SALTS SULPHATES. 329 



(c.) Soluble in concentrated sulphuric acid, especially if boiling, 

 but again precipitated by water.* 



(d.) Decomposed by ignition with charcoal ; its oxygen is separated 

 in the form of carbonic acid, and sulphuret of barium is left. 



(e.) Pulverized, kneaded up with flour and water, formed into a 

 thin cake and exposed to ignition, it becomes phosphorescent in the 

 dark.f 



5. COMPOSITION. Dr. Henry, after citing several analyses, con- 

 cludes that the true composition is, 



Acid, 33.90, 1 proportion, - 40 



Earth, 66.10, " - 78 



100.00 11 8, its equivalent. 



As baryta is used to separate sulphuric acid from all its combina- 

 tions, this salt is very important in analysis. The quantity is deter- 

 mined by weighing the precipitate, previously washed and dried, and 

 allowing 33.9 per cent, of its weight, " for real sulphuric acid," thus 

 shewing the quantity in any sulphate. J Sulphuric acid or any solu- 

 ble sulphate occasions a sensible precipitate in a solution containing 

 sVo o f baryta, or of any of its soluble salts. 



6. DECOMPOSITION. The mode by charcoal has been already 

 mentioned. 



Sa.^ Not decomposed by any acid or alkali.\\ 

 b.) Readily by double elective attraction, with carbonate of po- 

 tassa, or of soda, or ammonia, IT after long continued boiling. 



(c.) But much more readily, by ignition with the carbonate of an 

 alkali. Mix pure, decrepitated and pulverized sulphate of baryta, 

 with twice its weight of dry, pure carbonate of fixed alkali, and ex- 

 pose them in a crucible to a violent heat. A double decomposition 



* Easily shown by adding sulphuric acid to solution of baryta, or any of its soluble 

 salts ; the precipitate will be redissolved by more sulphuric acid, and then thrown 

 down by water, and thus it may be alternately redissolved and precipitated by acid 

 and water. 



t First observed, in the variety called Bologna stone, by an Italian shoemaker, 

 named Vincen/o Casciarolo. This man found a Bologna stone at the foot of mount 

 Paterno, and its brightness and gravity made him suspect that it contained silver. 

 Having heated it to extract the silver, he observed that it was afterwards luminous 

 in the dark, and on repeating the experiment, it constantly succeeded. It is evident 

 that by the calcination, it must be converted, at least in part, into sulphuret. Prof. 

 Olmsted informs me, that a granular sulphate of baryta from North Carolina, (Crow- 

 der's mountain,) when heated, phosphoresces with a clear white light. 



t Henry, 10th Ed. Vol. I, p. 604. Thenard, III, 171. 



|| Fourcroy asserts, (III, 32,) that the phosphoric and boracic acids, decompose it 

 by ignition. 



IT After boiling for two hours, about one fourth of it will be found to be decomposed, 

 and the result will be carbonate of baryta, sulphate of the alkali, and undecomposed 

 sulphate of baryta. 



42 



