A swan neck flask, also known as a gooseneck flask, is a flask with a particular shape of tube leading into it. A test tube or other vessel may also have a "swan neck". The swan neck significantly slows down the motion of air through the tube, and particles in the air, such as bacteria, become trapped on moisture on its inner surfaces. As a result, sterile liquid in the vessel itself remains sterile as long as the liquid does not contact the contaminated liquid in the tube. Louis Pasteur developed and used this apparatus in 1859 to prove that particles in the air (germ theory), rather than the air itself (spontaneous generation), led to fermentation.

 

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