Spallanzani placed fluids, which easily became putrid, in glass tubes, which he then hermetically sealed and boiled.

He found that the fluid remained clear and unchanged; if, however, he broke the sealed point of such a tube and allowed the air to enter, putrefaction, or in some cases fermentation, of the contents took place.

He concluded that boiling the substances destroyed the living germs which they contained, the sealed tubes prevented the air from entering, and when putrefaction or fermentation of the contents took place the organisms to which this was due, being contained in the air, entered from without.

Objection was made to the conclusions of Spallanzani that heating the air in the closed tubes so changed its character as to prevent development of organisms in the contents.

This objection was finally set aside by Pasteur, who showed that it was not necessary to seal the end of the tube before boiling, but it could be closed by a plug of cotton wool, which mechanically removed the organisms from the air which entered the tube, or if the tube were bent in the shape of a _U_ and the end left open, organisms from the air could not pass into the tube against gravity when air movement within the tube was prevented by bending.