CHAPTER II 



FILTERABLE VIRUSES 



Too Minute for the Most Powerful Lenses 



The very smallest of living things are apparently too minute 

 to be identified as individual organisms, even with the most 

 powerful lenses. These, known as the "ultra-microscopic organ- 

 isms," also as the "filterable viruses," cannot be seen, but are 

 known only through their activities as causative agents of differ- 

 ent diseases; as such their full consideration is not within the 

 scope of this volume. 



The filterable viruses derive their name from their ability to 

 pass through porcelain or diatomaceous earth filters, the latter 

 made of the silicious shells of diatoms.* Our knowledge of 

 filterable viruses is very limited and certain authorities have even 

 questioned whether these minute particles are endowed with life. 

 They seem in some way to be related to the formation of various 

 cellular and nuclear inclusions,'*' and are very definitely the cause 

 of diseases, not only in man but also in fishes, fowls, insects, and 

 plants. 



Their Activities 



Knowledge of these filterable viruses is a fairly recent devel- 

 opment. Although vague references to them had been made 

 before, it was only in 1892 that .the Russian botanist, Dimitri 

 Iwanowski, found that the leaf mosaic disease of tobacco was 

 caused by a filterable infecting agent. After that the progress 

 was rapid. In 1898 a causative virus of filterable nature was 



* Diatom (from the Greek, meaning "cut in two") is any microscopic one-celled' 

 alga belonging to the order Bacillariales ; diatoms are remarkable for a silicified 

 cell wall which persists as a skeleton after the death of the organism and forms 

 kieselguhr. Diatoms are always found on submerged objects (wood, stones, etc.) 

 to which they impart a slimy feeling. Kieselguhr is a fine, usually white, powder, 

 and is used as an absorbent in dynamite and as a polishing material. 



t An inclusion is a foreign body, usually of minute size, inclosed in a larger 

 mass. In biology, it is a foreign body inclosed in a cell. 



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