1899J MICROSCOPICAL JOURNAL. 343 



teria cannot grow down in the enamel, not because the 

 hole is too small, but because the water in the tooth has 

 become too insoluble or inaccessible to the digestive ac- 

 tion of bacteria. The remains of the dead bacteria act- 

 ually serve as a tilling and protection against the entrance 

 of other bacteria. 



We carry into actual practice the application of the 

 principle, viz : protecting the water in enamel and den- 

 tine by causing the tooth, and the bacteria as far as they 

 have penetrated the tooth, to take up certain substances 

 dissolved in water, like formaldehyde, nitrate of silver, 

 chloride of gold, sulphate of copper, chloride of zinc, 

 chloride of tin, and a whole host of substances of a like 

 nature that I believe have the power of protecting the 

 water in the tissues from the growth of bacteria. 



Formaldehyde is used about as follows : After cleans- 

 ing the surface to be hardened with pyrozone (three per 

 cent medicinal) make several applications of the formal- 

 dehyde, varying in strength from two to forty per cent 

 (forty per cent being full strength as it comes to us from 

 the shops) to the cavity, carious surface, and healthy por- 

 tion of the tooth and teeth under the rubber-dam, from 

 ten to forty minutes. The cavity is then dried out and 

 coated with a saturated solution of paraform in chloro- 

 form, to which has been added sufficient hard Canada bal- 

 sam to make the solution a thin varnish, into this, after 

 waiting for the varnish to nearly dry, may be burnished 

 amalgam, stuck gold, gutta-percha or cement. 



Formaldehyde should never be applied to the surfaces 

 of the teeth, except the rubber-dam be in position, fitted 

 evenly around the necks of the teeth, so that there shall 

 be no holes whereby the mucous surfaces of the mouth 

 may become exposed to the action of formaldehyde, as it 

 produces an ugly slough. In the application of the for- 

 maldeyde it is important to have the surfaces of the teeth 

 free from all adhering colonies of bacteria, so as to be 



