Alternative theory variation of P with S

The negative sign occurs because the added layer causes a decrease in S. This equation applies to an apparatus where the liquid is run off and the deposited layer is retained for analysis. In the type of apparatus where the 

By plotting P against S and finding the slope of the curve, y(rf)dr/ may be evaluated. Equations (8.78) and (8.79) can also be derived by differentiating equation (8.69) which may be written  

Differentiating this with respect to S leads directly to equation (8.65). 

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