The immediate bleaching effect will be observed on the lower third of the tooth where the dentine is the thinnest. In the majority of cases this will be effected by one application. The greatest trouble will be found at the gingival border. Here the dentine is very thick, and it will be slow work, and in some cases end in failure, to restore normal color. The great objection made to this operation is, that the tooth will re-discolor, but if the subsequent operations are properly performed, this danger will be reduced to a minimum. The fact that dentine is permeated by pulp prolongations throughout the tissue, increases the difficulty of bleaching, and also increases the liability of a return of discoloration; but if the oxidation of the soft contents of the tubes has been properly effected, and then an agent used to fill the canal, and also act directly upon this microscopic tissue, there is but little reason to fear a return of discoloration. The operation, simple as it is, requires close attention to details and a clear comprehension of possible results. The tooth having been restored to a good color, the next consideration is the proper filling to place in it. In this connection the before mentioned fact still remains an important factor, that the tubuli are still filled with decomposable matter. To allow this to remain without attention to future contingencies, must result in eventual failure. To effect any good results the antiseptic must not only operate in the main canal, but penetrate deeply into the minuter conduits. This quality is possessed in a remarkable degree by chloride of zinc, and maintains the same effect when combined with the oxide of zinc, forming the oxychloride of zinc. The canal and pulp chamber should be thoroughly filled with this paste, or, it is better to line the whole cavity with it, and then finish with the oxyphosphate, using gutta-percha at the cervical margin. Chlorine acts as a bleaching agent by reason of its strong affinity for hydrogen. Vegetable and animal colors when brought in contact with chlorine in the presence of water, is seized upon by the chlorine, and the oxygen set free, oxidizes the color and destroys it. Chlorine in this case acting indirectly as an oxidizing agent. If you wish to try to bleach more rapidly, a solution of oxalic acid is used to liberate the chlorine. Oxalic acid is more rapid than either tartaric or acetic. Sulphurous acid is said to be a good bleaching agent, and acts by an entirely different method from chlorine. It is therefore of great interest from a chemical point of view. As before mentioned, chlorine acts as a bleaching agent by reason of its strong affinity for hydrogen. Sulphurous acid, on the contrary, is a reducing agent by reason of its affinity for oxygen, in combining with which it becomes _sulphuric_ acid. On the chemical character, therefore, of the coloring matter depends the choice of the agent to be used.