14 I. INTRODUCTION TO VIRUSES 



in a state of the particle of over a certain size in order to exhibit the virus 

 action and that the action is stronger as the particle becomes larger. 



The unstabihty of smaller sized particles could be proved by the 

 writer also with vaccinia virus protein. Galloway and Elford found 

 that foot-and-mouth disease virus became more unstable after "purifi- 

 cation" through a collodion membrane having a small pore diameter 

 (29). This fact indicates that the small sized particles of this virus are 

 also more unstable than the larger ones. Tobacco mosaic virus parti- 

 cles appear likewise to have various sizes. Bawden and Pirie (30) have 

 suggested that the small particles may be virus that has become non- 

 infective without losing serological activity or that they are incomple- 

 tely formed or mal-formed virus particles, although it has been claimed 

 by a number of workers that particles are 280 m/z long, and that such 

 are only the virus. Crook and Sheffield (31) found that a virus prepa- 

 ration containing only particles shorter than 280 m^ had even some infec- 

 tivity. Takahashi and Rawlings (32) also stated that particles shorter 

 than 225 mjn could be infective. 



Before the universal application of electron microscope, the size of 

 virus was usually measured by collodion membranes ; the size was 

 estimated from the average pore diameter of the most porous mem- 

 brane that prevents the passage of the virus particles. Thus, it should 

 naturally follow that the virus will show the smallest diameter when 

 measured by this method. In short, there seems little doubt that the 

 particle size estimated by collodion membranes is of the smallest 

 particle in which the virus protein can preserve its activity ; in other 

 words, when the protoplasm protein is coagulated into particles smaller 

 than those of this size the virus activity may fail to be revealed. 



By electron micrographs the particle size of various phage strains 

 has been estimated and reported to be a little less than 0.1 ju, while 

 by the filtration method some phage was previously reported to be so 

 small as having a diameter of the order of 0.01 jn. Also from our ex- 

 perimental data, phage appeared sometimes to retain its activity even 

 when the protein existed in so small a particle as of this order (16). 



Of the smallest viruses known are those of mouth-and-foot disease 

 and of poliomyelitis, and both of which are accepted to be also of the 

 order of 0.01/^. This may be ascribed to their stable nature, capable 

 of retaining their activity even when decomposed to such a small size. 

 According to Andrewes and Horstmann (33) the viruses that are patho- 

 genic to animals and known to have very small sizes show a high resi- 

 stance against a variety of chemicals. 



At present, the sizes of many viruses are mostly measured by 

 electron miroscope, and it has been recognized that the sizes thus 

 measured are larger than those estimated from the filtrability as is 



