119 



consideration may nevertheless have been responsible for the fact 

 that evidently he soon resigned himself to another negative result. 

 After having become an academic teacher, he felt quite free in the 

 choice of the subject of his researches, and, since the opening of the 

 Bacteriological Institute had provided him with all means necessary 

 for the investigation in question, he returned in 1897 to the problem 

 offered by tobacco mosaic. 



This time he was able to provide definite proof that the juice obtain- 

 ed by expressing the leaves of diseased plants contained a principle 

 \vhich passed through a porcelain filter retaining all visible micro- 

 organisms, which principle on being inoculated into a healthy tobacco 

 plant, transmitted the disease to it. 



Moreover, it was demonstrated that the principle actually multi- 

 plied in the living tissues of the host, so that infection in series could 

 be obtained. In addition it was shown that the principle shared with 

 most living cells the property of being destroyed by heating the juice 

 to 90 C. Great stress \vas laid by BEIJERINCK on the outcome of the 

 experiment in which he proved that on bringing a drop of the juice of 

 diseased plants on the surface of an agar gel the contagious principle 

 diffused into this gel, so that after a week or ten days its presence 

 could be demonstrated in a layer at least two millimeters beneath the 

 surface. For BEIJERINCK this meant a convincing proof of the non- 

 corpuscular nature of the principle, which, therefore, should occur in 

 the liquid state in the juice l }. This led him to the characterization of 

 the principle as a "contagium vivum fluidum" 2 ). When to the fore- 

 going we add that BEIJERINCK also proved that the contagium 

 multiplied only in tissue in which cell division took place, and that, 

 moreover, it could be dried at low temperature or precipitated with 

 alcohol from the aqueous solution without loss of infectivity, it will 

 be obvious that he succeeded in establishing the main properties 

 characteristic for all viruses. 



The great merit of this pioneer investigation in the virus field is not 

 diminished by the fact that shortly after the appearance of the pre- 

 liminary communication a note was published by IWANOWSKI 3 ) in 

 which this author rightly claimed the priority for the discovery of the 

 filtrability of the contagious agent of mosaic disease. In a paper 

 which had been published already seven years before IWANOWSKI had 

 indeed proved this fact beyond doubt 4 ). BEIJERINCK, to whom this 

 publication had remained unknown, readily acknowledged this claim 

 both in a separate note 5 ), and in an addendum to the French version 

 of his extensive publication 6 ). 



1) The expressions "liquid state" and "dissolved state" of the virus were apparently 

 employed by BEIJERINCK interchangeably. 



2 ) It is, however, noteworthy that BEIJERIXCK uses this indication only in the title 

 of the paper, but not in the text, wherein the term "virus" is used throughout. 



- 1 ) D. IWANOWSKI, Centralbl. f. Bakt. u. Parasitenk. II, 5, 250, 1899. 

 ) D. IWANOWSKI, Bull, de 1'Acad. Imp. d. St. Petersbourg 13, 237, 1892, 

 s) Centralbl. f. Bakt. u. Parasitenk. 1 1, 5, 310, 1899. 

 6 ) Cf. footnote 2 on page 116. 



