PRECAUTIONS 99 



six units of stain will be required, or 0.6 cc. of Unna's 

 dye. 



The foregoing examples show how the coefficient of 

 diffusion is determined, and how, after it has been 

 ascertained, one can, by means of the equation, find 

 out other factors, which may be unknown quantities. 

 It follows that by this procedure other substances can 

 be made to diffuse into the cells. This method of 

 calculation has been used throughout these researches, 

 and it will be seen that further examples will be given 

 in the future chapters of this book. 



The factors most often employed are alkali and heat. 

 Salts are not usually varied a greal deal, although their 

 amounts can be altered if necessary by adding more of 

 them to the second solution. 



The determination of the units of any other sub- 

 stance is carried out on the principle that all units must 

 be equal. Let us take a substance like urea, for in- 

 stance. It delays the diffusion of other substances, 

 such as Unna's stain. All that has to be done is to 

 find out how much urea must be contained in the 

 10 cc. of jelly to neutralize the increasing action of a 

 unit of alkali. Having found out the unit of the fresh 

 substance, that unit is added to the equation in the 

 usual w r ay. If it increases diffusion it is placed in 

 the bracket with the alkali and heat; if it delays diffu- 

 sion it is bracketed with the salts. 



Lastly, having obtained the coefficient of diffusion 

 of a class of cells by measuring the rate of diffusion 

 of the stain, the stain may be omitted and any other 

 substance substituted for it. If more than one sub- 



