152 THE GERM THEORY OF DISEASE. 



tooth is decomposed by an acid ; this acid acts more readily 

 on dentine than upon the enamel, therefore the tendency to 

 the enlargement of the cavity toward the internal portions of 

 the tooth. Some writers, as Dr. Watt, have attempted to 

 define the acids thus acting, and to divide decays into 

 classes, according as this or that acid is active in its produc- 

 tion. The acids that have been thus pointed out are: Nitric 

 acid (white decay), Sulphuric acid (black decay) and Chloro- 

 hydric acid (intermediate colors). 



Most of those who have written on this subject, however, 

 have been content without specifying the particular acids so 

 definitely; while some have been of the opinion that these 

 particular acids have little or nothing to do with the matter, 

 and seek to show that other acids are more likely to do the 

 ugly work. 



The origin of the acids which produce caries has been a 

 subject of much inquiry. A great amount of labor has been 

 bestowed upon this point. It has been supposed that the 

 saliva contained it, and very careful examinations have been 

 made in this direction, which have contributed to our know- 

 ledge of this secretion, but have thrown very little light on 

 the point in issue, other than to demonstrate that the cause 

 of decay is not to be found in this direction. It has shown 

 that decay occurs in mouths in which the saliva is habitually 

 normal; and that decay does not occur in some mouths in 

 which the saliva is habitually acid. 



The hypothesis that the acid is furnished upon the spot, 

 through the decomposition of the food lodging between the 

 teeth, or in crevices and imperfections in the enamel, seems 

 much more feasible, and gives a much more satisfactory 

 explanation of the phenomena. According to this hypothesis, 

 the acid is formed in juxtaposition with the portion of the 

 tooth suffering from its effects, even within the cavity itself, 



