Ozone is a colourless gas which possesses a characteristic smell. When strongly cooled it condenses to an indigo blue liquid which is extremely explosive (see Liquid Gases). In ozonizing oxygen the volume of the gas diminishes, but if the gas be heated to about 300° C, it returns to its original volume and is found to be nothing but oxygen. The same change of ozone into oxygen may be brought about by contact with platinum black and other substances. Ozone is only very slightly soluble in water. It is a most powerful oxidizing agent, which rapidly attacks organic matter (hence in preparing the gas, rubber connexions must not be used, since they are instantly destroyed), bleaches vegetable colouring matters and acts rapidly on most metals.