It is by the French chemist Pasteur, how ever, that the most complete and brilliant series of experiments was performed that disposed of the theory of spontaneous generation.
He was led to these experiments by studying the action of yeast. Leeuweuhoeck had noticed little round bodies in beer, and in 1837 Cagniard Latour had observed that the fermentation of beer was accompanied by the growth of these little bodies. They are called torulee, and are not bacteria. They are round or oval, and multiply by budding. But Pasteur proved that the fermentation was the expression of the active life of this small cell ; that, to obtain certain materials requisite for its nourishment, it attacked the sugar present in the fluid, and, as a result of splitting it up, produced carbonic acid and alcohol. From the fermentation of beer he passed to the butyric and lactic fermen tations—actions whose results anyone may see by watching the process of the souring of milk. He demonstrated that in each case the fermen tation was the work of an organism introduced from without. The production of vinegar he showed to be also the work of a bacterium. Introduce yeast into beer-wort, it grows and flourishes, and alcohol and carbonic acid gas are formed. But the yeast soon exhausts the material on which it grows, and immediately its own activity ceases. But it has not exhausted the capacity of the liquor to nourish various other kinds of living things. This is the golden opportunity of bacteria, which may then become active, and, by the fermentive process they in turn set up, beer becomes sour. Beer, vinegar, and wine in turn received Pasteur's attention, and in each case lie separated the living agent, different in each, whose growth was the direct cause of the change. Let bacteria foreign to the fermentation proper in each case be intro duced, and beer becomes sour, vinegar becomes flat and tasteless, wine is converted into vinegar. These questions were inextricably interwoven with the wider one of spontaneous generation.
This Pasteur also attacked by means of such experiments as have been indicated, though net in their historical order, and which Tyndall's experiments, performed later, amply verified, so that Pasteur was able to declare: "There is not one circumstance known at the present day which justifies the assertion that microscopic organisms come into the world without germs or without parents like themselves. Those who
maintain the contrary have been the dupes of illusions and of ill-conducted experiments, tainted with errors which they knew not how either to perceive or to avoid. Spontaneous generation is a chimera." But many observers have taken organic in fusions—an infusion of hay, for example,—have boiled them in flasks, duly sealed the necks, and have laid them aside, and yet, after a suf ficient time, bacteria have been found in them multiplying. Such results have been obtained by the most experienced observers, when no doubt existed as to the experiments being well conducted. The explanation in due time was forthcoming. The fully-developed bacteria are destroyed by a temperature much below that of boiling water; not so their spores. These seeds, or eggs as one may call them, resist the tern• perature of boiling water, prolonged even for several minutes, and in some cases even for hours, so that, when set aside, by and by the boiled infusion will give way, owing to the growth of the eggs to the fully active bacteria. Indeed they are unusually resistant, defying the action not of heat-only, but also of extreme cold. One may freeze the solution containing the bacteria, one may keep them for hours at a temperature many degrees below zero: the bacteria themselves die, but their spores are only dormant, and will awake up to life soon after the usual temperature is restored. Drying they successfully encounter as well as the action of many chemical agents.
Thus one objection after another has been set aside, till it becomes conclusively evident that there is present in the atmosphere a vast number of germs of various kinds, each kind capable of setting up a fermentation peculiar to itself; that putrefaction is only one kind of fermentation, the expression of the life and growth of a particular germ whose activity liberates from the organic substances on which it lives sulphurous and other badly-smelling gases; and that, if by any means the organic substance is kept free of the living things, it will not putrefy, will not break down, but will remain in its organized though lifeless condition.