White caries, the most formidable variety known, may be produced by nitric acid, and in these cases all the components of the tooth are acted upon and disintegrated as far as the action extends. In proximal cavities attacked by this kind of caries, separate freely on the lingual side, and fill with tin. When such fillings have been removed the dentin has been found somewhat discolored and greatly solidified as compared to its former condition; this solidification or calcification is more frequent under tin than gold, which is partly due to the tin as a poor conductor of heat. Nature will not restore the lost part, but will do the next best thing--solidify the dentin. In some cases, under tin, the pulp gradually recedes, and the pulp-cavity is obliterated by secondary dentin. In other cases the pulps had partly calcified under tin. It has been known for years that tin would be tolerated in large cavities very near the pulp without causing any trouble, and one reason for this is its low conducting power. Attention is called to the fact that gold is nearly four times as good a conductor of heat as tin, and more than six times as good a conductor of electricity. Where tin fillings are subject to a large amount of attrition, they wear away sooner or later, but this is not such a great detriment, for they can easily be repaired or replaced, and owing to the concave form produced by wear the patient is liable to know when a large amount has been worn away. That portion against the wall of the cavity is the last removed by wear, so that further caries is prevented so long as there is any reasonable amount of tin left. If at this time the tooth has become sufficiently solidified, proper anchorage can be cut in the tin or tooth, one or both, as judgment dictates, and the filling completed with gold. A tin filling, confined by four rather frail walls, may condense upon itself, but it is so soft and adaptable that the force which condenses it continually secures the readaptation at the margin; thus there will be no leakage or caries for years. Owing to its softness and pliability, it may be driven into or onto the tubuli to completely close them from outside moisture, and with a hand burnisher the tin can be made to take such a hold on dry, rough tubuli that a cutting instrument is necessary to remove all traces of it.