1. Head, J.: Modern Dentistry, Philadelphia, 1917, W. B. Saunders Company, p. 46.
2. Pickerill, H.P.: The Prevention of Dental Caries and Oral Sepsis, ed. 3, New York, 1924, Paul B. Hoeber, Inc., p. 113.
3. Surface Resistance of Human Enamel to Acid Decalcification