'240 



J. A. GRAY AND M. D. FRANCIS 



Figs. 34 and 35. A section of enamel after exposure for 7 hours to an rui- 

 agitated aqueous solution of 0.05 m lactic acid at its natural pH of 2.6. The 

 right side was protected w ith varnish to serve as a control. 



Fig. 34. A light micrograph shows the complete loss of enamel to a depth 

 of 50 microns and the damage extending slightly into the remaining sample. 

 (X2000.) 



Fig. 35. A microradiograph also shows the complete loss of the enamel 

 surface and confirms the presence of a decalcification front. (X 2000.) 



Fig. 36. An electron micrograph of a section cut from the sample of Figs. 

 34 and 35. This enamel sample was exposed 7 hours to an aqueous solution of 

 0.04 M lactic acid at the "as is" pH of 2.6. Although most of the enamel was 

 disso]\'ed awav as shown in Figs. 34 and 35, a slight advancing front of de- 

 calcification extended into the enamel about 5 microns. (X 5000.) 



