78 DIFFUSION OF SUBSTANCES INTO LIVING CELLS 



polychrome dye. Hence the unit of alkali is . 1 cc. 

 of a 5-per-cent solution of sodium bicarbonate. 



Time is a factor. One unit of time is ten minutes; 

 and since time increases diffusion in arithmetical pro- 

 portion, therefore in twenty minutes (two units) the 

 diffusion of one or more units of the dye or other 

 substance will be doubled. 



The unit of heat is 5 C. ; unity is 10 C., because 

 one cannot conveniently work at a temperature below 

 this point; 20 C. is three units, etc. 



Salts delay diffusion. The two commonly employed 

 are sodium citrate and sodium chloride. Their units 

 respectively are . 03 gramme and . 08 gramme. One 

 unit of sodium citrate or sodium chloride will prevent 

 the increase of diffusion due to one unit of alkali, heat, 

 or time; an extra unit of stain will neutralise a unit 

 of salt. 



Hence the units of all the factors are so measured 

 experimentally that they are as nearly as possible equal 

 in value. The delay in diffusion caused by a unit of 

 a substance which decreases diffusion is equal to the 

 acceleration occasioned by one which increases diffusion. 

 It will therefore be realised that the units can be 

 substituted for each other. A unit of alkali will double 

 the diffusion of the dye, but so will another unit of the 

 dye itself. Again, the unit of time is ten minutes; 

 since time increases the diffusion by arithmetical pro- 

 gression, another ten minutes of time is equal to a unit 

 of alkali or another unit of dye. Salts delay diffusion; 

 we have found out how much of a salt, such as sodium 

 citrate, is required to decrease this diffusion in equal 



