COLOUR INPHOSPHORESCENCE OF THE SEA The natural colour of perfectly pure water is most probably blue, but in small quantities the bluish tint cannot be perceived, and the water appears absolutely colourless. The colour of the sea varies from a greenish tint in shallow water, to a rich indigo blue in the open ocean, the depth or intensity of the colour increasing with the depth of the water. The greenish colour of shallow water is probably an effect pro duced by the reflection of the generally yellow colour of the sandy bottom through the natural blue tint of the water. The degree of saltness as well as the depth of the water seems to affect the richness or depth of the colour. Thus the waters of the trade-wind region, having a maximum degree of saltness, are dark blue, while the comparatively fresher and northern seas are light green. The Mediterranean generally appears of a deep azure blue, and the Gulf Stream can be readily distinguished by the deep indigo blue of its waters.
Besides these variations of colour in the open sea, due to varying depth and saltness, certain areas have peculiar colours due to local causes, e.g., the Yellow Sea off China, Vermilion Sea (Californian Gulf), Sargasso Sea in mid-Atlantic, Red Sea, &c. The Yellow Sea is so called from the tint given to its waters by the vast amount of mud brought down by large rivers entering it. The Red Sea is a misnomer, the general colour of the channel being deep blue, the reddish tint being observable only along its coralline shores. Occasionally a reddish tint has been observed in the south Atlantic, off the mouth of the La Plata, as well as in certain parts of the Pacific. Not unfrequently the waters of the Gulf of Guinea are of a whitish colour; while in the northern seas, off Green land, alternate streaks of green and blue have been observed.
The streaked appearance is probably due to streams of fresher surface water, derived from melting icebergs. The
peculiar colours of the other areas mentioned are most probably due to the presence of vast numbers of minute marine organ isms, both animal and vegetable.
The phosph,orescence or luminosity of the sea is one of the most striking and beautiful of all its numerous phenomena Although varying with the degree of light, state of the weather, and time of the year, it is probably observable in every part of the ocean. Mr. Skertchley, of Her Majesty's Geological Survey, thus describes' this truly beautiful phenomenon : " We have seen it fitfully gleaming over the sleeping waters, and again gilding with burning light every wave and ripple within sight, sometimes so pale as to seem spectral in its whiteness, sometimes glowing in a ship's wake so powerfully that books may be read by its light. . . . Great globes of fire well up from beneath the ship's keel, or irregular patches, constantly changing in outline, float around. Every now and then a flash of fire is propagated across the surface, and all remains dark until illuminated by another similar display." This luminous appearance, generally of a pale greenish colour, is much more perceptible under certain conditions— as on a dark night, when the still, calm water is disturbed by a passing vessel or boat ; the sharp bow of the ship seems, as it were, to strike fire from the formerly dark glassy surface of the water, and the oars seem to dip into a halo of light. These beautiful effects are due to the presence near the surface of vast numbers of various minute luminous animals, and probably also to floating particles of decaying animal matter. The phosphorescent appearance of decaying fish by night is a well known fact, and may afford a partial explanation of this phenomenon, which is, however, generally due to living organisms, and not to decaying matter.