SALTS 



Hydrous. (2.) Hydrous sulphate of lime, long familiar 

 to chemists and mineralogists under the names 

 of gypsum and selenite, crystallizes in right 

 oblique angled prisms ; and, when formed arti- 

 ficially, it crystallizes in small silky needles, so 

 minute, that their shape is not easily determined. 

 The constituents of this salt are 



1 atom acid . 5 



1 atom lime . 3-5 



2 atoms water . 2-25 



/ 



10-75 



The water of crystallization may be driven on 

 by a temperature not exceeding 500. In a red 

 heat, some of the sulphuric acid is driven off; 

 and, if water be poured on the salt thus treated, 

 it will be found to give a violet colour to cud- 

 bear paper. This circumstance long perplexed 

 me in the analyses of minerals, when I was 

 searching for an alkali. The liquid containing 

 the supposed alkali was filtered, evaporated to 

 dryness, and water digested upon the residual 

 matter. This water, by giving a violet colour 

 to cudbear paper, gave an indication of an al- 

 kali j but the most careful examination of what 

 the water had dissolved frequently discovered 

 nothing but lime. I found at last, that all the 

 filtering paper which I could procure contained 

 sulphate of lime ; a little of this was dissolved 



