VIEOLOGICAL METHODS 281 



virus a high-speed centrifuge may serve the purpose adequately. The 

 gravitational force and time employed will vary with the virus, and 

 published work will often serve as a guide for the individual viruses 

 under study. Certain generalities, however, are worth noting. The 

 virus particles within a given suspension may not be uniform in size; 

 therefore, to calculate the amount of virus sedimented from the amount 

 left in the supernatant fluid may or may not be correct, depending on the 

 qualitative identity of the two fractions. Inhibitive products may be 

 left in or removed from the supernatant viral particles, causing them to 

 produce a misleading titer. 



The resuspension of virus from high-speed-centrifugation sediments 

 may be more difficult than ordinarily assumed. Usually best results 

 are obtained by repeated and gentle agitation over a period of time. 

 This may be especially difficult in concentration procedures where less 

 than the original volume is used for resuspension. Resuspending virus 

 in a menstruum different from the one from which it was removed may 

 change its stability and the functions by which it is recognized and 

 quantitated. It is well to control such changes carefully. So-called 

 soluble substances as well as small viral particles will not sediment at 

 the same speed as larger viral particles; therefore, this presents a method 

 of separation. It seems preferable for the novice to learn the methodology 

 for operating a high-speed centrifuge from an experienced operator. 

 Therefore, such operation will not be discussed here. 



Evaporation. Certain viral preparations, such as an allantoic fluid 

 harvest, can be concentrated by evaporation. If it is desirable to have 

 the whole preparation concentrated, this method offers some advantages. 

 Since some of the salts in allantoic fluid are present almost to the point 

 of saturation, it frequently is desirable to dialyze the preparation against 

 distilled water at 4°C before evaporation. The time of dialysis and the 

 number of water changes depend upon the amount of salt to be removed. 

 Most of the salts can be removed by six exposures to 20 vol of water 

 over a 72-hr period. Approximately 80 per cent of this time can be 

 saved by employing a stirring apparatus during the dialysis. If a 

 precipitate forms in the dialysate, it can be removed by low-speed cen- 

 trifugation. The precipitate usually contains some of the virus. 



The preparation is placed in a viscose tube of suitable diameter and 

 length with a wall thickness of 0.18 mm. The tube should be washed 

 inside and out. After double-knotting one end, fill it with water to 

 test for leaks. If the bag is intact, empty it and tie a funnel in the open 

 end. The apparatus can be sterilized in flowing steam. This bag can be 

 used for dialysis and for evaporation. To evaporate the preparation the 

 tube is hung in front of the air current from an electric fan. The evapora- 

 tion keeps the preparation reasonably cool, and several viruses will not 

 fall in infectivity during the process. If the column of flxiid is measured 



