From Spallanzani to Schultze, there were no further experiments to prove or disprove spontaneous generation. Schultze, in 1836, attempted to meet the second objection to Spallanzani’s experiment, _i.e._, the exclusion of air, by drawing air through his boiled infusions, first causing it to bubble through concentrated sulphuric acid to kill the “germs” (Fig. 3.). His flasks fortunately showed no growths, but his critics claimed that the strong acid changed the properties of the air so that it would not support life. This experiment of Schultze’s, though devised for a different purpose, was really the first _experiment_ in the use of _chemical disinfectants_, though Thaer (page 31) had used chemicals in a practical way. Schwann, in 1837, modified this experiment, by drawing the air through a tube heated to destroy the living germs (Fig. 4). His experiments were successful but the “spontaneous generation” theorists raised the same objection, _i.e._, the change in the air by heating. This was the first _experiment_ in which the principle of “_dry heat_” or “_hot air_” sterilization was used. Similar arguments were brought forward, also to the use of _cotton plugs_ as filters by Schroeder and Dusch in 1859 (Fig. 5). This was the first use of the principle of _sterilization by filtration_. It remained for Chevreuil and Pasteur to overcome this objection in 1861 by the use of flasks with long necks drawn out to a point and bent over. These permitted a full access of air by diffusion but kept out living germs, since these cannot fly but are carried mechanically by air currents or fall of their own weight (Fig. 6.). Hoffman, the year before (1860), had made similar experiments but these remained unnoticed. The Pasteur flasks convinced most scientists that “spontaneous generation” has never been observed by man, though some few, notably Dr. Charlton Bastian, of England, vigorously supported the theory from the early seventies until his death in November, 1915. [Illustration: FIG. 4.--Schwann’s experiment. After boiling, as shown in the diagram, and cooling, air was drawn into the flask by aspiration while the coiled tube was kept hot with the flame.] [Illustration: FIG. 5.--Schroeder and Dusch’s experiment. The aspirating bottle drew the air through the flask after it had been filtered by the cotton in the tube.] [Illustration: FIG. 6.--Pasteur’s flask.] [Illustration: FIG. 7.--Tyndall’s box. One side is removed to show the construction. The bent tubes at the top are to permit a free circulation of air into the interior. The window at the back has one corresponding in the front (removed). Through these the beam of light sent through from the lamp at the side was observed. The three tubes received the infusion and were then boiled in an oil bath. The pipette was for filling the tubes. (Popular Science Monthly, April, 1877.).]