1839 Justis Liebig
 

. . . I WISH NOW TO DRAW THE attention of natural scientists to a previously undiscovered cause for the changes and decompositions which in general are known as decay, putrefaction fermentation and rotting.
This cause is the ability which a chemical substance possesses, when in the process of decomposition or combination, to cause or enable another substance which is touching it, to undergo this same change which it itself is undergoing....
The generality of this causation can be demonstrated with countless experiences. It will suffice when I indicate only several of these.
Platinum, for example, does not possess the ability to decompose in nitric acid and hence to dissolve in it. Platinum alloyed with silver dissolves very easily in nitric acid. The ability which silver possesses is carried over to the platinum.... [Then follow a number of other examples of the same sort drawn from the chemical literature.]
The indicated examples suffice to prove the existence of this characteristic principle....
Before I go into more detailed considerations it is necessary to establish the essence of the above mentioned experiences for the cases at hand.
By decay, one understands in general the changes which organic materials undergo at ordinary temperatures. These changes take place only under moist conditions, they cease at the freezing point of water, they do not occur when oxygen is withheld.
If air is withheld from a substance in the process of decay, putrefaction sets in.
Decay is an oxidation at low temperature in which the elements of the substance take unequal parts, depending on their congeniality with oxygen.
Putrefaction is a decay in which the oxygen of the atmosphere takes no part. It is an oxidation of one or more elements of the substance, using the oxygen of the material itself, or the oxygen of water, or both.
By withholding oxygen and in a deficiency of water, putrefaction and decay begin simultaneously. This decay process is called rotting.
Fermentation is a putrefaction of vegetable substances which results in little or no unpleasant odour....
The so-called ferment arises in the course of a metamorphosis which begins after the entrance of air into a sugar-containing plant juice, and which can continue to a certain point without interruption in the absence of air. In the ferment is found all of the nitrogen of the nitrogen-containing substances of the plant juice....
. . . it appears that the ferment is not a characteristic cause of putrefaction or fermentation. It is rather that putrefaction and fermentation elicit the changes which the ferment undergoes.
The ferment is a substance which is in the process of putrefying or decaying. It transforms oxygen from the surrounding air into carbon dioxide and also produces carbon dioxide out of its own substance . . . its ability to cause fermentation disappears when putrefaction is completed.
To preserve the properties of the ferment . . . the presence of water is necessary. Through simple expression of water by squeezing, its ability to cause fermentation is reduced, and through drying this ability is destroyed. It is destroyed by boiling water temperature, alcohol, sodium chloride, acetic acid, an excess of sugar, mercuric oxide, mercuric chloride, sulphuric acid, silver nitrate, and esters that is, by all substances which prevent putrefaction.
The insoluble substances, which are called ferment, do not cause fermentation. If one washes beer or wine yeast with boiled cooled distilled water, always being careful that the material is covered with water, the residue does not cause fermentation when placed in sugar water. This ability is retained, however, by the wash water, which will lose the ability in several hours if exposed to air.
The ability to cause fermentation which the soluble part of the yeast possesses, does not depend on an effect brought about through contact with the yeast. The yeast loses its fermentation powers immediately upon contact with alcohol, without having taken up any of it. A hot, clear, watery extract of ferment does not cause any fermentation when mixed with sugar water in a closed vessel. The fermentation is effected by the soluble material, only after it itself has undergone a decomposition. If this hot extract is allowed to cool in the air and allowed to stand for several hours, it is able to cause a vigorous fermentation in sugar water. Without first contact with air, no fermentation occurs. During contact with the air, absorption of oxygen begins, and the extract contains after several hours a significant amount of carbon dioxide.
During the fermentation of sugar witlh ferment, two decomposition processes occur. If one takes a graduated bell jar filled with mercury and adds one cubic centimeter of a beer yeast suspension and 10 grams of a sugar solution containing one gram of pure sugar, after 24 hours at 20¡ to 25¡, one finds about 5s5l per cent of the weight as carbon dioxide.
Thenard found that from l00 parts of cane sugar, he obtained 51.27 parts of carbon dioxide, and 52.62 parts of alcohol. Thus one finds that the carbon of the sugar is distributed almost exactly between the carbon dioxide and alcohol.
The analysis of cane sugar has shown without a doubt that it contains the elements of 4 atoms of carbon dioxide, 2 atoms of ether, and 2 atoms of water.
The products of its fermentation show that 2/3 Of the alcohol and 1/3 of the carbon dioxide carbon comes from the sugar, but these products contain 2 atoms of hydrogen and l atom of oxygen more than the sugar. It is clear that these additional atoms have arisen from one molecule of water....
A certain amount of ferment is necessary in order to bring a certain amount of sugar into fermentation, but its effect is not one of mass action. Its influence is limited to its necessity to be present until the time when the last atom of sugar has been decomposed. The ferment is not the cause of fermentation. The insoluble part does not possess this property. The extracted part, which arises through the decomposition of the ferment, possesses this property. Both materials are able to cause fermentation from the moment when they suffer changes through the agency of air and water which results in their own destruction. It is however, no characteristic body, substance or material which causes this destruction. This charactcristic body or substance is only the carrier of an activity which serves to increase the amount of substance undergoing decomposition....
During the putrefaction of animal substances, one finds the elements of these substances in continual change, in a state of unstable equilibrium, during which even the weakest forces can alter them or modify them. Such a condition seems to be a fruitful base for the development of imperfect and lower animals, the microscopic animals, whose eggs are well known to be widely distributed. They develop in this putrefying material and multiply in myriads, making use of the products developing during the putrefaction as their nutrients.
Many natural scientists view the chemical process of putrefaction as only a result of the productions of these animals. This would be like saying that the cause of the decay of wood and its rotting arise from the plants which use these decaying products as nutrients.
None of these animals develop in putrefying materials, if one prevents these materials from contact with atmospheric air, which is a necessary condition for their presence, in the same way that one can prevent maggots from developing in putrefying cheese, when one protects this cheese from flies....
Organic chemistry recognizes two separate types of phenomena for the behavior of its compounds.
( l ) Substances develop from newly developed properties, in which the elements of manv atoms of simple compounds become transformed into molecules of a higher order.
( 2 ) Molecules of a higher order can decompose into molecules of a lower order, as a result of a neutralization of the equilibrium in the attraction of their elements.
The following disturbances will bring about decomposition:
(a) Heat
(b) Contact with different substances
(c) The influence of a substance which is in the process of changing.

Comments by Thomas Brock
I have translated only a small portion of this long, rambling account. Its inclusion in this collection is only warranted because Liebig was a powerful figure who was able, by sheer weight of prestige, to influence a number of lesser scientists. Because of this the controversy over spontaneous generation was extended for another generation. It will be noted that there is little experimental work in Liebig's paper. He starts from a position of pure prejudice, and consequently cannot but conclude as he does. This paper should be a continuous object lesson to all scientists, young and old, that one good experiment is worth one hundred bad theories.
This should not be taken to mean that Liebig did not make significant contributions to science. He was a great chemist, and was convinced that all vital activity could be explained in terms of chemistry and physics. Much of the history of biochemistry had its beginnings in the work of Liebig and his school. It is unfortunate that he took the view he did on the putrefactive process, since this probably delayed for a number of years the rise of microbiology as a science.