100 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS trations of methyl salicylate in mineral oil. Figure 3 gives similar curves showing the plasma levels of unhydrolyzed methyl salicylate and serves to demonstrate the relationship between plasma levels of methyl salicylate and salicylic acid. Figure 4 shows a logarithmic transformation of the data _ IOO% 20% - -- IO% i// ........•-___ • 5% .... , , 5 1530 I 2 3 (MIN.) TIME (HOURS) Fig. 3--Effect of varying concentration on the absorption of methyl salicylate in mineral oil. I I I I I I ! I I 70 -- /100% j200/. - 50-- - 40 20 I0 -/ ./.-'q I I • I" I I I - c7' O• :• • 1.10 1.201 .:30 1.40.50 .601 20 .eo I I I I I .08 .16 22 .:39 .6:3 5 I0 15 :30 I HR. MIN. TIME GENERAL E(3UATION: A= KS + (C-K) S= I-e -t A ß ABSORPTION ).Lcj/mi. K ß SLOPE CßVALUEOFAWHEN SßO Fig. 4.--Effect of varying concentration on the absorption of methyl salicylate in mineral oil transformed to linear form. 1.90 I 1.0• I I S .86 .95

THE PERCUTANEOUS ABSORPTION OF SALICYLATES 101 shown in Fig. 2 and the equation employed. Please notice that the inter- cepts at T = infinity give the theoretical maximum plasma levels for each concentration while the intercepts at the abcissa give the minimum times required for salicylic acid to become detectable in the blood. T,v. Evr•.cT or AR•.A When the same quantity of material was employed, but the area was re- duced by 50 per cent, a distinct decrease in absorption was obtained. This is shown in Fig. 5. 50.0 l I I FULL ARE 40.0 - L ,!! 20.0 j !/ J o.o I """" 0 '•" I I I TIME (HOUFIS) 5.•Effect of area on the percutaneous absorption o• 10• methyl salicylate in m•neral oil. Fig. 50 4O 30 2O t IO i I •IGM. - 2 GM. 0.5 GM. ,/, - 0 I I I TIME (HOURS) Fig. &--Effect of varying the amount of the mixture of 10% methyl salicylate in mineral oil on the percutaneous absorption of methyl salicylate.