It appears that when light is reflected from a cloud of fine colourless particles so as to give a predominant blue colour, the light so reflected is affected in that special way which physicists describe as being "polarized." It is possible by the use of certain apparatus (the polariscope) to distinguish polarized from non-polarized light, so that it should be possible to decide (or at any rate to gain evidence) whether blue water–a sheet of blue water–owes its colour to fine particles suspended in it or to the self-colour of the water. An admirable case for making this simple experiment is presented by the great tanks–some 20 ft. cube–which are used by the water companies which draw their water supply from the chalk, for the purpose of precipitating the dissolved chalk–"Clarking" the water, as it is called, after the inventor of the process–and so getting rid of its excessive "hardness." Such tanks are to be seen by the side of the railway near Caterham. The water in these tanks is of such a brilliant turquoise blue that many people suppose that copper has been added to the water to free it from microbes! Such, at any rate, was the conviction expressed by a friend in conversation with me only a few weeks ago. The water in these tanks, when seen from the railway, looks like a magnificent blue dye, and a very important point for those (not a few) who believe that the blue colour of seas and lakes is due to the reflection of the blue colour of the sky overhead is that the water in the tanks looks just as blue when the sky is overcast with cloud as when there is blue sky. The blue colour of water has, as a rule, nothing to do with the reflection of the sky, though it is the fact that a shallow film of water may at a certain angle reflect the sky to our eyes, just as a mirror may. The effect is quite unlike that due to light passing through deep water when reflected from below it. If we examine the tanks in question we find that they have been filled with water pumped from the chalk, and that then lime has been added to the water in order to combine with the carbonic acid dissolved in it and form chalk or carbonate of lime–which is insoluble in pure water and falls as an excessively fine white powder to the bottom of the tank. But the important fact is that water having carbonic acid dissolved in it can dissolve carbonate of lime or chalk to a certain amount: and this water pumped from the chalk, having carbonic acid naturally dissolved in it, has consequently also dissolved a quantity of chalk. It is this which gives the chalk-spring water the objectionable quality of "hardness." When lime is added to the chalk-spring water as pumped into the tanks, the carbonic acid in it is taken up by the lime, and the chalk previously dissolved by the carbonic-acid-holding water is, so to speak, "undissolved," and thrown down as a very fine white powder, together with the chalk newly formed by the union of the lime and the carbonic acid.