76 DIFFUSION OF SUBSTANCES INTO LIVING CELLS 



degrees, and seconds. It has been found preferable to 

 measure these factors in special units which can, if 

 necessary, be resolved into their proper ones of 

 grammes, degrees, and seconds. 



For instance, in order to remember the rate of stain- 

 ing of a class of cells it would be most inconvenient 

 to have to make a note of a statement such as this: 

 To stain the nuclei in twenty minutes, it is necessary to 

 keep the cells at 20 C. on a film made from a jelly con- 

 taining 0.5 cc. of Unna's stain, 0. 16 gramme of sodium 

 chloride, . 03 gramme of sodium citrate, and . 3 cc. of a 

 5-per-cent solution of sodium bicarbonate. It is much 

 simpler to say that the jelly contains so many "units" 

 of stain, salts and alkali, heat and time. One may go 

 farther and express these units as a simple equation, 

 thus: 



Stain. Alkali. Heat. Time. Slats. 



cf = (5s+3a 



A letter by itself means one unit of the factor; a 

 number before a letter means that there is that number 

 of units of the factor: c means a unit of sodium citrate, 

 3c would mean three units of it, and so on. 



It will be grasped that it is better to make "one 

 unit" of any factor a standard quantity, and these 

 quantities have been chosen with a special object. As 

 has been previously explained, the coefficient of diffusion 

 of a cell is the total number of units of the factors 

 required to cause staining of the nucleus. Some of the 

 factors increase the diffusion into the cell, and others 

 decrease it. A unit of a factor which increases diffu- 

 sion is so chosen that the increase it causes is equal to 



