STANDARDISATION OF FACTORS 77 



that of one unit of any other factor which also increases 

 diffusion. Likewise a unit of any factor which retards 

 diffusion is also equal to a unit of any other factor 

 which does the same thing. But further still, a unit 

 which increases the diffusion of a substance into a cell 

 is so chosen that the increase which it causes can be 

 exactly neutralised by a unit of a factor which retards 

 diffusion. The units are all equal in value, so to speak. 

 Some increase diffusion, and some decrease it. Any 

 number of units of factors which decrease diffusion 

 retard exactly the increase of diffusion due to the 

 same number of units of factors which cause increase of 

 diffusion. 



By the first law, if we double the quantity of the 

 dye in the jelly, we double the rapidity of its diffusion 

 into the cells. A convenient quantity was chosen, 

 namely, 0.1 cc., and this contained in 10 cc. of jelly 

 constitutes one unit of polychrome dye. 1 Let us 

 suppose that this quantity (one unit) causes staining 

 of the nucleus of a given cell in a certain time. If 

 now another unit is tried, the cell will stain in half the 

 time it did before. 



The alkali, sodium bicarbonate, increases the dif- 

 fusion of other substances into cells, and therefore it 

 greatly increases the rapidity of the staining by poly- 

 chrome methylene blue. Now, since all units must be 

 equal in value, it was ascertained experimentally that 

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

 exactly doubled the rapidity of diffusion of one unit of 



1 Unna's polychrome methylene blue (Griibler) is only supplied^in 

 solution, which is standardised. It cannot be made in a powder. 



