THE BIOLOGY OF BACTERIA 3 



tubes or drawn through strong acid the result was the same 

 as if no air entered at all, viz., no organisms and no decom- 

 position. The result of these investigations was that scien- 

 tific men began to believe that no form of life arose de novo 

 {abiogenesis), but had its source in previous life {biogenesis). 

 It remained to Pasteur and Tyndall to demonstrate this 

 beyond dispute, and to put to rout the fresh arguments for 

 spontaneous generation which Pouchet had advanced as late 

 as 1859. Pasteur collected the floating dust of the air, and 

 found by means of the microscope many organised particles, 

 which he sowed on suitable infusions, and thus obtained rich 

 crops of " animalculae." He also demonstrated that these 

 organisms existed in different degrees in different atmo- 

 spheres, few in the pure air of the Mer de Glace, more in 

 the air of the plains, most in the air of towns. He further 

 proved that it was not necessary to insist upon hermetic 

 sealing or cotton filters to keep these living organisms in the 

 air from gaining access to a flask of infusion. If the neck of 

 the flask were drawn out into a long tube and turned down- 

 wards, and then a little upwards, even though the end be left 

 open, no contamination gained access. Hence, if the infusion 

 were boiled, no putrefaction would occur. The organisms 

 which fell into the open end of the tube were arrested in the 

 condensation water in the angle of the tube ; but even if that 

 were not so, the force of gravity acting upon them prevented 

 them from passing up the long arm of the tube into the neck 

 of the flask. A few years after Pasteur's first work on this 

 subject Tyndall conceived a precise method of determining 

 the absence or presence of dust particles in the air by passing 

 a beam of sunlight through a glass box before and after its 

 walls had been coated with glycerine. Into the floor of the 

 box were fixed the mouths of flasks of infusion. These 

 were boiled, after which they were allowed to cool, and 

 might then be kept for weeks or months without putrefying 

 or revealing the presence of germ life. Here all the con- 



