Cataphoresis experiments show that the virus of foot-and-mouth disease carries, under ordinary conditions, an electropositive charge. Its isoelectric range is at the high point of pH = about 8. Although cultivable bacteria are, as a rule, electronegative, yet certain protozoa, such as trypanosomes and spirochetes are also electropositive. In respect to charge, then, the virus is different from ordinary bacteria, but there is nothing in this finding to indicate an inanimate character of the incitant. A knowledge of the charge, however, aids in the interpretation of certain filtration phenomena, and indirectly in delimiting the size of the virus. In addition, it serves to explain its remarkable resistance to certain chemicals—a subject to be dealt with in the next paper of this series. Finally, cataphoresis indicates the possible separation of the virus from protein particles.

Filtration experiments were made with different types of filters: Seitz, Berkefeld V and N, and Chamberland, of practically all sizes, collodion membranes, and Bechhold's ultrafilter. The results confirm the electropositive charge of the virus, as well as the minuteness of its size. Filtration was effected through the Seitz, Berkefeld, and Chamberland filters. In regard to the latter, the active agent passed through the L 11 only when its charge was shifted to negative: under ordinary conditions, carrying an electropositive charge, it failed to traverse this more dense wall, and was completely adsorbed in the oppositely charged barrier. Filtrations through electronegative collodion membranes, prepared in different ways and of varying thicknesses, resulted, as a rule, in failure, unless the thinnest and most permeable membranes were employed. But in these, the complication of microscopic holes was to be considered. Hence this method was regarded as impracticable. Success, however, was obtained with Bechhold's ultrafilter membranes of the most permeable type, and with these it was possible to measure relatively, by a system of "molecular" filtrations, the size of the incitant. This was found to be, in relation to other particles of like charge, between 20 and 100 mµ in diameter.

The filtration phenomena of the foot-and-mouth disease virus can be accounted for on the basis of the minute size of the particles of the incitant carrying an electropositive charge, and no evidence can be deduced therefrom that the virus is of a fluid character. For the relative size of the particles is constant and the invariability of the limits of measurement contradicts the notion that the incitant may be a "solute" varying in dimensions in different "solvents."

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