of powdered gold and silver ores, to swallow up and bring out every grain of the precious metals. How they get the gold and silver away from the Mercury when it swallows them, is another story. (See Mercury, Metallurgy, Gold.)
THUMB PRINT AUTOGRAPHS
Look at the ball of your right thumb very closely. It would be better to look at it through a reading glass or microscope. It is covered with very fine lines that sweep in curves around a center. Sometimes these lines form a circle in the middle; sometimes an oval, sometimes a letter U, or a spiral like a letter S. Press the ball of your thumb on an ink pad such as is used for stamping. Then press it onto a sheet of white paper. Try until you get a clear print of the lines with no smears. That is your thumb autograph. No other thumb in the world, not even your own left thumb, will make a print just like it. Take the thumb prints of all the members of your family and of your friends, and see if you can find any two alike. Thumbs are like faces. No two are alike. This is curious, but you can't see how it is of any use. It is. When a thief or other criminal is caught, a photograph is taken of his face and this is kept by the police. But it has been found that a man cannot always be known by his face. If a smooth-faced man grows a beard it changes his appearance. Age changes faces, too. But a thumb ball has lines that do not change. So now, besides the photograph, the police take thumb prints of the bad men they catch.
FAIRY PRINCE ECHO
Nearly all ancient peoples had pretty, poetic stories about the echo. Some, very likely, thought merry little fairies really lived in rocky caves and valleys, to shout and laugh back at them. No one really knew what an echo was until wise men discovered that sound travels in waves, just as water and light and electricity travel. (See Acoustics, Wave-Motion and "Sound Waves and the Telephone.") If a wave of water is stopped by a break-water or a cliff, it is thrown back into the sea. So if a sound is stopped by a wall it is thrown back to our ears. A big empty building, especially if windows and doors are closed so the sound waves cannot escape, is a line place for echoes. Best of all is a rock-walled glen in hilly country. The sound very seldom comes back just as it was made. When a wave of water is stopped it is broken up and thrown back
