The Micro-oryanisms of the Soil. 
845 
ferment, the heterogeneous particles are separated from each other, 
recombining so as to form more stable compounds, including the 
same principles but in different proportions. Putrefaction is but a 
particular case of fermentation.” This theory remained unchallenged 
eighty years. 
Lavoisier, by applying the new methods of organic analysis he 
liad invented, quantitatively ascei’tained the relations between the 
fermented matter and the products. 
Gay-Lussac considered oxygen the sole cause of fermentation, 
putrefaction, and decay, by transmitting its motion to the ferment, 
and this imparted its motion to the loosely combined fermentable mass. 
The present theories of fermentation originated with Schwann 
and Pasteur. It took a century and a half before the experiments 
which led up to Schwann’s theory found a scientific explanation by 
the work of this chemist. Leeuwenhoeck had in 1680 already 
noticed that beer-yeast was composed of small spheroidal globules. 
Cagniard de la Tour declared yeast to be a plant, and the exciter of 
fermentation. 
Schwann’s experiments were made to determine the possibility 
of spontaneous generation. He found that fermentable fluids, when 
first heated in closed vessels in the presence of oxygen to the tem- 
perature of boiling water, would not ferment. This disproved Gay- 
Lussac’s theory that oxygen caused fermentation. He next showed 
that purified air or oxygen passed into a sterilised fermentable fluid 
did not induce fermentation ; but that this set in with the intro- 
duction of ordinary air. He concluded from these experiments that 
the air was not the exciter, but simply the medium containing it, 
and that in the floating particles of the atmosphere were organisms 
capable of developing in the fluid ; should these be killed by heat, 
fermentation would not take place. In his examination of these 
organisms, although his methods were not absolute, his conclusions 
that alcoholic ferments are of a vegetable nature were correct. 
Instead of general acceptance, Schwann’s theory received but 
little recognition. 
Schultze’s method of first passing the air entering a sterilised 
fermentable fluid through oil of vitriol, and that of Schroeder and 
Dusch of filtering it through cotton, can be regarded as modifications 
of Schwann’s experiments. All these experiments conclusively show 
that the particles in the atmosphere are the exciters of fermentation, 
but do not render them visible. 
Pasteur, spurred on by the same motive as Schwann — namely to 
determine the question of spontaneous generation, — made a simple 
modification of Schroeder and Dusch’s experiment, by substituting 
gun-cotton, and achieved most remarkable results. The gun-cotton, 
containing the particles filtered from the air, was dissolved in ether 
under the microscope, and now for the first time the organisms 
could be thoroughly examined. 
Tyndall’s well-known experiments, with an air-tight box coated 
with glycerine, demonstrated that gravity alone can purify the 
atmosphere so as to debar fermentation from setting in. 
