Make a strong infusion of the leaves of the red cabbage, which will be of a beautiful _blue_ color; drop into it a few drops of dilute sulphuric acid, and the color will change to a bright red; add some solution of carbonate of potash, or soda, and the red color will gradually give way to the original blue; continue adding the alkaline solution, and the fluid will assume a bright _green_ color. Now resume the acid, and as it is dropped in, the color will again change from green to blue, and from blue to red. Now this simple experiment illustrates three points: first, that acids change the color of most vegetable blues and greens to red; second, that alkalies change most blues and reds to green; and third, that when the acid and alkali are united together, they both lose their property of changing color, and become what is called a _neutral_ salt, _i.e._ a compound possessing the properties of _neither_ of its constituents. ACOUSTICS. Acoustics is the science relating to sound and hearing. Sound is heard when any shock or impulse is given to the air, or to any other body which is in contact directly or indirectly with the ear. Difference Between Sound and Noise. Noises are made by the crack of whips, the beating of hammers, the creak of a file or saw, or the hubbub of a multitude. But when a bell is struck, the bow of a violin drawn across the strings, or the wetted finger turned round a musical glass, we have what are properly called sounds. Sounds, How Propagated. Sounds are propagated on all bodies much after the manner that waves are in water, with a velocity of 1,142 feet in a second. Sounds in liquids and in solids are more rapid than in air. Two stones rubbed together may be heard in water at half a mile; solid bodies convey sounds to great distances, and pipes may be made to convey the voice over every part of the house. To Show How Sound Travels Through a Solid. Take a long piece of wood, such as the handle of a hair broom, and placing a watch at one end, apply your ear to the other, and the tickings will be distinctly heard. To Show That Sound Depends on Vibration. Touch a bell when it is sounding, and the noise ceases; the same may be done to a musical string with the same results. Hold a musical pitch-fork to the lips, when it is made to sound, and a quivering motion will be felt from its vibrations. These experiments show that sound is produced by the quick motions and vibrations of different bodies. Musical Figures Resulting From Sound. Cover the mouth of a wine-glass, having a foot-stalk, with a thin sheet of membrane, over which scatter a layer of fine sand. The vibrations excited in the air by the sound of a musical instrument, held within a few inches of the membrane, will cause the sand on its surface to form regular lines and figures with astonishing celerity, which vary with the sound produced. To Make an Æolian Harp.