AS APPLIED TO DENTISTRY. 153 



It is argued by some, that none but distilled 

 mercury should be used for this purpose; and 

 others maintain that this is not essential. 



My attention has been called to these points 

 by statements, that when a machine had been 

 some time in use, the same results were pro- 

 duced, at the same indicated temperature, in a 

 shorter period than when the machine was first 

 put up. 



A certain time, and the use of a certain and 

 even degree of steam heat, seems now to be abso- 

 lutely necessary for good and practical work. 



Some important facts on this point have been 

 satisfactorily proved by others as well as by 

 myself : — 



1st. That an excess of heat will give a darker 

 colour to the material, and render it brittle ; while 

 not enough heat leaves it of a brighter colour 

 and not possessing sufficient solidity to hold the 

 teeth. 



Any endeavour to substitute increased heat for 

 length of time would be about as reasonable as to 

 expect to bake a loaf of bread or roast a round of 

 beef in ten minutes. For, unless a given time is 

 allowed, one part will be surely crisp and another 

 raw. 



2nd. That a piece vulcanized upon a plaster 

 model will afterwards fit its duplicate in tin, or 

 fusible metal, and vice versa. 



