In 1876 Pasteur published his Études sur la Bière (Studies on Beer). This illustration from his book demonstrates a method for examining the yeast in beer without exposing the sample to contamination from other microorganisms. His publication included plans for a fermentation tank that would prevent air-borne bacteria from entering and spoiling the brew. Here you will find individual descriptions of the different parts of the device.
A bowl contains mercury where the bent-neck end of the flask (A) is submerged. This setup minimizes the fermenting liquid in the flask from becoming contaminated by any microbes in the outside air.
The ball-shaped flask has two long necks. One bends to the side and bottom, and its end is placed in the mercury bowl (b). The other flask neck has a tap/faucet (r) connected to a long tube that runs inside of the neck and extends to the bottom of the flask. Liquid prepared for fermentation is placed in the flask.
A tap/faucet is connected to the tube that runs inside of the flask neck to its bottom. When the liquid inside the flask ferments, it produces gases that increase pressure in the flask and push the liquid up the inner tube toward the connected tap/faucet. The open tap allows the liquid to flow into another glass tube with a flat lens (l-l) that sits under the microscope.
This glass tube has a flat circular lens in the middle. One end of the tube is connected to the flask neck with the tap, the flat lens is placed under the microscope, and the other end is connected to a rubber tube. The fermenting liquid pushed by the increasing air pressure in the flask travels into the glass tube onto the lens. The lens is air-tight, preventing any air contamination, and is thin enough to view the liquid inside with a microscope. The other end of the glass tube is connected to another rubber tube that drains the liquid into a glass (V).