Lazzaro Spallanzani, (1729–1799)

In 1765 the Italian Lazzaro Spallanzani, joined the controversy about how life starts.  He put broth into two sets of flasks with slender necks which could be melted shut.  One set he boiled for half to three-quarters of an hour and sealed, the other he left open.  He observed that only the broth which had been sealed remained sterile.  He kept the sealed flasks, and found no signs of life, however long he left them, even when he examined them under a microscope.  But when he broke the necks of the flask, the infusions soon showed the usual life.

Proponents of spontaneous generation, however, remained unconvinced, arguing that the boiling had destroyed some "vital principle" in the air which explained why no microbes appeared in the closed container. 

Next, Spallanzani tried boiling the broth for different lengths of time. He set up five series of flasks.  One series was left open, the other four boiled, each for 30 seconds longer than the previous ones and then sealed.  After two days the open series was swarming, and the 30 second series contained some organisms, while the remainder contained almost none. He had shown that the duration of the boiling mattered.  (We now know this is because some organisms are more heat resistant than others.)  He argued that when Needham had carried out his experiments he had not boiled the gravy for long enough to kill all the organisms and when the broth was cooled they had reproduced and multiplied.

In 1767, Spallanzani published his account rebutting Needham and Buffon, saying:

"I sought to discover whether long boiling would injure or prevent the production of animalcules in infusions. I prepared infusions with eleven varieties of seeds, boiled for half an hour. The vessels were loosely stopped with corks. After eight days I examined the infusions microscopically. In all there were animalcules, but of differing species. Therefore long boiling does not of itself prevent their production".

Needham would not accept this explanation.  He maintained that the heat had killed the ‘active principle’ in the broth, necessary for life. 

So a stalemate was reached.  What was this ‘active principle’?  Nobody could prove its existence and nobody could disprove it either.  In those days, little was known about sterilization.  People could not control temperature accurately, nor could they filter air and liquid efficiently.  Spallanzani reasoned (correctly, as we now know) that the minute organisms must have a more minute early stage of growth, and so decided that the problem could not be resolved through the use of a microscope.  Scientists had gone as far as their apparatus and techniques allowed, and they were left only with the speculation.  Theory, not experiment, played the major part in the controversy, so it is not surprising that little progress was made.

Spallanzani’s microscope

Bibliography

Grace Monger and Richard Gliddon eds, The Perpetuation of Life: revised Nuffield biology text 4, Longman 1975

Robert Reid,  Microbes and Men, BBC 1974

G. Rattray Taylor,  The Science of Life, Thames and Hudson 1963