Mr. J. Wilson's Researches into the Alum Manufacture. 423 



At Whitby the calcining heaps are from 90 to 100 feet in 

 height. The stratum of aluminous shale is abut 29 miles wide. 

 Its specific gravity is about 2*40, and the upper part of the 

 rock is always the most abundant in sulphur. It contains a 

 large amount of bituminous matter, which renders great care 

 necessary to regulate the combustion, as it is apt to burn too 

 rapidly aud drive off" portions of sulphur. About one ton of 

 alum is produced from 130 tons of calcined shale. 

 • In some alum ores considerable quantities of magnesia exist. 

 W hen this is the case, Epsom salts are produced in the process 

 of calcination, and being more soluble than alum, remain in 

 solution after it has crystallized. To obtain the Epsom salts, 

 the liquor is evaporated sometimes by waste heat from the alum 

 boilers when the salt crystallizes, often in considerable quantities; 

 one work in Yorkshire producing annually about 800 tons of 

 this salt. 



In certain specimens of Hurlet shale magnesia is found, some- 

 times as much as Li or 2 per cent., but there was no appreciable 

 amount in what I examined. Formerly an efflorescence of nearly 

 pure sulphate of magnesia was to be found in the mines in con- 

 siderable abundance, but of late years its occurrence has been 

 les3 frequent ; it consists of long, transparent, brittle needles, 

 often more than 2 inches long. A specimen from Hurlet in 

 Dr. Thomson's museum at St. Thomas's Hospital, which he kindly 

 permitted me to analyse, I found consisted of, — ■ 



Per cent. Atoms. Epsom salt. 



Sulphuric acid . . 33- 17 1 32-52 1 



Magnesia .... 15-42 1-07 16-25 1 

 "Protoxide of iron . 1-28 

 Alumina .... -23 



Water ..... 49*91 72 51-23 7 



Ibo-oF 10(F~ 



The formula is (MgO FeO) SO 3 , 7HO. 



A specimen from Peru resembling this in appearance, also in 

 Dr. Thomson's museum, contained — 



Atoms. 

 Sulphuric acid .... 35-96 7-19 



Magnesia 2'54 1 



Alumina 15*88 2-2 



Water 45-80 396 



100-18 



MgO SO 3 + 2Al*O 3 5SO 3 + 40HO. 



Alum is also made from clay. For this purpose the purest 

 clay is calcined in u reverberatory furnace at a moderate heat. 



