Demonstrate how gases expand when heated with this amazing Hand Boiler. Charles's Law (also known as the law of volumes) is an experimental gas law which describes how gases tend to expand when heated. Hold the bottom bulb in the palm of your hand and wrap your fingers around it. The heat of your hand increases the temperature of the gas in the bottom bulb. The increase in temperature creates an increase in air pressure and the gas expands. This increased pressure pushes the liquid in the bottom bulb up the tube to the top bulb. When all of the liquid has transfered from the bottom bulb, air bubbles are forced up the tube from the bottom and this causes the liquid in the top to look like it is boiling. The liquid comes in different colours: red, blue, purple and green. A random colour will be sent.