Of Magnefian Earth . 467 
rf of Magnefia, viz. of not changing its dimen- 
* c fions, nor receiving any alteration in its con- 
“ fiftence, by the ftrongeft heat. I therefore 
“ went, the fird fair day, to the place where 
“ the lens was at work, carrying with me the 
“ very fame bottle with which you favoured me. 
,c I made a fimilar cube of your Magnefia, with- 
Ct in the fame metallic box that had been ufed 
<c in the former cafe 3 but 1 found that, by ex- 
tc pofing it, even for a longer time, to the force 
tc of the lens, than the other Magnefia had been, 
c< this of yours was not fenfibly diminifhed in 
<c bulk, nor did it become harder, as the other 
(C had done, which muft have had a confiderable 
“ portion of Argillaceous or Siliceous Earth in 
** its compofition, to occafion it to Ihrink in 
te fuc'n a manner. I was told it was bought of 
ct a very good and reputable Chemift. But, very 
tc probably, he may precipitate his Magnefia* 
“ with uncryftallifed Tartar, or with any other 
“ Alkali, whereas Bergman prefers the vola- 
“ tile. Or, perhaps, cryftallifed Tartar has 
“ always fome filiceous or other earthy ( Alfa— 
‘ c ftance, which, mingling with the Magnefia, 
“ prevents its adhering to its fpecific charafter. 
“ 1 wifh to know whether I am right iu this way 
,c of accounting for the difference; for as I am 
,c now preparing for the prefs, a fecond edition 
<c of Cronftedt’s Mineralogy, I wifh the facl to 
<c appear in its true and proper light.” —So far 
Mr. Magellan. 
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