192 Chemistry. [Lecture 30. 



composition of lime, but was afterwards disco- 

 vered to be a peculiar kind of earth. 



Magnesia has rarely been found native. It 

 may be procured from sulphat of magnesia, a 

 salt which exists copiously in sea water, and in 

 some springs in the neighbour hopd of Epsom, 

 whence it formerly obtained the name of Epsom 

 salt. jThe sulphat of magnesia used in medi- 

 cine is chiefly obtained from the mother ley of 

 common salt, that is, the liquor which remains 

 after the muriat of soda has been crystallized 

 and extracted. To obtain the earth in question, 

 the sulphat of magnesia is dissolved in water, 

 and carbonat of potass added. The potass 

 having therefore a stronger affinity for the sul- 

 phuric acid, the magnesia is precipitated, and is 

 afterwards washed in a quantity of water to ex- 

 tract whatever salt may remain, and dried. 



Magnesia is, I believe, only used in medicine 

 to remove acidities from the stomach and bowels. 

 In this case the carbonat of magnesia is prefer- 

 able to the pure, for all absorbent earths are ren- 

 dered somewhat caustic by being deprived of the 

 carbonic acid. 



It combines artificially with few substances, 

 but is found in nature united with silica and 

 alumina in a class of stony bodies, which Mr. 

 Kirwan has classed together ; these are talcs, as- 

 bestos, &c. 



TALC or MICA may be defined earthy or 

 stony bodies, so soft as to be scraped with a 



