employ it in making a close study of these forms of life. He was followed by a number of distinguished microscopists who all, more or less, added to our knowledge of the natural history of micro-organisms. All these were descriptive and systematic morphologists and not experimenters. Of the more distinguished microscopists who followed Leeuwenhoek we may mention the names of Otto Friedrich Müller (1730-85), in Denmark, and Ehrenberg (1795-1876), in Germany.

In the year 1822 Persoon gave to yeast the systematic name Mycoderma, a designation which seems to indicate that he regarded it as a fungus (mycoderma signifies fungoid film).

About the same time-in the middle of the thirtiesCagniard Latour (V. 1, 2), Schwann (VI. 1, 2) and Kützing (VII.) stated expressly that yeast is a plant. Meyen agreed with this view and gave to the new genus the systematic name of Saccharomyces (i.e., sugar fungus) which it has since retained.

Considering the state of knowledge at that time, very valuable contributions to the natural history of yeast fungus were made by Eilhard Mitscherlich. It is evident from a paper published by him in 1841 (IX. 1), that this investigator recognised the substance invertin. In 1843 (IX. 2) he read a paper on the multiplication of yeast; he had observed under the microscope the phenomenon of budding, and had followed the development from a single

cell.

Schwann, Cagniard Latour and Kützing expressed the opinion that it is the living yeast cell which excites alcoholic fermentation. In direct opposition to this vitalistic theory, Justus v. Liebig (1839-40) came forward with his theory of mechanical decomposition (VIII. 1). According to Liebig, every fermentation consists of molecular

motion which is transmitted from a substance in a state of chemical motion, that is, of decomposition, to other substances the elements of which are loosely bound together. In his last work on fermentation (VIII. 2), he sought to bring this theory into agreement with the observations of Louis Pasteur on auto-fermentation. Liebig's explanation of the latter is that the cells contain a decomposing substance which produces sugar for the auto-fermentation. Although he at first looked upon yeast as a lifeless mass, an albuminoid compound, yet he came gradually to the view that it consists of living cells. But, in his opinion, there could be no question of fermentation being a physiological process in this respect he held to his chemical conception.

At that time a vigorous dispute was taking place between the followers and the opponents of the doctrine of generatio æquivoca, i.e., of spontaneous generation. Let us look somewhat closer at this doctrine. By spontaneous generation we understand the development of organisms from lifeless material without eggs, seeds or embryos. Needham (1745), an energetic supporter of this doctrine, was the first to make experiments endeavouring to prove it. For this purpose he heated meat extract in closed flasks, and, on organisms appearing in the flasks, he assumed that they had been produced by spontaneous generation.

Spallanzani (1765) showed, however (I.), that certain errors were made in these experiments; he sealed his flasks hermetically and boiled them for an hour, after which treatment no development of micro-organisms could be observed. From his experiments he concluded that the eggs" of the micro-organisms are present in the air and only develop after they have found their way into the liquid. On these experiments the foundation of the technique

of sterilisation was laid, and a substantial addition made to the methods of cultivation.

The Swedish chemist and apothecary Scheele put Spallanzani's experiments to practical use in the sterilisation of vinegar by heating (II.). Appert, in France (1810), went a step further and used the method for preserving soup, beer, wine, etc. (III.).

In 1836-37 Franz Schulze (IV.) and Theodor Schwann (VI. 1) published the results of researches in which they sought to prove that air, when freed from its germs, that is, rendered sterile, can come into contact with a sterilised nutritive liquid without micro-organisms developing in the latter. The experiments of the last-named investigators were made in the following way: flasks containing nutritive liquid were closed with plugs fitted with bent glass tubes through which sterilised air was sucked. In order to free the air from its germs, Schulze passed it through sulphuric acid whilst Schwann subjected it to a high temperature. Their opponents, however, would not accept such proofs, but asserted that, in these experiments, the air had been violently treated, and, as a consequence, had suffered such a change that the inert matter could no longer be vitalised by contact with it.

Then in 1854 H. Schroeder and Th. v. Dusch (X.) showed that air can be freed from germs by filtration through cotton wool; thus the above-mentioned contention was disposed of. In fact, this method is still employed when we wish to sterilise air.

Belief in generatio æquivoca, however, did not yet die out. Not until 1860 was the victory won by Pasteur (XI. 4), who exposed all the failings of the experiments made by his opponents to prove the existence of spontaneous generation; in every case without exception he could prove that either an omission or an error had been made. In

consequence of these brilliant researches the theory of generatio æquivoca fell more and more into ill repute. Up to the present time no single case of spontaneous generation has been experimentally proved.

It has been remarked above that the principles of sterilisation and also a substantial part of our culture methods are the result of the experiments made in relation to the doctrine of generatio æquivoca. For the recent development in this direction we are indebted chiefly to Pasteur and his school. The appearance of Pasteur marks a very great and important epoch.

Besides the above researches relating to the doctrine of spontaneous generation, we might also refer to another of Pasteur's important researches which interests us here, viz., his investigation on lactic acid bacteria (1857). He describes lactic acid fermentation and finds microbes, which, as he assumes, cause this fermentation in milk (XI. 1); later on he mentions the same fermentation in beer and worts. He further proved (1861) that butyric acid fermentation is brought about by a special micro-organism (XI. 5). In addition we might mention his researches on acetic acid fermentation (1864 and 1868). Kützing had shown in 1837 that this fermentation is caused by a bacterium (VII.); but important progress in this direction was first made when Pasteur published his experimental studies on the subject (XI. 7).

Pasteur also made (1861) the discovery that certain micro-organisms thrive in the absence of free oxygen (XI. 5). He calls such forms anaerobic to distinguish them from those organisms to which free oxygen is necessary and which he terms aerobic.

In 1807 Chaptal had announced that the formation of a film on the surface of wine always precedes the souring of the wine; and, as already stated, Kützing had described

the acetic acid bacteria (VII.), which form acetic acid in beer and wine; but Pasteur was the first to spread the idea that bacteria excite diseases in fermented liquids (XI. 8).

The method of preservation invented by Scheele and Appert is, thanks to Pasteur's works, becoming of increasing practical value. His name has also been connected with it, as the method is known as "Pasteurisation".

Pasteur's doctrine that bacteria are responsible for the diseases of fermented liquids gave rise to a demand for the use of pure yeast. Pasteur communicated (XI. 8) a process for the purification of brewer's yeast, recommending that it should be cultivated either in sugar solution with the addition of tartaric acid or in wort containing a small quantity of carbolic acid. He seeks thus to gain the desired end by chemical means. (In most cases indeed this process is successful as a means of purifying the yeast from bacteria.) It was then unknown that some of the most dangerous diseases of fermented liquids are brought about by foreign, "wild" yeasts, and that this process favours these particular forms at the cost of the good yeast, as was shown later by Emil Chr. Hansen (XIX. 2, 3, 6). In fact the Pasteur process led in a direction exactly opposite to that in which the desired end lay.

A practical consequence of the doctrine of bacterial diseases was the construction of apparatus by means of which it was attempted to keep the living germs present in air away from the brewery worts, and so to ward off any bacterial infection which might be contracted in this way. For this purpose Velten, a co-worker of Pasteur's, constructed closed cooling apparatus for the aëration and cooling of worts. That this apparatus did not then come into general use was a simple consequence of the fact that, as mentioned above, the yeast which was added to the