THE OERM THEORY OF CARIES. 159 



view. It is admitted that decay is brought about by acids, 

 developed, or Dr. M. supposes them to be developed by fer- 

 mentation of some kind. The object of this course of experi- 

 ments is to find and examine this supposed ferment. 



It is not necessary that I should describe in detail all the 

 apparatus with which the experimenter provided himself; it 

 is sufficient to say that all the appliances for the prevention 

 of error were used. The first question to determine was 

 whether or not the ptyaline of the saliva could so change 

 starch as to produce an acid. This question was soon decided 

 in the negative. The starch was promptly changed into sugar, 

 but here the reaction ceased ; the fluid remained permanently 

 sweet when the proper precautions were observed to prevent the 

 ingress of germs. This proves that the acidifying power does 

 not belong to the saliva. It must then be something foreign. 



Now, a freshly extracted carious tooth was taken, all food 

 removed, the outer portions of the decayed mass saturated 

 with a ninety per cent, solution of carbolic acid, to destroy 

 any accidental germs that might be in this portion. Then, 

 with an instrument, purified by heat after each cut, layer 

 after layer of the softened dentine was removed until the 

 inner portions were reached. Then a slice was quickly con- 

 veyed to a sterilized culture medium, composed of sterilized 

 saliva, water, sugar and starch, and placed in an incubator, 

 together with another test-tube of the same culture fluid un- 

 infected, to serve as a check. In twenty-four hours the 

 infected culture became acid, while the other did not. This 

 remained constant in a sufficient number of experiments to 

 establish the fact that the acidity was due to the infection. 

 From the cultures that had become acid, other cultures were 

 infected, which also became acid, thus proving that the ex- 

 perimenter was dealing with a ferment that was capable of 

 propagating itself; an organized ferment. 



