"If a person eats an oyster stew at 130 deg. F., a gold filling would carry the difference between the temperature of the stew and that of the mouth, 130 - 98 = 32 deg., almost undiminished to the bottom of the cavity; allowing 2 deg. of diminution, then the cavity around the gold filling has assumed 128 deg.; now the person feels warm and drinks ice-water at 32 deg.. Taking into consideration the specific heat of the gold filling, it will assume about 40 deg., which it carries with a diminution of the cold of about 4 deg.,--that is, as if it was 44 deg.,--into the interior of the cavity; then the cavity will assume 44 deg., the difference within one-tenth of a minute being 128 - 44 = 84 deg., a change which would produce a violent inflammation in any organ which was not accustomed to it. This derangement in the tooth means interruption of circulation, and young teeth will be most affected. "Thermal effect depends on heat-conducting power [gold is nearly four times as good a conductor of heat as tin] and also on specific heat, so the more the latter approaches that of the tooth the less it is liable to produce sudden changes [thus favoring tin]. Specific heat manifests itself by the speed of changes, while the heat-conducting power influences the intensity [then the intensity of heat in a gold filling would be three or four times as much as in a tin filling]. In speed gold produces this change in one-tenth of a minute" [tin in one-fifth,--that is, gold absorbs heat and expands about twice as fast as tin]. In 1838 Dr. J. D. White introduced sharp-wedge-shaped instruments for filling teeth, and he claims to have been the first to use them; they pack laterally as well as downward, and present as small a surface to the filling as possible, so that the greatest effect may be produced upon a given surface with a given power. Rolls of either tin or gold are made by cutting any desirable portion from a sheet of No. 4 foil; cut this portion once transversely, place on a napkin or piece of chamois, then with a spatula fold a very narrow portion of the edge once upon itself; then with the spatula resting on the thickened edge draw the spatula away from it with gentle pressure, and the foil will follow in a roll. [Illustration: FIG. 9.]