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 distillation 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 temperature 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
