2 Heat Is Carried From the Pail Partly by Convection
, except where solid insulating material, such as wood or indurated fibre, is used; and that manner of packing which best entangles the air and prevents air currents will, therefore, most increase the effectiveness of the insulation.
Experiment: Convection
Into a glass flask of cold water drop a few crystals of potassium permanganate, being careful not to agitate the flask. Apply a flame to the bottom of the flask. As the water becomes heated its density is reduced and it rises, forming convection currents which are coloured by the permanganate and may be distinctly seen.
Convection currents may be formed in any liquid or gas; for instance, air. By means of them heat will be carried from one part of the liquid or gas to another. Thus air heated by contact with a kettle of food will, if allowed to flow freely, carry the heat away from the food.