ON GUTTA-PERCHA AS A PERMANENT STOPPING. 151 



attempt it who has not had some little experience 

 in chemical manipulation. There is one essential 

 point, however, that I must mention, which is not 

 likely to be found in books ; this is the necessity 

 of exposing the silica when thoroughly dry to a red 

 heat, in order to bring out the pure white colour; 

 when simply dried, it has a bluish or greyish hue. 

 The heat must not be allowed to rise too high, or 

 the silica will shrink up to a much smaller bulk, 

 becoming semi-crystalline and gritty, like that pre- 

 pared from the silicate of potash. This has hap- 

 pened once or twice to myself, to my great chagrin. 

 If the silica has been properly prepared, it is almost 

 inconceivably light and bulky, of snowy whiteness, 

 perfectly soft and impalpable when taried between 

 the teeth, and so ef&cacious in whitening the gutta- 

 percha, that cement can be made with it to match 

 in colour with teeth of every hue, from the pearly 

 white to the dirty brown. 



The cement is made by thoroughly incorporating 

 the ingredients in a hot mortar, in the proportion 

 of one part by weight of silica to four of gutta- 

 percha. Care must be taken to prevent the access 

 of dust, &c., during the process. The cement when 

 cold is extremely hard and firm, and yet so tough that 

 it cannot be disintegrated by any amount of masti- 

 cation, which has only the efiect of extending it into 

 a thin membrane. 



In using it, the cavity should be prepared as for 



