Now, bacteria are the very smallest living organisms of which we have cognizance. Millions of them can lie on a penny; therefore, to produce the gleaming appearance recognized by us as phosphorescence, they must be present in numbers too enormous even to contemplate with our finite minds. It would be immeasurably easier to reckon with the stars for multitude than with these phosphorescent bacteria. They are colorless, rod-like bodies, only known to us in the land revealed by the highest powers of the microscope, and careful comparison shows minor differences among them. For instance, some of them are capable of independent motion--we can hardly call it swimming--others are non-motile, some are inclosed in a jelly-like covering, others are without this sheath. Their power of motion is probably due to excessively fine hairs at their extremities, which, moving to and fro in the water, act the part of oars. These cilia have not been found in all forms of bacteria which move, but their presence is inferred, since every advance in the study of motile forms increases the number of bacteria which are seen to possess them. These light-producing bacteria are known as photo-bacteria, and so far some half-dozen varieties have been distinguished and named. The names in such cases are usually either given from the locality of their appearance (thus, photo-bacterium Balticum, found in the Baltic), from their discoverer (for example, photo-bacterium Fischeri, after Professor Fischer), or from some striking attribute (to wit, photo-bacterium phosphorescens, the commonest light-giving species). A Dutchman named Beyerinck has made a special study of these photo-bacteria, and has experimented with them in a great number of ways to determine, if possible, why they should thus become illuminated, and if the light plays any notable part in their life-history; but his results are, seemingly, all more or less of a negative nature. He can not find that it has any very important function. The breathing of these tiny organisms is not, apparently, in any way bound up with it; their nutrition, growth, and development go on quite well even if they are placed under such conditions that their luminosity is arrested; in no way, indeed, is it a vital process. It only seems to depend on the food which the bacteria feed upon and the presence of oxygen. Given suitable food and plenty of fresh air, and they exhibit their characteristic light; deprive them of one or the other and they no longer shine.