Like other organs of the human body, the teeth are more or less subject to constitutional change. The condition in which we find tooth-structure which needs repairing or restoring should be a sure indicator to us in choosing a filling-material. Up to the age of fourteen, and sometimes later, we find many teeth which are quite chalky. In some mouths also, at this period, the fluids are in such a condition that oxychlorid and oxyphosphate do not last long; for some reason amalgam soon fails, while gutta-percha is quickly worn out on an occlusal surface. In all such cases we recommend tin, even in the anterior teeth, for as the patient advances in years the tooth-structure usually becomes more dense, so that, if desirable, the fillings can be removed, and good saving operations can be made with gold. By treating cases in this manner very little, if any, tooth-structure is lost. The teeth of the inhabitants of Mexico and Guatemala are characteristic of their nervous and nervo-lymphatic temperaments; children ten years of age often have twenty-eight permanent teeth, and they are generally soft or chalky, but our dentists there report good success in saving them with tin. In filling this class of teeth, we should be very careful not to use force enough to injure the cavity-margin, for if this occurs, a leaky filling will probably be the result. Still, we have seen some cases where _slight_ imperfections at the margin, which occurred at the time of the filling or afterward, did no harm, because the deposit of tin oxid filled up the ends of the tubuli, thus preventing caries. We believe that this bar to the progress of caries is set up more frequently when tin is used than with any other metal under like conditions. CHAPTER V. In some mouths tin does not discolor, but retains a clean, unpolished tin color, yet when there is a sesquioxid of the metal formed, fillings present a grayish appearance. In the same mouth some fillings will be discolored, while others are not. As a general rule, proximal fillings are most liable to show discoloration. Perhaps one reason is that on occlusal and buccal surfaces they are subject to more friction from mastication, movements of the cheeks, and the use of the brush. We have seen a large number of fillings which were not blackened, yet were saving the teeth perfectly, thus proving to a certainty that blackening of tin in the tooth-cavity is not absolutely essential in order to obtain its salvatory effects as a filling-material.