dissolved. Do not
add any more water than you have to, and keep stirring the
alum with a glass rod while you are adding the water. Pour
half of the solution into another test tube for the next
experiment. Hang a string in the first test tube so that
it touches the bottom of the tube. Set it aside to cool,
uncovered. The next day examine the string and the bottom of
the tube.

EXPERIMENT 84. While the solution of alum in the second test
tube (Experiment 83) is still hot, hold the tube in a pan of
cold water and shake or stir it until it cools. When white
specks appear in the clear solution, pour off as much of the
clear part of the liquid as you can; then pour a little of
the rest on a glass slide, and examine the specks under a
microscope.

[Illustration: FIG. 148. Alum crystals.]

In both of the above experiments, the hot water was able to dissolve
more of the alum than the cold water could possibly hold. So when the
water cooled it could no longer hold the alum in solution. Therefore
part of the alum turned to solid particles.

When the string was in the cooling liquid, it attracted the particles
of alum as they crystallized out of the solution. The force of
adhesion drew the near-by molecules to the string, then these drew the
next, and these drew more, and so on until the crystals were formed.
But when you kept stirring the liquid while it cooled, the crystals
never had time to grow large before they were jostled around to
some other part of the liquid or were broken by your stirring rod.
Therefore they were small instead of large. Stirring or shaking a
solution will always make crystals form more quickly, but it will also
make them smaller.

HOW ROCK CANDY IS MADE. Rock candy is made by hanging a string in a
strong sugar solution or syrup and letting the water evaporate slowly
until there is not enough water to hold all the sugar in solution.
Then the sugar crystals gather slowly around the string, forming the
large, clear pieces of rock candy. The sugar around the mouth of a
syrup jug is formed in the same way.

You always get crystallization when you make a liquid too cool to
hold the solid thing in solution, or when you evaporate so much of
the liquid that there is not enough left to keep the solid thing
dissolved.

When you make fudge, the sugar forms small crystals as the liquid
cools. When a boat has been on the ocean, salt crystals form on t