[ 287 ] 



On this experiment I remark, that it does not clearly appear 

 that the chalk was thoroughly calcined, but on the contrary there 

 is great reafon to think it was not, becaufe chalk has never been 

 known to contain fo large a proportion of lime as tWV} it is true 

 he fays, it did not effervefce with acids, but furely it heated and 

 bubbled, and fuch bubbles are not diftinguilhable from real effer-. 

 vefcence, where the quantity of fixed air is fmall, but by weigh- 

 ing before and after the addition of an acid, which he does not 

 fay he had done. 



Dr. Black found it impoffible to calcine any confiderable quan- 

 tity of lime in an earthen crucible, but was obliged to ufe one of 

 black-lead to avoid vitrifadion, 2 Ed. Efl'aj's, 219. Smith found 

 the fame difficulty to effed the entire expulfion of fixed air, 

 Differt. de Acre fixo, p. 40, 43. Chalk in general contains no 

 more than 49 or 5c per cent, of fixed air, and the chalk he ufed, 

 if it was purified, as he mentions in the 2d fedlion, muft have 

 contained abundance of moifture ; it commonly contains but 41 

 per cent, of fixed air, and the proportion of earth in fuch cafe is 

 only 50 per cent, or -y/-^, therefore -^\%- grains of fixed air re- 

 mained unexpelled. 



SeB'ion T^d. 



5760 grains of fp. of fait were faturated with 2393 of the afore- 

 mentioned chalk, and the whole being evaporated to drynefs and 



heated 



