2. Fill a Florence flask up to the neck with a strong solution of sulphate of soda, or Glauber’s salt, boil it, and tie the mouth over with a piece of moistened bladder while boiling, and set it by in a place where it cannot be disturbed. After twenty-four hours it will probably still remain fluid. Pierce the bladder covering with a penknife, and the entrance of the air will cause the whole mass instantly to crystallize, and the flask will become quite warm from the latent caloric, of which we have spoken before, given out by the salt in passing from the fluid to the solid state. It is better to prepare two or three flasks at the same time, to provide against accidents, for the least shake will often cause crystallization to take place before the proper time.
Changes of Color Produced by Colorless Liquids.
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.