a skilled experimental physiologist, in a brilliant series of experiments showed the imperfect character of Needham’s work and the fallacy of his conclusions.  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.  The possibility of spontaneous generation cannot be denied, but that it takes place is against all human experience.

It was not possible to attain any considerable knowledge of the bacteria discovered by Loewenhoeck until more perfect instruments for studying them were devised.  Lenses for studying objects were used in remote antiquity, but the compound microscope in which the image made by the lens is further magnified was not discovered until 1605, and when first made was so imperfect that the best simple lenses gave clearer definition.  With the betterment of the microscope, increasing the magnifying power and the sharpness of the image of the object seen, it became possible to classify the minute organisms according to size and form and to study the separate species.  The microscope has now reached such a degree of perfection that objects smaller than one one hundred thousandth of an inch in diameter can be clearly seen and photographed.

Great impetus was given to the biological investigation of disease by the discoveries which led to the formulation of the cell theory in 1840 and the brilliant work of Pasteur on fermentation,[1] but it was not until 1878 that it was definitely proved that a disease of cattle called anthrax was due to a species of bacteria.  What should be regarded as such proof had been formulated by Henle in 1840.  To prove that a certain sort of organism when found associated with a disease is the cause of the disease, three things are necessary: 

1.  The organism must always be found in the diseased animal and associated with the changes produced by the disease.