REVERSING THE EQUATION 97 



of salts. The (3c+n), however, is the usual content of 

 salts which is always present in the "coefficient jelly," 

 but more salts may be added in the shape of solutions 

 to the 5 cc. which also contains the stain and alkali. 

 Whatever is added must be put down in the equation. 

 The only substance not in the equation is agar, 

 which, as already noted, does not affect the cells, and 

 which is always present in the same strength in every 

 experiment. 



Since the units of the factors are equal and inter- 

 changeable, and since their sum is equivalent to the 

 coefficient of diffusion, the numeral which constitutes 

 the coefficient of diffusion in the equation can be inter- 

 changed with an equivalent number of units of one 

 or more of any of the factors. We may reverse the 

 equation, therefore, and, provided we already know the 

 coefficient of diffusion of the cell experimented with, 

 we can, by this reversal, determine in a moment the exact 

 quantity of any factor required to obtain staining of the 

 nucleus. That is to say, that if the coefficient of diffusion 

 is known, and if all the other factors, except one, are 

 given quantities, then we can determine the required 

 quantity of the one unknown factor simply by reversing 

 the equation; always remembering the well-known 

 algebraic law that in bringing one factor from one side 

 of the equation to the other, we must change the signs. 

 For instance, suppose a strain of Spirochceta refringens 

 has a coefficient of diffusion of 20, and one wishes to 

 stain a sample of them : Let us suppose there is a jelly 

 to hand which contains 6 units of stain, 8 units of alkali, 

 and the usual content of salts in the "coefficient jelly" 



7 



