in the Cavities of Minerals. 41 



not enable him to form such a substance, a plate of topaz, with 

 particular longitudinal cavities, might be used, as a delicate ther- 

 mometer, for certain ranges of temperature ; and where slight 

 variations require to be observed, several such plates might be of 

 essential service in many researches, both of a chemical and a 

 physical nature. 



VOL. x. P. i. 



