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by obtaining their nitrogen from ammonia. The energy liberated during the oxidation of the nitrogen is regarded as splitting the carbon-dioxide molecule,—in green plants it is the energy of the solar rays which does this. Since the supply of free oxygen is dependent on the activity of green plants the process is indirectly dependent on energy derived from the sun, but it is none the less an astounding one and outside the limits of our previous generalizations. It has been suggested that urea is formed by polymerization of ammonium carbonate, and formic aldehyde is synthesized from CO„ and OH2. The Nitro-bacteria are smaller, finer, and quite different from the nitroso-bacteria, and are incapable of attacking and utilizing ammonium carbonate. When the latter have oxidized ammonia to nitrite, however, the former step in and oxidize it still further to nitric acid. It is probable that important consequences of these actions result from the presence of nitrifying bacteria in rotten stone, decaying bricks, &c., where all the conditions are realized for preparing primitive soil, the breaking up of the mineral constituents being a secondary matter. That “ soil ” is thus prepared on barren rocks and mountain peaks may be concluded with some certainty. In addition to the bacterial actions which result in the oxidization of ammonia to nitrous acid, and of the latter to nitric acid, the reversal of such processes is also brought about by numerous bacteria in the soil, rivers, &c. Warington showed some time ago that many species are able to reduce nitrates to nitrites, and such reduction is now known to occur very widely in nature. The researches of Gayon and Dupetit, Giltay and Aberson and others have shown, moreover, that bacteria exist which carry such reduction still further, so that ammonia or even free nitrogen may escape. The importance of these results is evident in explaining an old puzzle in agriculture, viz., that it is a wasteful process to put nitrates and manure together on the land. Fresh manure abounds in denitrifying bacteria, and these organisms not only reduce the nitrates to nitrites, even setting free nitrogen and ammonia, but their effect extends to the undoing of the work of what nitrifying bacteria may be present also, with great loss. The combined nitrogen of dead organisms, Fig. 6.—Stages iu the formation of a colony of a variety of Bacillus (Proteus) broken down to ammonia by putrefactive bacteria, the vulgaris (Hauser), observed in a hanging drop. At 11 a.m. a rodlet appeared (A)

at 4 p.m. it had grown and divided and brokenofupurea,

into eight rodlets andunder thea results of the fixation of free (B)

C shows further development atammonia

8 p.m., D at 9.30 p.m.—all high power. At E, F, and G further stages are drawn, as seen under much nitrogen, together with traces of nitrogen salts due to lower power. (Original.) meteoric activity, are thus seen to undergo various quantities of nitrate (saltpetre) are formed on exposed vicissitudes in the soil, rivers, and surface of the globe heaps of manure, &c., and it was supposed that direct generally. The ammonia may be oxidized to nitrites and oxidation of the ammonia, facilitated by the presence of nitrates, and then pass into the higher plants and be porous bodies, brought this to pass. But research showed worked up into proteids, and so be handed on to animals, that this process of nitrification is dependent on tempera- eventually to be broken down by bacterial action again ture, aeration, and moisture, as is life, and that while to ammonia; or the nitrates may be degraded to nitrites nitre-beds can infect one another, the process is stopped and even to free nitrogen or ammonia, which escapes. Another important advance is in our knowledge of the by sterilization. Warington, Schloesing, Muntz, and others had proved that nitrification was promoted by part played by bacteria in the circulation of carbon in some organism, when Winogradsky hit on the happy idea nature. The enormous masses of cellulose de- ellutose of isolating the organism by using gelatinous silica, and posited annually on the earth’s surface are, as bacteria. so avoiding the difficulties which Warington had shown to we know, principally the result of chlorophyll exist with the organism in presence of organic nitrogen, action on the carbon-dioxide of the atmosphere decomposed owing to its refusal to nitrify on gelatine or other nitro- by energy derived from the sun; and although we know genous media. Winogradsky’s investigations resulted in little as yet concerning the magnitude of other processes the discovery that two kinds of bacteria are concerned in of carbon-assimilation—e.g., by nitrifying bacteria—it is nitrification: one of these, which he terms the Nitroso- probably comparatively small. Such cellulose is gradually bacteria, is only capable of bringing about the oxidation reconverted into water and carbon - dioxide, but until of the ammonia to nitrous acid, and the astonishing result recently nothing positive was known as to the agents was obtained that this can be done, in the dark, by which thus break up the paper, rags, straw, leaves, and bacteria to which only pure mineral salts—e.g., carbonates, wood, &c., accumulating in cesspools, forests, marshes, and sulphates, and chlorides of ammonium, sodium, and elsewhere in such abundance. The work of van Tieghem, magnesium—were added. In other words these bacteria van Senus, Fribes, Omeliansky, and others has now shown can build up organic matter from purely mineral sources by that while certain anaerobic bacteria decompose the subassimilating carbon from carbon-dioxide in the dark, and stance of the middle lamella—chiefly pectin compounds—

instance, if protected from access of free oxygen by an envelope of aerobic bacteria or fungi, and provided with the carbohydrates and minerals necessary for its growth, fixes nitrogen in proportion to the amount of sugar consumed. This interesting case of symbiosis is equalled by yet another case. The work of recent observers leaves no room for doubt that the free nitrogen of the atmosphere is brought into combination in the soil in the nodules filled with “ bacteroids ” on the roots of Leguminosese, and since these nodules are the morphological expression of a symbiosis between the higher plant and the bacterialike “ bacteroids,” there is evidently here also a case similar to the last, though it is as yet impossible to say where and how the nitrogen is fixed. As regards the ammonium carbonate accumulating in the soil from the conversion of urea and other sources, we know from Winogradsky’s researches that it undergoes oxidation in two stages owing to the activity of the socalled “nitrifying” bacteria (an unfortunate term inasmuch as “nitrification” refers merely to a particular phase of the cycle of changes undergone by nitrogen). It had long been known that under certain conditions large