N 



116 ON SOME OF THE CAUSTICS 



lessened. It not nnfrequently happens, however, 

 that several applications are necessary; and I 

 have observed that if pain does not occur during 

 the action of the chloride, the benefit is seldom so 

 great. This caustic is peculiarly useful in cases 

 where the friction of artificial work has set up 

 great irritability about the necks of the teeth. 



We have next to consider arsenious acid as an 

 agent in reducing the sensitiveness of dentine. 

 There can be no question but that this is by far 

 the most efficacious substance for this purpose. 

 If it can only be thoroughly kept in contact for a 

 certain length of time with the part to be acted 

 upon, it will, I believe, scarcely ever fail in bring- 

 ing about the desired effect. It is, however, 

 much slower in its operation than chloride of zinc, 

 and cannot, therefore, for obvious reasons, be em- 

 ployed in mere surface cases, as the latter can ; 

 but it is far superior to it where there is a cavity 

 into which it can be inserted and leffc without dis- 

 turbance for a given time. The mode of applying 

 arsenious acid is too well known to render it 

 necessary that I should enter much into that part 

 of the subject. The preparation I use is the 

 common one, I believe, of arsenious acid, creosote, 

 and morphia, and I leave it in the cavity, covered 

 in with wool moistened in gum mastic, for a 

 length of time, in proportion to the nature of the 

 case, the age and constitution of the patient, &c. 

 But there is one serious objection to the use of 



