182 Chemistry. [Lecture 30. 



oblong crystals, and is called selenite. Some are 

 long and regular, like crystals of salt. 



The fifth species, or gypseous spar, is of a 

 platey texture. The ores of metals ore often 

 found in this spar. 



Sixthly, It is often found in waters of springs in 

 a dissolved state. When the water is evaporated, 

 it is deposed in crystals very small, and which 

 appear like a white powder; but by the micro- 

 scope are found to be oblong regular concre- 

 tions. 



Lime electrized negatively in contact with 

 mercury yields a white metal, which exists in 

 lime, in the proportion of 19 to 7,5 of oxygen. 



II. ALUMINA means the earth of alum ; but 

 though the salt which bears that name was well 

 known to the antients, they were entirely igno- 

 rant of its component parts. The alchymists 

 discovered that it was composed of sulphuric 

 acid and an earth ; and it was afterwards found 

 that this earth is clay in its purest form. Alum 

 then is a compound of sulphur, oxygen, and 

 clay. 



Alumina may be obtained pure by dissolving 

 alum in water, and pouring in ammonia or pot- 

 ass, as long as any precipitate is formed. The 

 nature of the process must be obvious. The al- 

 kali seizes on the sulphuric acid which the alum 

 contains, and the alumina is thrown to the bot- 

 tom. This, when rendered still purer by a fur- 



