632 M. Stockdale Table II. (continued) Source R.H.external (Reference) (%) Cx 102 Co D x 10 a (mg cm -a) Co-- C (cm" h -1) a 10 13 (Guinea-pig) 13 (Human) 80 4-13 16'88 198.2 4'89 20 3.34 4.18 39.3 1 '04 40 3.60 5.56 69.5 1.84 80 4.13 16.88 182.8 4'88 20 3.34 4'18 40-1 1'04 40 3.60 5.56 62-1 1.58 80 4.13 16.88 146-9 3.77 D x 10 aa cm sec -x 0.44 1.11 2.2 0.37 0.88 1-25 2.5 0.42 1.32 1.43 2.7 0.45 1.76 1.67 3.3 0-55 2.20 2.00 3.6 0.60 2.63 2.50 4.8 0.80 3.07 3.33 5.3 0.90 3.51 5.00 5.0 0.85 3.95 10.00 3-3 0-55 0'44 1'11 5.6 0'47 0'88 1.25 7'6 0'64 1.32 1 '43 8'0 0'67 1 '76 1 '67 8'6 0.72 2'20 2.00 8'9 0.75 2'63 2'50 10'4 0.87 3-07 3.33 11'5 0.96 3'51 5.00 11'0 0-92 3.95 10-00 9.4 0.79 Using the data in Table H (excluding the two highest humidity values of E1-Shimi and Princen and the data of Grice et al.) the correlation becomes: R -- 0.175(coC•øC)-3-0.46 n = 61 r ----- 0.902. (6) This correlation is illustrated in Fig. 1 and Student's t test shows that the correlation could not have arisen by chance with a confidence of greater than 99'9•o. The water activity profile equation using Equation 6 and derived as before, for the case where R.H.external = 0•o, is as follows: x = 1.54x 10-•Ci-2.62 x 10 -4 In (Co-C-t)-8'23 x 10 -4 (7) This profile is illustrated graphically in Fig. 2. In this case C approaches Co to within 3'4•o at x = 15 x 10 -4 cm. This is a considerable improvement on Equation 5. For different relative humidities, a predicted value of D can be obtained from Equation 6, and hence a new value of J. By substituting this and the new boundary conditions into the corrected version of Equation 4, a new equation for the different condition can be derived.

Ifater diffusion coefficients and activity 633 lO I 4o Figure 1. Plot of relative diffusion coefficient (R) versus Col(Co- C). ¸, data from ref. 10 A, data from ref. 12 +, data from ref. 11' ED, data from ref. 13 O, data from ref. 9 a,, data from ref. 4. The data is plotted on a log-log scale for convenience. Figure 2. Water activity profile for normal skin. (A) R.H.externa I = 0%, Equation 7. (B) R.I-I.external = 50•o, Equation 8. % 2 Co 0 4 8 12 16 x • i0 4 (cm) The equation for R.H.exter•a• = 50• is as follows' x = 1'92 x 10-•'Ci- 3'21 x 10 -• In (Co- Ci)--1'65 x 10 -a (8) This is also illustrated graphically in Fig. 2 and again Ci closely approaches Co at x •- 15 x 10 -• cm. The area under the curve in Fig. 2A is 65• of the area bounded by the lines Ci = 0 Ci = Co x = 0 and x = 15 x 10 -• cm. The area under the curve and above the line Ci = 0.5 Co in Fig. 2B is 72•o of the area bounded by the lines Ci = 0.5 Co Ci = Co x = 0 and x = 15 x 10 -• cm. Thus, the use of a second iterative