THE elastic and mechanical properties of dentin were studied as early as 1895 by Black' who measured the compressive strengths of cubes of dentin and reported that a cube, 0.08 inch square, had an average compressive strength of 37,200 psi and an elastic deformation of 2.09 per cent when subjected to a load of 100 pounds (elastic modulus-0.8 x106 psi). Peyton, Mahler, and Hershenov2 reported average values of 23,400 psi, 36,100 psi, and 1.67 X106 psi for the proportional limit, compressive strength, and elastic modulus, respectively. Stanford, Paffenbarger, and Kumpula3 reported average values for these properties of 25,100 psi, 50,400 psi, and 4.1 x106 psi, respectively. Neumann and Di Salvo4 reported values from 1.1 to 1.7 X106 psi for the elastic modulus using large transverse sections of teeth. The values for the proportional limit and compressive strengths of dentin were in reasonable agreement, but those for the elastic modulus differed widely. The difficulties involved in handling small specimens of dentin and the problems connected with correcting the deformation used in the calculation of the elastic modulus values possibly account for the differences obtained in the various studies. It was felt that a critical re-evaluation of the technics and procedures used to obtain the elastic modulus, with particular emphasis on flow properties, would help to establish a better experimental value for the elastic modulus as well as the other physical properties of dentin. EXPERIMENTAL Equipment.-The equipment used to measure the elastic properties of dentin was the same as that described by Peyton, Mahler, and Hershenov,2 except for the compression testing machine and the type of steel plungers used. The stress was applied to the samples by a Riehle testing machine. The load scales of the testing machine were calibrated for loads in compression with a recently calibrated dynamometer ring. The calibration data for the 250 lb. scale, which was used for most of the measurements, showed that the load indicator was accurate to ± 0.5 per cent at the low end of the scale and to ± 0.1 per cent for the high portion of the scale. The actual error was generally ± 0.2 lb. over the entire scale. The accuracy on this and higher scales was greater than reading errors, and, therefore, the recorded scale readings were used directly in the calculation of the stress. This report represents the partial results of studies supported by Contract No. D-462

THE hardness of enamel and dentin has been determined by a variety of methods including abrasion," 2 pendulum,' scratch,4-7 and indentation" teehnics. Since the hardness of enamel and dentin has been shown to have considerable local variations, the methods using a microscratch or microindentation have been preferred. One of the more common types is the Knoop diamond indenter14 which has been used by a number of investigators. ', 12, 15, 16 It should be mentioned, however, that in spite of the fact that the indentations are extremely small, they still represent a macroindentation when compared to the microstructure of enamel and dentin.The majority of the published hardness data for enamel and dentin has been measured on ground sections, although several papers'0 13 reported the hardness of intact enamel surfaces. The conclusions in regard to the difference in hardness from one section of a tooth to another are at times in variance with each other. This study of dentin and enamel was undertaken in an attempt to establish any trends in hardness existing from one area of a tooth to another or between different types of teeth. With this purpose in mind, this research did not attempt to relate the hardness values to the histologic tooth structure, but a sufficiently large number of hardness measurements were made so that the data could be treated on a statistical basis. EXPERIMENTAL Specimen Preparation.-Mature, freshly extracted, noncarious teeth were imbedded in Ward's Bio-Plastic by suspending them in a Vaughn ring containing the polymer mixed with the catalyst and accelerator. The incorporated air was removed by degassing in a vacuum chamber, after which the polymerization was hastened by placing the ring in an oven at 500 C. for 4 hours or more.The imbedded teeth were sectioned by using a water-spray cooled carborundum wheel, 0.33 mm. thick. The specimens were cut so that the sections were 1 to 2 mm. thick, the first cut being made at or slightly below the ocelusal surface or incisal edge. Specimens cut in a direction mesial to distal or buccal to lingual were cut so that the first section was the lingual or mesial surface, respectively. The thickness of each section was measured with a micrometer and the surfaces to be tested were

The compressive properties of human enamel and dentin have been reported by Stanford, Paffenbarger, Kumpula, and Sweeney.' The elastic modulus of occlusal, side, and cusp enamel was reported to be 1.8, 6.0, and 8.2 X 106 psi, respectively. The corresponding values for the proportional limit were 16,800, 21,000, and 34,200 psi and, for the compressive strength, 19,400, 28,300, and 40,200 psi. An improved procedure for preparing compressive specimens of hard tooth tissues and some restorative materials was published by Stanford, Weigel, Paffenbarger, and Sweeney.2 The compressive properties of enamel were within the experimental error of the earlier values, and additional values relating compressive properties to environment of development and orientation were reported. In addition, the compressive properties of plastics, amalgam, silicate cement, zinc phosphate cements, and dental golds were listed.Tyldesley3 determined the mechanical properties of enamel by using a transverse type of loading system. The elastic modulus of enamel was reported to be 19 X 106 psi in bending. The proportional limit and compressive strength were found to coincide at an average value of 11,000 psi.The published values for the compressive strength of enamel appear low when compared with the values listed for human dentin. Craig and Peyton4 reported an average compressive strength for dentin of 43,100 psi; Stanford et al.1 gave a value of 50,400 psi; and Tyldesley3 published a value of 38,800 for the breaking stress. The highest average compressive-strength value of 40,200 psi reported for cusp enamel is, in general, lower than those reported for dentin.' 2 These results do not appear reasonable when the hardness and general working characteristics of enamel and dentin are compared.The principal purpose of this investigation, therefore, was to re-evaluate the compressive properties of proportional limit, compressive strength, and elastic modulus of human enamel, using improved procedures for sample preparation.In addition, the dental literature included little information concerning the compressive properties of restorative materials measured on specimens approaching the size normally used in dentistry.2 Investigations of the effect of sample size on the compressive properties of amalgam indicated that higher values were obtained with smaller specimens. The second object of this study, therefore, was to determine the compres-

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The thermal conductivity of human dentin has been reported by Lisanti and Zander,' Simeral2 Phillips, Reinking, and Phillips,3 and Soyenkoff and Okun4 to be 2.29 X 10-3, 2.35 X 10-3, 0.257 X 10-3, and 0.96-1.07 X 10 3 cal/sec/cm2/0C/cm, respectively. The thermal conductivity of human enamel has been reported by Soyenkoff and Okun3 to be 1.55 X 10-. More limited data are available for zinc phosphate and silicate cements. Simeral2 reported values of 2.81 X 10-3 for zinc phosphate and 2.00 X 10-3 for silicate cements. Phillips and co-workers3 5 at different times listed the thermal conductivity of zinc phosphate cement to be 3.91-5.37 X 10-3 and 0.311-0.388 X 10-3 cal/sec/cm2/0C/cm, which represents the centimeter, gram, second (c.g.s.) units of measurement. Silicate cement was found to have a thermal conductivity of 0.458 X 10-3 c.g.s. units.5 A survey of the dental literature revealed that no values have been reported for amalgam. All the literature values indicated that dentin, enamel, zinc phosphate cements, and silicate cements were good thermal insulators, although the numerical values for the various materials often disagreed by a factor of 10. It was the purpose of this investigation to establish more accurately the thermal conductivity of tooth structure and dental cements, as well as to determine a value for dental amalgam. MATERIALS AND METHODS Specimen preparation.-Dentin and enamel specimens were cut from human teeth by means of a diamond-core drill mounted on a jeweler's lathe. The cylindrical blanks obtained were held in the lathe by a chuck, and the sides of the cylinder were cut down by a tungsten carbide tool. The dentin specimens were cut to a diameter of 0.234 inch (15/64") and the enamel specimens to a diameter of 0.156 inch (5/32"). These finished cylinders were then placed in appropriate collets in the lathe and the ends ground flat and parallel by a grinding attachment and an India stone. The thickness of the enamel specimens was 0.030 inch, and those for dentin varied from 0.045 to 0.060 inch. The zinc phosphate* and silicatet cement specimens were prepared by placing the mixed cement in a split stainless-steel mold, which was 0.