PHYSICAL AND LITERARY. 199 



either in the (hape of an exceedingly fubtile 

 powder, or more probably in that of an ela- 

 ftic fluid. To this I have given the name of 

 fixed air, and perhaps very improperly j but 

 I thought it better to ufe a word already fa- 

 miliar in philofophy, than to invent a new 

 name, before we be more fully acquainted 

 with the nature and properties of this fub- 

 flance, which will probably be the fubjed: of 

 my further inquiry. 



It is, perhaps, needlefs to mention here, 

 that the calcarious fubftances ufed in making 

 the above experiments fhould be of the pureft 

 kind, and burnt with the utmoft violence of 

 heat, if we would be fure of converting them 

 into perfedt quick-lime. I therefore made ufc 

 of chalk burnt in a fmall covered crucible with 

 the fierceft fire of a Black-fmith's forge, for 

 half an hour, and found it neceiTary to employ, 

 for this purpofe, a crucible of the Aujirian 

 kind, which refemble black lead -, for if any 

 calcarious fubftance be heated to fuch a degree 

 in an ordinary or He/Jian crucible, the whole 

 of it is melted down> together with part of the 

 veffel, into glafs. 



I now prepared to inquire into the pro- 

 perties of the cauftic alkali 3 in order to which, 



I 



