TRANSACTIONS OF THE SECTIONS. 61 ° 
‘coal, and fused again. This destroys 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 
carbonate 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 the sulphate 
of soda into carbonate, is not very obvious, and would require an ex- 
perimental investigation to throw light on it. 
5. Another chemical manufacture, which may be seen, is alum- 
making. There are two establishments, one at the Hurlet, about six 
miles south-west, by the Paisley canal; another at Campsie, about 
eight miles off, near Kirkintulloch, on the Great Canal, and near the 
foot of the Campsie hills. The alum is made from the shale, which 
exists in great abundance in the exhausted coal beds. This shale is a 
clay mixed with some coal, and with that variety of iron pyrites, 
which undergoes decomposition, and is converted into sulphate of iron 
by exposure to the air. Thesulphate of iron, thus formed, acts slowly 
on the clay, and in process of time converts it into sulphate of 
alumina. The alum-maker washes this altered shale, and obtains a 
solution of sulphate of iron and sulphate of alumina. When suffi- 
ciently concentrated and cooled, the liquor yields an abundant crop of 
sulphate of iron, which is removed, dried, and sold at a cheap rate. 
The sulphate of alumina does not crystallize till it is mixed with sul- 
phate of potash or sulphate of ammonia; because alum is a double 
salt, composed of three atoms of sulphate of alumina and one atom of 
sulphate of potash, or sulphate of ammonia. Formerly, nothing but 
chloride of potassium, bought from the soap-makers, was used. But 
of late years (at least at Hurlet), sulphate of ammonia, from the liquor 
obtained during the preparation of gas, has been employed. In 
general, the alum made at Hurlet contains both potash and ammonia; 
but the manufacturer can supply it free from potash. Such alum is 
convenient to chemists, because when it is heated to redness, every- 
thing is driven off except pure alumina. At Hurlet and at Campsie 
the mode of concentrating the liquid by a current of heated air pass- 
ing over its surfaces, deserves attention. 
6. At Campsie alum-works may be seen another interesting che- 
mical manufacture, the fabrication of prussiate of potash, a beautiful 
well-known yellow salt, which crystallizes in truncated octahedrons. 
It was here that the manufacture of this salt, on a great scale, first 
began. Before that time it was only prepared in laboratories for 
scientific purposes, and sold at a high price. Mr. Macintosh intro- 
‘duced it to the calico-printers, who used it extensively, to produce 
very beautiful blues and greens. It is prepared by burning the hoofs 
and horns of cattle in iron pots, along with a quantity of potash. The 
hoofs and horns of a hundred head of cattle are consumed every day 
in the works. For some time no iron was added, the requisite quan- 
tity for forming the salt being corroded from the pots during the com- 
bustion. But the last time that the author visited the works, he found 
that iron was mixed with the hoofs, &c. during the combustion. The 
