18 I. INTRODUCTION TO VIRUSES 



The decrease in the sugar-insoluble space, as indicated in Fig. 3, 

 when the concentrations of the sugar are higher, may be due to the 

 penetration of the sugar into the water layer. On the thickness of 

 this water layer as well as on the force which attracts the water mole- 

 cules around the particles, a due discussion will be made in a later 

 chapter. This force appears to play an important part in the accom- 

 plishment of the virus action, a matter which will likewise be fully 

 discussed later. 



To estimate the water quantity in virus particles usually the spe- 

 cific gravity is at first to be measured. For this purpose a certain 

 amount of a substance is dissolved into a virus suspension, and the 

 change raised thereby in the sedimentation rate of the particles is 

 investigated. Sugars or inorganic salts dissolved for such a purpose, 

 however, will penetrate into the particle when the concentrations 

 are high as indicated in the experiment in Fig. 3. Consequently, the 

 specific gravity estimated by such a method may be erroneous ; pre- 

 sumably the obtainable values are too large, and accordingly the water 

 contents are too small. 



A linear relation is expected between the sedimentation rates and 

 the concentrations of a dissolved substance, if the dissolved substance 

 do not penetrate into the water which is associating with the particle. 

 However, this is not the case, and the sedimentation rate is not a 

 linear function of the density of the medium (36). In additon, a slirink- 

 age may occur in the particle when the density of the medium is high 

 (37). In order to avoid this shrinkage. Sharp et al. (38) used a protein 

 solution of a low osmotic pressure, and estimated the water content of 

 influenza virus A and B and also the swine type to be respectively 52, 

 34.4, and 43.3 per cent. They claimed that the relation between the 

 sedimentation rate and the solvent density fell on a straight line 

 when protein solutions were used ; but it may be unreasonable to 

 assume that the protein, regardless of its concentration, can dissolve 

 only into the water layer outside the particle without penetrating 

 into the water present inside. Hence, the reported values do not seem 

 to the writer to be legitimate. 



According to Schachmann and Lauffer (39), the specific gravity of 

 tobacco mosaic virus particles was calculated to be 1.13 in serum 

 albumin solutions, while 1.27 in sucrose solutions, indicating, also in the 

 case of the plant virus which is known to have peculiar properties 

 distinct from usual animal viruses, sugars appear to penetrate more 

 readily than do proteins. They calculated the water content of the 

 tobacco mosaic virus to be 65 per cent from the specific gravity in 

 the protein solutions. From X-ray measurements, Pirie (40) concluded 

 that the virus is normally associated with about its own weight of 



