Schwann's apparatus was similar in construction, except that the bent tube on one side was surrounded by a jacket of metal and was subjected to a very high temperature while the air was being drawn through it, the effect being to kill any floating germs that might exist in the air. Great care was taken by both experimenters to have their flasks and fluids thoroughly sterilized, and the results of their experiments were to show that the nutrient fluids remained uncontaminated. These experiments proved that there is something in the atmosphere which, unless it be removed or rendered inactive, produces life within nutrient fluids, but whether this something is solid, fluid, or gaseous did not appear from the experiments. It remained for Helmholtz to show, as he did in 1843, that this something will not pass through a moist animal membrane, and is therefore a solid. The results so far reached satisfied the minds of scientific men, and the question of the spontaneous origin of life was regarded as having been finally set at rest. III. The Third Period. Pouchet.--We come now to consider the third historical phase of this question. Although it had apparently been set at rest, the question was unexpectedly opened again in 1859 by the Frenchman Pouchet, the director of the Natural History Museum of Rouen. The frame of mind which Pouchet brought to his experimental investigations was fatal to unbiased conclusions: "When, _by meditation_," he says, in the opening paragraph of his book on _Heterogenesis_, "it was evident to me that spontaneous generation was one of the means employed by nature for the production of living beings, I applied myself to discover by what means one could place these phenomena in evidence." Although he experimented, his case was prejudiced by metaphysical considerations. He repeated the experiments of previous observers with opposite results, and therefore he declared his belief in the falsity of the conclusions of Spallanzani, Schulze, and Schwann. He planned and executed one experiment which he supposed was conclusive. In introducing it he said: "The opponents of spontaneous generation assert that the germs of microscopic organisms exist in the air, which transports them to a distance. What, then, will these opponents say if I succeed in introducing the generation of living organisms, while substituting artificial air for that of the atmosphere?"