tasteless and in small quantities, colorless, transparent 

 liquid. In bulk it becomes blue in color and very nearly opaque. It 

 never occurs pure in nature, the nearest approach to it being rain-water 

 after it has rained for some time (at first the rain gathers up a large 

 amount of impurity from the atmosphere) ; and melting snow. Water 

 can be readily purified by distillation. For ordinary purposes one dis- 

 tillation is enough, but for certain scientific work a special method of 

 distillation must be resorted to. In this degree of purity it is almost 

 a non-conductor of electricity. 



Water is only slightly compressible. For every atmosphere (15 

 pounds per square inch) of additional pressure, it is made smaller by 

 0.0005 of its volume. The effect of pressure upon its freezing point is 

 also exceedingly small — only 0.00757° C. lowering for each atmosphere. 

 Nevertheless, it can be prevented from freezing by a pressure of 138 

 tons to the square inch at 1.11° C. Any further lowering of tempera- 

 ture requires a proportional increase of pressure. In passing from the 

 liquid to the solid state there is an increase in volume equivalent to one 

 eleventh that of the liquid. 



The boiling point is affected to a much greater extent. Under a 

 normal pressure of 760 mm., water boils at 100° C, or rather, this value 



