taining
cold water. This condensation is dew. It always forms when the
conditions are right, summer and winter. In cold weather we call it
frost. It has been stated in a former chapter on evaporation that the
capacity of the air for holding moisture in a transparent form depends
upon its temperature. If the temperature is at the freezing point it
will contain the 160th part of the atmosphere's own weight as aqueous
vapor. If it is 60 degrees Fahrenheit the air will retain six grains of
transparent moisture to the square foot of air, while at 80 degrees it
will contain nearly eleven grains. When the air is charged with this
vapor to the point of saturation (which point varies with the
temperature) a slight depression of the temperature is sufficient to
condense this vapor into cloud or drops of water. Between 1812 and 1814
Dr. Wells made a series of experiments with flocks of cotton wool. He
weighed out pieces of equal weight and attached a number of them to the
upper side of a board and as many more to the lower side, and exposed it
to the night air under varying conditions. One experiment was made with
a board four feet from the earth, so that half of the bunches of cotton
faced the ground and the other half the sky. He found upon weighing
these after a night's exposure under a clear sky that the cotton wool on
top of the board had gained fourteen grains in weight from the moisture,
or dew, that had formed upon it, while the same amount of cotton on the
under side of the board had only increased four grains. He tried further
experiments by making little paper houses, or boxes, to cover a certain
portion of grass or vegetation. He found that while there would be a
heavy dew on the grass outside there was little or none within the
inclosure. These experiments were conducted in various ways and closely
watched to see that none of the phenomena were in any way connected with
falling rain. It has been determined that substances like grass and
green leaves of all kinds, hay and straw, while they are poor
conductors of heat, are excellent radiators. In another chapter we have
referred to this quality of straw, that is taken advantage of by the
inhabitants of hot countries in the manufacture of ice and in our own
land for storing it.

Perhaps everyone who has lived in the country has noticed that on a
summer's morning when the grass is laden with dewdrops a gravel walk or
a dusty road will be perfectly dry. This is due to the