342 THE AMERICAI^ MONTHLY [Nov 



peculiar black or reddish line, and when present ti is 

 quite impossible to find decay. From this it would seem 

 that there are certain bacteria wliich of themselves have 

 the power to confer immunity from attacks of those bacte- 

 ria that cause the destruction of the teeth. 



In mouths where the decay is slow the decayed por- 

 tion is always discolored. The cause of this discolora- 

 tion is an important question, and in many mouths one 

 quite difficult of solution. In many instances I have 

 found the decoloration is tobacco stain (seep. 341). This, 

 I observed, had penetrated quite deeply the enamel and 

 dentine, and, by the aid of direct sunlight reflected by 

 the mirror up through the Abbe condenser, and the speci- 

 men under the microscope, one could see many bacteria 

 which had taken on the stain from tobacco. This would 

 suggest that the discoloration seen in decay is partly due 

 to bacteria having become stained. 



The use of direct sunlight in the examination of speci- 

 mens stained to show bacteria will make visible bacteria 

 as far as they have penetrated the tooth structure. This 

 idea came from observing how a ray of sunlight would 

 make visible the bacteria and particles of dust in a dark 

 room. I immediately made use of it in my study of speci- 

 mens of teeth under the microscope. It was not neces- 

 sary to grind them so thin to show bacteria penetrating 

 both enamel and dentine. 



This immune layer is sometimes due to the cracks and 

 spaces in and between the cells having become filled with 

 insoluble compounds, as is seen in teeth stained with to- 

 bacco or nitrate of silver, chloride of gold, sulphate of 

 copper, etc., similar to the manner that oil covers the 

 surface of water and retards its evaporation. In other 

 words, the water already in the enamel, dentine and bac- 

 teria has attracted compounds that have a greater affini- 

 ty for the water in the tooth substance than have the bac- 

 teria. So decay stops for a time, simply because the bac- 



