The demonstration that infectious diseases were produced by organisms so small as to be beyond demonstration with the best microscopes was made possible by showing, that some fluid from a diseased animal was infectious; and capable of producing the disease when inoculated into a susceptible animal. The fluid was then filtered through porcelain filters which were known to hold back all objects of the size of the smallest bacteria and the disease produced by inoculating with the clear filtrate. There are a number of such filters of different degrees of porosity manufactured, and they are often used to procure pure water for drinking, for which use they are more or less, generally however, less efficacious. The filter has the form of a hollow cylinder and the liquid to be filtered is forced through it under pressure. For domestic use the filter is attached by its open end to the water tap and the pressure from the mains forces the water through it. 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. When the coarser of such filters have been long in domestic service in filtering drinking water, bacteria may grow in and through them giving greater bacterial content to the supposed bacteria-free filtrate than in the filtering water.