Most people know and admire the splendid expanse of blue colour offered by the clear sea water on many parts of our coasts, and by that of lakes at home and abroad. I find that there is still a sort of a fixed determination not to believe that this colour is due (as it is) to the actual blue colour of pure water. Pure, transparent water is blue. Those who think they know better will point to a glass of pure water, hold it up to the light, and affirm that it is colourless. But this apparent colourlessness is due to the small breadth of water in the glass through which the light passes. It is definitely ascertained that if water as pure and as free from either dissolved or suspended matter as it is possible to make it (by distillation and the use of vessels not acted upon by water) be made to fill an opaque tube 15 ft. long, closed at each end by a transparent plate, and then a beam of light be made to traverse the length of the tube, so that the eye receives the light after it has passed through this length of 15 ft. of water, the colour of the light is a strong blue. Water is blue in virtue of its own molecular character, just as sulphate of copper is. Liquid oxygen, prepared by the use of intense cold, is also transparent blue, and the peculiar condensed form of oxygen known as "ozone" is, when liquefied, of a darker or stronger blue than oxygen. At one time (some thirty years ago) there was still some doubt as to whether water was self-coloured blue, or whether its blue colour was due to the action on light of excessively minute solid white particles of chalk suspended in the water. Such fine suspended particles in some cases act on the light which falls on to them so as to reflect the blue rays. This occurs in certain natural objects which have a blue colour. But these can be distinguished from transparent self-coloured blue substances by the fact that whilst the light reflected from their surface is blue, the light which is made to traverse them (when they are held up to the light so that they come between one's eye and the sun's rays) is brown. This is the case with very hot smoke, and can be well seen when a cigar is smoked in the sunlight. The smoke which comes off from the lighted end of the cigar is very hot, and its particles are more minute than those of cooler smoke. The hot smoke shows a bright blue colour when the sunlight falls on it and is reflected, but when you look through the smoke-cloud at a surface reflecting the sunlight, the cloud has a reddish-brown tint. As the smoke cools its particles adhere to one another and form larger particles, and the light reflected from the cloud is no longer blue but grey, and even white. Thus the smoke which the smoker keeps for half a minute in his mouth is cooled and condensed, and reflects white light–is, in fact, a white cloud–when he puffs it out, and contrasts strongly with the blue cloud coming off from the burning tobacco at the lighted end of the cigar.