30 HOW^ — ON MINERALO&Y OF NOVA SCOTIA. 



great distinction made in the qualities of gypsum is between 

 blue and white plaster. The former is chiefly used for agricul- 

 tural purposes, probably the greater part of that exported is so 

 employed ; it is sent to Boston, New York, Philadelphia, Richmond 

 and Baltimore. The chief consumption I understand to be in 

 Virginia, Maryland and Pennsylvania, where the ground plaster 

 is used as manure for tobacco and Indian corn ; before the late 

 war gypsum was becoming a favourite fertilizer in cotton grow- 

 ing and large orders were sent, but the war interfered, and as 

 yet, trials have not, I believe, been further made as regards this 

 application. The white gypsum is sought for boiling and burn- 

 ing by which it is prepared for the making of plaster for walls, 

 ceilings, cornices, etc. Gypsum consists of — 



Lime, 32.55 



Sulphuric acid, 46.51 



Water, 20.94 



100.00 

 and its property of furnishing plaster depends on the fact that 

 its water can be expelled and afterwards taken up again. 

 Calcined gypsum is what remains after burning or boiling ; the 

 burning is effected by building up lumps of the rock into heaps 

 with cord-word intermixed and maintaining a very moderate fire 

 for some hours, the burnt plaster is afterwards beaten to powder 

 and is fit for use : in boiling, the ground gypsum is heated in 

 caldrons and the peculiar agitation caused by the escape of 

 water is so like that of a liquid in ebullition that the plaster is 

 said to boil. The burnt or boiled gypsum is mixed with water 

 to a paste and when left it soon hardens. The best plaster, that 

 which sets most quickly into a hard mass, is got by heating to 

 about 500° Fah., if the heat attains redness, the gypsum becomes 

 very dense and does not set with water. Most important pro- 

 perties are gained by the addition of one or two per cent, of 

 certain salts, such as borax and sulphate of potass. Gypsum 

 which has been thus treated will endure a red heat without 

 losing its power of setting with water ; it becomes much more 

 dense than common plaster, sets in a few hours, becomes hard 

 and takes a fine polish. Keene's, Martin's and Keating 's 



