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Show 76 ORIGINATION OF LIVING BEINGS. mouth of the vessel with a red-hot tube in such a way that the air would have to pass through the tube before reaching the infusion, that then you would get no anilnalcules. Yet another thing was noticed : if you took two flasks containing the same kind of infusion, and left one entirely exposed to the air, and in the mouth of the other placed a ball of cotton wool, so that the air would have to filter itself through it before reaching the infusion, that then, although you might have plenty of animalcules in the first flask, you would certainly obtain none from the second. These experiments, you see, all tended towards one conclusion- that the infusoria were developed from little minute spores or eggs which were constantly floating in the atmosphere, which lose their power of germination if subjected to heat. But one observer now made another experiment, which seemed to go entirely the other way, and puzzled him altogether. He took some of this boiled infusion that I have been speaking of, and by the use of a mercurial bath-a kind of trough used in laboratories-he deftly inverted a vessel containing the infusion into the mercury, so that the latter reached a little beyond the level of the mouth of the inverted vessel. You see that he thus had a quantity of the infusion shut off from any possible communication with the outer air by being inverted upon a bed of mercury. He then prepared some pure oxygen and nitrogen gases, and passed them by means of a tube going from the outside of the vessel, up through the mercury into the infusion; so that he thus had it exposed to a per- ORIGINATION OF LIVING BEINGS. 77 fectly pure atmosphere of the same constituents as the external air. Of course, he expected he would get no infusorial animalcules at aU in that infusion '· but, to his great dismay and discomfiture, he found he almost always did get them. Furthermore, it has been found that experiments made in the manner described above answer well with m·ost infusions; but that if you fill the vessel with boiled milk, and then stop the neck with cotton-wool, you will have infusoria. So that you see there were two experiments that brought you to one kind of conclusion, and three to another; which was a most unsatisfactory state of things to arrive at in a scientific inquiry. Some few years after this, the question began to be very hotly discussed in France. There was M. Pouchet, a professor at Rouen, a very learned man, but certainly not a yery rigid experimentalist. He published a number of experiments of his own, some of which were very ingenious, to show that if you went to work in a proper way, there was a truth in the doctrine of spontaneous generation. Well, it was one of the most fortunate things in the world that M. Pouchet took up this question, because it induced a distinguished French chemist, M. Pasteur, to take up the question on the other side; and he has certainly worked it out in the most perfect manner. I am glad to say, too, that he has published his researches in time to enable me to give you an account of them. He verified all the experiments which I have just mentioned to youand then finding those extraordinary anomalies, as in the case of the mercury bath and the milk, he set |