SALTS OF LIME. 4I>7 



in some mineral waters to a considerable extent. It is depo- 

 sited from the latter, when exposed to air, in a gradual manner 

 and in possession of a crystalline structure, forming stalactites 

 in mountain caverns, and calcareous petrifactions, when it flows 

 over wood and other organic and destructible matters, of which 

 it preserves the form. It is decomposed with effervescence by 

 acids. At a red heat it parts with carbonic acid and is con- 

 verted into quicklime in the manner already described. 



A crystalline mineral was discovered by Boussingault at 

 Merida in America, which he ascertained to be a double carbo- 

 nate of soda and lime, with 5 eq. of water, and named goy- 

 lussite, in honour of Gay Lussac. It may be made anhydrous 

 by heat, and its two salts are then separated by water. 



Sulphate of lime, gypsum; CaO, SO 3 + 2HO ; 857-2+225, 

 or 68.69 -f 18. This salt precipitates as a bulky and gritty 

 powder, when sulphuric acid is added to a soluble salt of lime. 

 Sulphate of lime appears to have nearly the same degree; 

 of solubility at all temperatures, and requires 461 parts of 

 water for solution. It occurs in nature in well-formed crys- 

 tals, and also in large crystalline masses, forming beds of 

 gypsum ; a mineral which contains 2 eq. of water, and of 

 which the density is 2.322 (Roget and Dumas). Mr. John- 

 ston has likewise obtained small prismatic crystals of sulphate 

 of lime, deposited in a steam boiler, which contain only half an 

 equivalent of water (page 330). Sulphate of lime occurs in a 

 crystalline form, without water, forming the mineral anhydrite, 

 of which the density is about 2.96. Sulphate of lime fuses at a 

 strong red heat, without decomposition, and on cooling assumes 

 the crystalline form of the last mineral. To form plaster of 

 Paris, gypsum, in pieces about the size of a pigeon's egg, is 

 heated in an oven till it is nearly anhydrous, and then reduced 

 to powder. When this is made into a paste with a little water, 

 it forms a hard coherent mass, or sets, in a minute or two, with 

 a slight evolution of heat. This artificial hydrate, or stucco, has 

 the same composition as native gypsum. If sulphate of lime 

 has been heated above 300, in drying, it refuses to set when 

 mixed with water. 



Hyposulphite of lime is formed by adding sulphurous acid to 

 a solution of sulphuret of calcium, till the solution is neutral 

 and colourless. The solution is decomposed wher heated above 

 140 (60 cent.) into sulphur and sulphite of lime. Ifevapo- 



K K 2 



