CHEMICAL MANUFACTURES IN GLASGOW, ETC. 
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abundance in the exhausted coal beds. This shale is clay, 
mixed with some coal, and with that variety of iron pyrites 
which undergoes decomposition, and is converted into sul- 
phate of iron by exposure to the air. The sulphate of iron, 
thus formed, acts slowly on the clay, and in process of time 
converts it into sulphate of alumina. When sufficiently con- 
centrated and cooled, the liquor yields an abundant crop of 
sulphate of iron, which is removed, dried, and sold atacheap 
rate. The sulphate of alumina does not crystallize till it is 
mixed with sulphate of potash, or sulphate of ammonia. For- 
merly, 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 
manufacturers 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 
Campsie the mode of concentrating the liquid by a current of 
heated air passing over its surfaces, deserves attention. 
6. At Campsie alum-works, may be seen another interest- 
ing chemical manufacture, the fabrication of prussiate of pot- 
ash, 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 introduced 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 quantity 
for forming the salt being corroded from the pots during the 
combustion. But the last time that the author visited the 
works, he found that iron was mixed with the hoofs, &c, dur- 
ing combustion. The residue after this combustion is lixiviat- 
