x P R E F A C E. 



form ; common fait, fluor, zeolyte, galena, 

 in a cubic, &c. if the nature of any fubftance 

 could be determined by its form, it would 

 undoubtedly be that of falts, yet there is 

 fcarce any of thefe which in different circum- 

 fiances, may not affume a different figure ; 

 Mr. Pot't affures us, that microcofmic fait 

 aflumes the figure of almoft all other falts, 

 nitre, vitriol, falammoniac, allum, glauber's 

 falts, &c. 4 Pott) 49. According to Ma- 

 quer, if fublimate corrofive be cryftalized, by 

 cooling it forms needles, but if by mere eva^- 

 poration, cubes or lozenges. Mem. Par. 1755* 

 p. 540. Digeftive fait will form cubes if it 

 be exactly neutral, but if the alkali predo- 

 minates quadrangular prifms, common fait 

 is generally looked upon as the moft con- 

 ftant in its figure, yet Mr. Cadet has found 

 it cryftalized in needles, 9 Mem. bcav. Etrang. 

 p. 555. and Gerhard^ 4 Berlin^ Schrift. 292, 

 Very many, if not moft of the miftakes to 

 be found in chymical writers on falts, arofe 

 from their having denominated them from 

 their figure. 



Laftly, Specific Gravity.) which is certainly 

 one of the beft external tefts, frequently va- 

 ries, by reafcn either of the different texture 

 of the fame fpc.cies of mineral, into whofe 

 jnterftices water cannot equally penetrate, or 

 by reafon of the greater proportion of fome 



or 



