In laboratory uses, denser filters of smaller diameters are used, and the filter is surrounded by the fluid to be tested.

The open end of the filter passes into a vessel from which the air is exhausted and filtration takes place from without inward.

The test of the effectiveness of the filter is made by adding to the filtering fluid some very minute and easily recognizable bacteria and testing the filtrate for their presence.

These filters have been studied microscopically by grinding very thin sections and measuring the diameter of the spaces in the material.

These are very numerous, and from 1/25000 to 1/1000 of an inch in diameter, spaces which would allow bacteria to pass through, but they are held back by the very fine openings between the spaces and by the tortuosity of the intercommunications.