In surgical spirit, the hydrometer floats at a lower level compared to ordinary water. This shows that surgical spirit is lighter than water. Stir with a spoon and keep adding salt until you feel a gritty layer building up on the bottom of the pan. Leave the salty water for several hours until it is no longer cloudy. Then it is ready to use.
Half fill a large jar with the salty water and put the egg into the water.
Now carefully pour some ordinary tap water down the side of the jar. The egg will seem to float in the middle of the jar. The tap water is lighter than the salty water so it floats on top. The egg sinks down through the tap water but floats on top of the salty water. It looks as if it is floating in the middle of the jar.