“So you see that outside our bodies we have bacteria working for or against us. The nitrifying bacteria are helping to pile up further supplies of nitrogen compounds upon which the plants can draw and whereon, indirectly, we ourselves can be supported. The denitrifying bacteria, on the other hand, are continually nibbling at the basic store of our food; decomposing the nitrogen compounds and freeing the nitrogen from them in the form of the pure gas which is useless to us from the point of view of food.” “You mean that a large increase in the numbers of the one set would put us in clover, whereas multiplication of the other lot would mean a shortness of supplies?” “Exactly. And we have no idea of the forces which govern the reproduction of these creatures. It’s quite within the bounds of possibility that some slight change in the external conditions might reinforce one set and decimate the other; and such a change would have almost unpredictable influences on our food problem.” At this moment the thunder-clouds, which had grown heavier as time passed, evidently reached their full tension. A tremendous flash shot across the sky; and on its heel, so close as to be almost simultaneous, there came a shattering peal of thunder. We looked out; but I had been so dazzled by the brilliance of the flash that I could see little. The air was very still; no rain had yet fallen; and my skin tingled with the electrical tension of the atmosphere. Wotherspoon felt it also, he told me. It was evident that we were in the vicinity of some very powerful disturbance. “Awfully hot to-night, isn’t it?” I said. “Suppose we have some more air? It’s stifling in here.” Wotherspoon pushed the broad leaves of the French windows apart; but no breeze came to cool us; though in the silence after the thunder-clap I heard the rustle of leaves from the trees below us. We stood, one at either end of the bench with the cultures on it, trying to draw cooler air into our lungs; and all the while I felt as though a multitude of tiny electric sparks were running to and fro upon the surface of my body. Suddenly, over St. Katherine’s House, a sphere of light appeared in the air. It was not like lightning, brilliantly though it shone. It seemed to hover for a few seconds above the roof, almost motionless. Then it began slowly to advance in a wavering flight, approaching us and sinking by degrees in the sky as it came. To me, it appeared to be about a foot in diameter; but Wotherspoon afterwards estimated it at rather less. In any case, it was of no great size; and its rate of approach was not more than five miles an hour. For some seconds I watched it coming. It had a peculiar vacillating motion, rather like that which one sees in the flight of certain kinds of summer flies. Now it would hover almost motionless, then suddenly it would dart forward for twenty yards or so, only to resume its oscillation about a fixed point.