Pasteur's experiment, illustration. This experiment, undertaken in 1859 by French microbiologist Louis Pasteur (1822-1895), disproved spontaneous generation, the belief that living organisms could arise from non-living matter. Pasteur added nutrient broth to two flasks and boiled them to sterilise the broth. Both flasks were open, but one had a straight neck and the other an s-shaped, or swan, neck that prevented airborne microbes from entering. The broth in the straight-necked flask quickly became cloudy, but the broth in the swan-necked flask remained clear. If spontaneous generation was possible, the swan-necked flask would have eventually become cloudy as life formed in it.