960 THE ANIMAL VIRUSES 



. . . serve to influence~the results of all filtration experiments." The thickness 

 of the filter also makes a considerable difference. 



The ordinary porous bacterial filter consists of a positively charged alkaUne earth 

 cation, and a negatively charged silicate anion. If a simple basic dye, such as methylene 

 blue, which consists of an organic coloured cation united to an inorganic anion, is passed 

 through a filter, a large amount of the dye wiU be adsorbed. The explanation of this is 

 that the organic coloured cation enters into combination with the sihcate anion in the filter, 

 forming an insoluble dye-silicate, which is retained ; the soluble salt, such as NaCl or 

 KCl, formed by the union of the alkahne earth cation in the filter with the inorganic 

 anion of the dye, passes through. A similar phenomenon is observed in protein solutions. 

 In solutions more acid than the isoelectric point of the protein, the dissociated protein 

 is chiefly in the form of multivalent cations capable of entering into combination with the 

 silicate anions in the filter, and forming an insoluble compound, which is retained. 

 On the other hand, in solutions more alkahne than the isoelectric point, the dissociated 

 protein is chiefly in the form of multivalent anions, capable of entering into combination 

 with the alkaline earth cations in the filter with the formation of soluble salts, which pass 

 through. This is probably why enzymes, toxins, and viruses appear to pass more readily 

 through filters in weakly alkahne than in acid solutions (see Mudd 1922-23, 1928). 



The nature of the suspending fluid plays an important part in determining the 

 result. Several workers have noted that viruses pass much more readily through 

 filters when the suspension is made up with broth or serum than with saline or 

 phosphate buffer (Grinnell 1929, Ward 1929, Sawyer and Frobisher 1929, Tallerman 

 1929, Marie and Urbain 1930, Galloway and Elford 1931). The mode of action of 

 the broth is not known with certainty, but according to Elford (1933) it appears to 

 be closely related to the ability of this medium to stabilize the dispersion of 

 a lyophilic colloid. Soap has the opposite effect. Another factor, which is of special 

 importance in comparing the filtrability of two different strains of virus, is the 

 initial concentration of virus. The greater the number of virus particles present 

 in the suspension, the more likely is virus to be found in the filtrate (Galloway and 

 Elford 1931). 



Cataphoresis experiments on such viruses as vaccinia, fowl-pox, foot-and-mouth, 

 rabies, yellow fever, myxoma, and Rous sarcoma have agreed in showing that most 

 viruses carry a negative charge in neutral or nearly neutral suspensions (Douglas and 

 Smith 1928, Findlay 1930, Hindle and Findlay 1930, Poppe and Busch 1930, Nata- 

 rajan and Hyde 1930, Sichert-Modrow 1930, Sankaran et al. 1934). It is true that 

 Olitsky and Boez (1927) stated that the foot-and-mouth virus carried a positive 

 charge up to pH 8-0, but the results of these workers have not been confirmed. 

 Most viruses have been studied over a range of about pH 5-0 to 9-0 ; they have 

 been found to be negatively charged up to about pH 7-6, though the exact location 

 of the isoelectric point has varied from pH 7-0 with the yellow fever virus to pH 9-3 

 with the virus of myxoma. Not too much attention, however, should be paid 

 to these measurements, since most of them have been carried out in the presence 

 of tissue protein. Beard, Finkelstein and Wyckoff (1938), who worked with a 

 relatively pure suspension of vaccinial elementary bodies, found their isoelectric 

 point to be between pH 4-6 and 4-3. The nature of the charge carried will affect, 

 to some extent, the passage of the virus through a filter. Incidentally, use may 

 be made of the electric charge carried by the virus to free it from other material 

 in a tissue suspension, or at any rate to obtain it in a more concentrated form. 

 (Douglas and Smith 1928, Sankaran et al. 1934). 



