CHEMICAL MANUFACTURES ON THE TYNE. 177 



sulphate of magnesia on the Tyne. The mineral is a tolerably pure 

 double carbonate of lime and magnesia, containing about 21 per cent, 

 of magnesia. 



The following is an analysis by Mr. Clapham : — 



Silica .... 



10-00 



Alumina 



1-60 



Oxide of Iron . . 



0'50 



Carbonate of Magnesia . 



35-33 



Carbonate of Lime . 



52-50 



The process formerly employed was to calcine the limestone 9993, and 

 wash it repeatedly with water, by which, however, the lime is only im- 

 perfectly removed, the residue being dissolved in acid and crystallised. 

 The principal source of sulphate of magnesia for many years past has 

 been the rough Epsoms, obtained from the residual mother liquors of 

 the Yorkshire Alum Works. In these salts protoxide of iron replaces a 

 variable proportion of magnesia, forming a double salt, and an excess 

 of sulphuric acid is always present. 



The following is an analysis of Rough Epsom salts, by Dr. Richard- 

 son: — 



Sulphuric Acid . . 23-26 



Magnesia . . . 15-35 



Protoxide of Iron . . 1 73. 



Oxides of Nickel and Cobalt 0-12 



Lime .... 0-09 



Alumina . . . 1-33 



Potash .... 0-83 



Water .... 48-29 



Formerly these salts were mixed with washed magnesian lime, 100-0, 

 and then calcined in order to peroxidise the iron. It is found, however, 

 (as first suggested by Dr. Richardson), that calcination is unnecessary 

 when the solution is sufficiently diluted, and when space is provided in the 

 precipitating tank for the bulky precipitate of protoxide of iron, which 

 is formed by the gradual addition of magnesian lime. This is pro- 

 bably the only chemical manufacture of the district, with the exception 

 of prussiate of potash, which has greatly fallen off in extent, a more 

 rational system of medicine having diminished the use of purgatives, 

 and reduced the demand for Epsom salts to about one-third of what it 

 was twenty years ago. The annual production is still 1,500 tons, two- 

 thirds of which are made from the rough salts. 



Carbonate op Magnesia. — This compound has long been produced 

 in this district, where it was formerly, and is still to a limited extent 

 manufactured from the mother liquors of the salt pans, known as Bittern, 

 to which carbonate of soda is added to precipitate the magnesia in the 

 form of carbonate. This old process has been largely superseded by 

 VOL. iv. o 



