150 



KNOWLEDGE 



[July 1, 1898. 



c and d) the difference between the produce of the best 

 and the worst season approached, and in two cases actually 

 exceeded, the average produce of the plats. 



More than two thousand years ago the Romans recognized 

 the fact that leguminous crops enriched the soil for succeed- 

 ing crops — in short , discovered what is termed the ' ' rotation 

 of crops," a practice which is admitted to be the foundation 

 of the improvements in our own agriculture. How, then, 

 are the admittedly beneficial effects of alternate, as dis- 

 tinguished from continuous, cropping to be explained ? 

 Liebig's first definite theory on this subject assumed that 

 the excreted matters of one description of crop were 

 injurious to plants of the same description, but that they 

 were not so, and might even be beneficial, to other kinds 

 of plants. Later, he considered that, as the dift'erent plants 

 had such diverse mineral requirements, the alternation of 

 one kind with another relieved the soil from exhaustion, and 

 discerned after many years that nitrogen probably played 

 some important part in the matter. Boussiugault, in 

 chemical statistics extending over ten years, came to the 

 conclusion that the difference in the amounts of nitrogen 

 taken up by various crops constituted a very important 

 element in the explanation of the benefits of rotation. 

 Prof. Daubeny, of Oxford, in testing De CandoUe's theory 

 that the excretions of one kind of plant were injurious to 

 plants of the same description, arrived at a negative 

 conclusion, and recognized the validity of Boussingault's 

 argument that the same kind of plant may continue to 

 grow healthier on the same land for long periods of time ; 

 and experience at Rothamsted also is conclusive against 

 the theory of injurious or poisonous excretions. Upon the 

 whole the results at Rothamsted show that the benefits 

 of rotation are very various. The opportunities which 

 alternate cropping affords for cleaning the land constitute 

 a prominent element of advantage. The difference in the 

 amounts available within the soil of the various mineral 

 constituents is one element in the explanation ; but the 

 facts relating to the amount and to the sources of the 

 nitrogen of the different crops are of still greater signifi- 

 cance. The varying requirements of the different crops, 

 habits of growth, and capabilities of gathering and assimi- 

 lating the necessary constituents have to be considered ; 

 with a variety of crops the mechanical operations of the 

 farm, involving horse and hand labour, are better distri- 

 buted over the year, and are, therefore, more economically 

 performed. 



.^ 



"THE MIMIC FIRES OF OCEAN." 



By G. Clabke Nuttall, b.sc. 



NATURE dazzles the eye of man with many wonder- 

 ful phenomena, but perhaps never more so than 

 when she turns the gloomy night waters of the 

 sea into a sheet of silvery fire. At these times 

 every movement of the wave, every cleavage of 

 the water by oar or prow, reveals in its dark depths a 

 hidden fire which scintillates and sparkles with weird 

 and mysterious light. The spectacle is one of absolute 

 fascination, for the Spirit of Enchantment rests upon the 

 waters and reality becomes fairyland. 



The ancients, keenly alive to a sense of the supernatural, 

 saw in this lunnnosity a manifestation of some unknown 

 power, and wondered ; the ignorant read in it a portent of 

 judgment and terror ; while in all ages the curious and the 

 searchers after knowledge have speculated as to its cause. 

 But just as nature has invested its appearance with a halo 

 of mystery, so she has also wrapt in much obscurity its 

 immediate cause ; and thus, though in the course of 

 "cehturies varying suggestions have been put forward, 



nothing with any finality about it has been arrived at. 

 It was asserted truly that certain fishes were luminons ; 

 sharks have glowed and shone, shoals of herrings, pilchards, 

 or mackerel have been moving masses of light, and the 

 fish drawn out of the water have lain in great shining 

 heaps, the glow of which vanished as they dried and died. 



Many writers have described the passages of ships 

 through such shoals — the sheet of moving flames — the 

 beautiful pale greenish elf-light that the fish exhibited ; 

 while poets have apostrophized the " mimic fires of ocean " 

 and the " lightnings of the wave," and scientists and 

 naturalists have in turn tried to account for their power of 

 luminosity. Some have attributed it to the presence of 

 certain substances of a fatty nature excreted by the fish 

 and adhering to the surface of their bodies ; others have 

 declared that it is due to a subtle power of the fish itself 

 — a form in which the energy of life shows itself under 

 certain conditions, just as this energy may be exhibited 

 in heat, or motion, or electricity; others, again, have 

 ascribed it to direct absorption and transmission of the 

 light of the sun, and so on. Many theories have been 

 elaborated, but none convincingly. 



But now, it is asserted, the secret is laid bare. 



It is wonderful how many secrets the searching light of 

 the nineteenth century is claiming to reveal. It is, perhaps, 

 a matter for still more wonder whether in the far future our 

 descendants will endorse all our solutions, or whether they 

 will not smUe at some of them just as we, half contemp- 

 tuously, discredit those of our ancestors. However that 

 may be, we have, in this case, a solution offered to us that 

 apparently approaches nearer the heart of truth than any 

 yet put forward, in that it satisfies the various phases of 

 the phenomenon and gives a unity and coherence to its 

 manifestations. 



It is only lately that any very serious effort has been 

 made to study this phenomenon, but the research has 

 been abundantly rewarded, for it is now pretty certain 

 that the luminosity is due to the presence in the water 

 of various kinds of bacteria. 



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 phosphores- 

 cent bacteria. They are colourless, rodlike 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 swim- 

 ming — others are non-motile, some are enclosed 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 Prof. Fischer), or from some striking attribute (to 

 wit, photo -bacterium phosphorescens, the commonest light- 

 giving species). 



