The blue colour of the sky is held by many physicists (though other views have been of late advanced) to be due to the same action on the part of the very finest particles of watery vapour, which are diffused through vast regions of our atmosphere above the condensed white-looking clouds consisting of larger floating particles of water. Vapours are given off by many liquids, and even by some solids, varying in their production according to the heat applied in different cases. They are gases, and quite transparent and invisible at the proper temperature, like the atmospheric air. Thus water is always giving off "water-vapour," which is quite invisible. When water is heated to the boiling point it is rapidly converted into transparent invisible vapour. Steam, as this vapour is called, is invisible, and we all habitually make a misleading use of the word "steam" when we apply it both to this and to the slightly cooled and condensed cloud which we can see issuing from the spout of a kettle or from a railway engine. It seems that the fault lies with the scientific writers, who have applied the word "steam" to the invisible water vapour or gas before it has condensed to form a cloud. The old English word "steam" certainly means a visible cloudy emanation, and not a transparent invisible gas. A cloud is not a vapour, but is produced by the coming together or condensation of the minute invisible particles of a vapour to form larger particles, which float and hang together, and reflect the light, and thus are visible.