210 CHEMICAL MANUFACTURES IN GLASGOW, ETC. 
must have come along with the chlorine gas, or been absorbed 
from the atmosphere. 
4. After the chlorine has been extricated, there remains in 
the still a semi-liquid mass, consisting partly of the impuri- 
ties of the manganese, and partly of sulphate of soda, and sul- 
phate of manganese. If the manganese were pure binoxide, 
and only the quantity of salt and sulphuric acid necessary 
for the decomposition were used, the sulphate of manganese, 
(abstracting the water) would weigh nine and a half, and the 
sulphate of soda nine. But in order to save the stills by pro- 
ducing the decomposition with little heat, twice as much sul- 
phuric acid is used as is necessary, and this excess is after- 
wards saturated by means of common salt ; so that the quan- 
tity of sulphate of soda in the residue is at least twice as great 
as that of the sulphate of manganese. To get rid of the sul- 
phate of manganese, the residue from the still is fused in a re- 
verboratory furnace at a red heat ; this drives off the sul- 
phuric acid and leaves the manganese in the state of sesqui- 
oxide. 
The whole is dissolved, and the insoluble manganese thrown 
away. The solution of sulphate of soda is evaporated to 
dryness, mixed with small coal, and fused again. This de- 
stroys the sulphuric acid and converts the soda into sulphuret. 
This sulphuret being mixed with sawdust, &c., and exposed 
to an incipient red heat, the sulphur is driven off, and carbo- 
nate of soda remains, which is obtained in crystals by solution 
and crystallization, or in the state of soda ash, by a more 
rapid process. The theory of the last step of the process, in 
converting sulphate of soda into carbonate, is not very obvious, 
and would require an experimental investigation to throw light 
on it. 
5. Another chemical manufacture, which may be seen, is 
alum making. There are two establishments, one at Hurlet, 
about six miles southwest, by the Paisley canal ; another at 
Campsie, about eight miles off, near Kirkintullock, on the 
great canal, and near the foot of the Campsie hills. 
The alum is made from the shale, which exists in great 
