98 "COEFFICIENT OF DIFFUSION OF CELLS" 



from which it was prepared. The total contents of the 

 tube has already been made up to 10 cc. as usual. The 

 specimen is prepared and incubated at 37 C. Then the 

 question must be asked, How long must the specimen 

 remain in the incubator before the spirochsetes will be 

 stained ? We could, of course, keep taking the speci- 

 men out and looking at it, but every time we did this 

 we should lower the temperature and spoil the experi- 

 ment. It is much simpler to plot the equation. The 

 coefficient of diffusion of the spirochsetes is a known 

 quantity, i.e. 20; the time is now the unknown factor. 

 We therefore exchange the places of the symbols cf 

 and t, thus: 



=3. 

 t= 3, or 3 units of time, i.e. half an hour. 



Likewise, since the units of all the factors are equal, 

 we may interchange any of them. Another example 

 may be given. A certain strain of Amoeba coli from 

 a "culture" has a cf of 13. We want to stain the 

 nuclei of these parasites in 10 minutes with a jelly 

 which contains 7 units of alkali. But we want to 

 stain them at the room temperature of 20 C. The 

 jelly contains its usual content of sodium citrate and 

 sodium chloride i.e. 3 units of the former and 1 of 

 the latter. How much Unna's stain must we add to 

 the tube of jelly to obtain the required result ? 



The number of units of s is the quantity required, 

 hence : 



