Cohesion is the power to resist separation, and it acts at insensible distances. The integral particles of a body are held together by cohesion, the constituent parts are united by affinity. The attraction between atoms of pure tin represents cohesion. Marble is composed of lime and carbonic acid, which are united and held together by affinity. The condition which obtains in the tin may be called cohesion, adhesion, welding, or interdigitation, but the fact remains that layers of tin foil can be driven together into a solid mass, making a tight filling with less malleting than is required for gold; if it is overmalleted, the receiving surface is injured. On account of its pliability it is easily adapted to the walls and margins, and a perfect fit is made, thus preventing capillary action and preventing further caries. Of all the metals used for filling it is the best tooth-preserver and the most compatible with tooth-substance, and the facility with which a saving filling can be made largely commends it. Tin has great possibilities, and has already gained a high position as a filling-material. Upon the knowledge we possess of the possibilities and limitations of tin as a filling-material, and our ability to apply that knowledge, will largely depend our success in preserving teeth. It is a good material for filling many cavities in the temporary teeth, and children will bear having it used, because it can be placed quickly, and but little force is required to condense one or two layers of No. 10 foil. The dentin in young teeth has a large proportion of organic material, for which reason, if caries takes place, many believe it is hastened by thermal changes. Gold fillings in such teeth might prevent complete calcification, on account of the gold being so good a conductor; but if tin is used, there is much more probability of calcification taking place, because of its low conductivity and its therapeutic influence. It does not change its shape after being packed into a cavity. Under tin, teeth are calcified and saved by the deposit of lime-salts from the contents of the dentinal tubuli. This is termed progressive calcification.