First experiments

Following the first incorrect experiments such as Van Helmont’s one, many others were performed.
In 1668 Doctor Francesco Redi led a series of experiments that were supposed to prove that spontaneous generation does not exist. Redi placed in containers some samples of veal and fish,some he sealed while others he left in the open air. As time went by, he noticed that in the open containers on the decomposed meat there were worms (that were in fact larvae!), flies and other insects, while there were no signs of life in the sealed containers.
In that same period, the Dutch naturalist Anton Van Leeuwenhoek (1632 – 1723) built a rudimental microscope which enabled him to watch microorganisms. This made it possible to see that all substances contained a large number of living organisms, and this obviously brought back the spontaneous generation idea which had apparently been abandoned following Redi’s experiments.
Following numerous examinations with Leeuwenhoek’s instrument new arguments arose among those who sustained the abiogenesis theory (life is born from lifeless substances) and the biogenesis one (life is generated only from living beings).
In 1745 the English naturalist John Needham invented new experiments to prove the abiogenesis theory. He filled some test tubes with chicken broth and herb infusions and then he closed them with some gauze. The test tubes had been sterilized by the heat, nevertheless after a few days hundreds of living beings could be seen inside them. This result reinforced the spontaneous generation theory.
A few years later the abbot Lazzaro Spallanzani, who was not convinced by Needham’s experiment, tried to repeat it heating the nutritious liquid for much longer and at much higher temperatures, actually making it boil for a few minutes. He sealed the test tubes and the result was that even several days later there was no sign of any living organisms in them. As a result, the naturalist Needham, criticized Spallanzani saying that the nutritious liquid had been heated too much, and this had killed the active elements and that sealing the test tubes had not allowed the presence of air which is indispensable for life.
Discussions went on for a long time until half way through the nineteenth century, when a French biologist Louis Pasteur, ran a new experiment that settled the matter. Pasteur created some glass containers with a long curved neck (called “swan neck balloons”). Inside these the nutritious liquid was boiled for over an hour, letting the vapour out through the container’s curved neck. After boiling, the broth inside was left to cool slowly , while the contaminated air carrying microorganisms entered from the outside as a result of the post heating depression. Thus the microscopic organisms that came in contact with the boiling vapour of the liquid inside , would die and even after some months there was no trace of life to be found, instead on the outer part of the container’s neck, one could see dust and microorganisms that were coming in from the outside.
This experiment put a definite end to the abiogenesis theory, those that claimed that the long boiling of the nutritious liquid killed the active element. Pasteur instead, proved that once the curved neck of the container was broken, air in contact with the substance would bring germs and microorganisms inside, shortly after. Furthermore, the unsealed container allowed air to enter, even if through a tortuous neck, disproving the objections of those who supported that the active element needed air to generate life.

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