J. A. GRAY AND M. D. FRANCIS 



0.1 0.2 0.3 0.4 0.5 



TOTAL LACTATE CONCENTRATION (moles/liter) 



Fig. 29. Enamel dissolution rate as a function of lactate concentration dur- 

 ing exposure at 37° C to a medium containing lactic acid and CaCL plus 6 per 

 cent hydroxyethyl cellulose adjusted to either pH 3.5 or 4.5. The rate was linear 

 and increased with increasing lactate concentration. Decreasing pH or CaCU 

 concentration increased the slope of these linear functions. The intercept at zero 

 lactate concentration resulted from the effect of the buffer content of the hy- 

 droxyethyl cellulose sample. 



order, which appears to be equal to about %. The fractional aspect 

 of the order is quite certain, but further data are required to establish 

 the quantitative exactness. 



It should also be pointed out that under conditions giving very 

 high enamel dissolution rates, involvement of the surface became 

 more and more extensive, leading to the condition, described by 

 Hals et al. ( 1955 ) , of an "inner" and an "outer" spot. However, there 

 appeared to be no detectable discontinuity in the rate when this 

 effect was encountered and included in the measurement of enamel 

 dissolution ( Fig. 29 ) . The very highest rates resulted in severe sur- 



