556 



FILTERABLE VIRUSES 



The passage of a virus through a filter of the type mentioned does 

 not necessarily indicate that the virus is too small to be visible with 

 the highest powers of the microscope, although the filtrates of the 

 so-called " ultramicroscopic viruses" are clear and do not contain 

 particles demonstrable with the ultramicroscope. Filters used for 

 the study of filterable viruses should be new, sterile, and tested for 

 permeability with suitable known bacteria. A preliminary test, 

 forcing air under pressure through the submerged filter, will reveal 

 "pin holes." The virus to be tested should be forced through at a 



FIG. 91 FIG. 92 FIG. 93 FIG. 94 



FIGS. 91 to 94. Types of unglazed porcelain filters. (Park.) 



temperature of about 20 C., and the process should be completed 

 within one and a half hours, using as little pressure or suction as 

 possible. The filtrate, proved to be free from visible particles (best 

 by adding a known organism to the fluid to be filtered), should repro- 

 duce the disease in susceptible animals; the virus should be recovered, 

 again filtered, and again reproduce the disease. Some of the filterable 

 viruses will pass only the coarser filters, others go through those with 

 finer pores. 



Ultramicroscopic viruses with few exceptions are of unknown 

 morphology, and, with the exception of their resistance to desiccation 

 and physical agents, but little is known about them. The viruses of 



