The investigation of the question requires more critical methods than was at first supposed, and more factors enter into its solution than were realized by Spallanzani and Schwann. Pasteur demonstrated that the floating particles of the air contained living germs, by catching them in the meshes of gun cotton, and then dissolving the cotton with ether and examining the residue. He also showed that sterilized organic fluids could be protected by a plug of cotton sufficiently porous to admit of exchange of air, but matted closely enough to entangle the floating particles. He showed also that many of the minute organisms do not require free oxygen for their life processes, but are able to take the oxygen by chemical decomposition which they themselves produce from the nutrient fluids. Jeffries Wyman, of Harvard College, demonstrated that some germs are so resistant to heat that they retain their vitality after several hours of boiling. This fact probably accounts for the difference in the results that have been obtained by experimenters. The germs in a resting-stage are surrounded by a thick protective coat of cellulose, which becomes softened and broken when they germinate. On this account more recent experimenters have adopted a method of discontinuous heating of the nutrient fluid that is being tested. The fluids are boiled at intervals, so that the unusually resistant germs are killed after the coating has been rendered soft, and when they are about to germinate. After the brilliant researches of Pasteur, the question of spontaneous germination was once again regarded as having been answered in the negative; and so it is regarded to-day by the scientific world. Nevertheless, attempts have been made from time to time, as by Bastian, of England, in 1872, to revive it on the old lines. [Illustration: Fig. 91.--Apparatus of Tyndall for Experimenting on Spontaneous Generation.] Tyndall.--John Tyndall (1820-1893), the distinguished physicist, of London, published, in 1876, the results of his experiments on this question, which, for clearness and ingenuity, have never been surpassed. For some time he had been experimenting in the domain of physics with what he called optically pure air. It was necessary for him to have air from which the floating particles had been sifted, and it occurred to him that he might expose nutrient fluids to this optically pure air, and thus very nicely test the question of the spontaneous origin of life within them.