PHYSICAL CHEMISTRY OF ENAMEL DISSOLUTION 235 



niicroradiograph\' and light microscopv and as a prcliminaiv step 

 in sample preparation for electron microscop\'. The thickness of the 

 sections xaried between 50 and 100 microns. The sections were 

 mounted on glass slides with Permount after microradiographs were 

 taken. 



The rate of enamel dissolution during incipient carious lesion 

 formation followed a typical solubility curve (Figs. 25, 26, and 27). 

 In every case, the rate of dissolution decreased with increasing 

 length of exposure time, and the curves appeared to be approaching 

 equilibrium. The decrease was not due to saturation of the bulk 

 solution in all cases, as identical rates were found when the volume 

 of the decalcification medium was increased tenfold. It is apparent 

 that the dissolution rate is limited bv changes in the composition 

 of the medium in the vicinitv of the enamel because of lack of time 

 for equilibration with the bulk solution. 



Adding calcium chloride to a medium containing 0.05 m lactic 

 acid plus 6 per cent hvdroxyethvl cellulose at pH 3.5 (Fig. 26) 

 retarded the rate ( Fig. 27 ) . As pointed out previously, however, this 

 addition caused a better-defined "relatively sound outer layer." In- 

 creasing pH of this same medium at a constant buffer strength 

 caused a marked reduction in the rate of enamel dissolution (Fig. 

 27). Increasing temperature from 4° to 37 °C had almost no effect 

 (Figs. 25 and 26). 



Further quantitative evaluation of the different variables was done 

 bv a differential method measuring the rate of dissolution at only 



Fig. 23. An election micrograpli of a section of normal enamel cnt with a 

 diamond knife and then demineralized with 0.005 n hydrochloric acid. The 

 residue, which appears to be organic, is believed to be the remains of the or- 

 ganic matrix of enamel, but little of the original enamel rod pattern is recog- 

 nizable. (X 5000.) 



Fig. 24. An electron micrograph of a section cut from the sample of Fig. 

 21 and demineralized with 0.005 n hydrochloric acid. The incipient carious 

 lesion was produced bv a 72-hour exposure at 37°C to a medium consisting of 

 0.05 iM lactic acid and 0.03 m CaCL plus 6 per cent hydroxvethyl cellulose at 

 pH 3.5. The enamel rod pattern is very apparent and resembles the picture in 

 Fig. 21. The organic residue is located primarily in the rods and is more highly 

 concentrated than in normal enamel. The highest concentration is always found 

 at the surface (upper left-hand corner) in the relatively sound outer laver. (X 

 5000.) 



