Page:Life in Motion.djvu/29

Rh the temperature of the fluid rises, and the yeast cells grow and multiply, living, as it were, on the sugar and other nutritive matters in the fluid.

Let us pass to another experiment. I have prepared a saturated solution of acetate of sodium—that is, a solution which cannot be made stronger. You observe it is a clear fluid like water. I now drop in a crystal of acetate of sodium. You see at once crystals shooting through the fluid, and in a few moments the mass in the flask has become solid. The flask has also become perceptibly hotter. The agitation excited by dropping in the crystal has caused a rapid change in the position of the particles, the solution passing from the fluid to the solid state with the evolution of heat. This is another example of a molecular movement.

Consider next an experiment in which a state of movement can be appreciated by the eye. Look at the limbs of this large tuning-fork. You observe they are stationary. I strike the fork, and you see it is at once thrown into a state of vibration, as shown by the fuzzy appearance of the limbs when placed