Part 18

Time and space (or rather the want of them) compel us to conclude with a few experiments of a miscellaneous character.

To Form a Solid From Two Liquids.

Prepare separately, saturated solutions of sulphate of magnesia (Epsom salts) and carbonate of potash. On mixing them the result will be nearly solid.

Solutions of muriate of lime and carbonate of potash will answer as well.

To Form a Liquid From Two Solids.

Rub together in a Wedgewood mortar a small quantity of sulphate of soda and acetate of lead, and as they mix they will become liquid.

Carbonate of ammonia and sulphate of copper, previously reduced to powder separately, will also, when mixed, become liquid, and acquire a most splendid blue color.

The greater number of salts have a tendency to assume regular forms, or become crystallized, when passing from the fluid to the solid state; and the size and regularity of the crystals depend in a great measure on the slow or rapid escape of the fluid in which they were dissolved. Sugar is a capital example of this property; the ordinary loaf-sugar being rapidly boiled down, as it is called: while to make sugar-candy, which is nothing but sugar in a crystallized form, the solution is allowed to evaporate slowly, and as it cools it forms into those beautiful crystals termed sugar-candy. The threads found in the center of some of the crystals are merely placed for the purpose of hastening the formation of the crystals.

Experiments.

1. Make a strong solution of alum, or of sulphate of copper, or blue vitriol, and place in them rough and irregular pieces of clinker from stoves, or wire-baskets, and set them by in a cool place, where they will be free from dust, and in a few days crystals of the several salts will deposit themselves on the baskets, etc.; they should then be taken out of the solutions, and dried, when they form very pretty ornaments for a room.

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.

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.

This instrument consists of a long, narrow box of very thin deal, about six inches deep, with a circle in the middle of the upper side of an inch and a half in diameter, in which are to be drilled small holes. On this side seven, ten or more strings of very fine catgut are stretched over bridges at each end, like the bridges of a fiddle, and screwed up or relaxed with screw-pins. The strings must all be tuned to one and the same note, and the instrument should be placed in a window partly open, in which the width is exactly equal to the length of the harp, with the sash just raised to give the air admission. When the air blows upon these strings with different degrees of force, it will excite different tones of sound. Sometimes the blast brings out all the tones in full concert, and sometimes it sinks them to the softest murmurs.

A colossal imitation of the instrument just described was invented at Milan in 1786 by the Abbate Gattoni. He stretched seven strong iron wires, tuned to the notes of the gamut, from the top of a tower sixty feet high, to the house of a Signor Moscate, who was interested in the success of the experiment; and this apparatus, called the “giant’s harp,” in blowing weather yielded lengthened peals of harmonious music. In a storm this music was heard at a greater distance.