Some Noctilucæ emit a clear blue light, but when the same animal is disturbed it appears white with green and blue flashes of great beauty and intensity--a telling illustration of the boundless, and marvelous, resources of nature. Many interesting experiments have been tried with these dainty light givers. A tube fifteen millimeters in diameter was filled with them, and by shaking this novel lamp a printed page was read a foot distant; yet when a delicate thermometer was thrust into the fiery mass, the mercury was not affected in the slightest, showing that here was that wonder of wonders--vivid light without heat--a secret which man has vainly endeavored to wrest from nature. The vast number of these minute creatures can be realized when it is said that the ship _Magenta_ sailed nearly five hundred miles among swarms of Noctilucæ, which gave splendid displays of phosphorescent light at night. Sometimes the light emitted was milky white; again it was green, or blue, the different species possessing different colors. II. THE SPONGES [Illustration: FIG. 13.--Living sponges.] Almost every day, for several years, I devoted one or more hours to the pastime of floating or drifting over a part of the great coral reef which constitutes the most westward portion of Florida where it reaches out in the direction of Yucatan. The islands composing the group are the Tortugas Keys, and are just above water; indeed some disappeared when a particularly heavy hurricane came, and in the center of the island upon which I lived, the water at very high tide appeared above the surface.