212 CHEMICAL MANUFACTURES IN GLASGOW, ETC. 
ed with water, and when the solution is sufficiently concentra- 
ted, the prussiate of potash crystallizes. Connected with this 
manufactory of prussiate of potash, is another of Prussian 
blue. It is made by mixing sulphate of iron, alum and prus- 
siate of potash, and precipitating the whole by an alkali. The 
precipitate is at first a light blue. But it is washed with new 
portions of water every day, for several weeks. At every 
washing the color deepens, and when it has acquired the re- 
quisite shade, the prussian blue is allowed to subside, the 
water is drawn off and the powder allowed to dry. The co- 
lor varies according to the proportion of alum employed ; and 
it has the finest color of all, with the coppery lustre which is 
so much admired, when no alumina whatever is mixed in it. 
7. Another beautiful chemical product may be seen at 
Shawfield near Rutherglen, about two miles from Glassgow, 
in the manufactory of Mr. White. This is hichromate of 
potash, a salt very much used by the calico-printers, and 
forming the finest and most indelible yellows, oranges, and 
greens. Its introduction constituted quite a new era in cali- 
co-printing. This salt was originally made by heating chro- 
mium ore with saltpetre, dissolving out the chromate of pot- 
ash, and adding the requisite quantity of nitric acid to deprive 
the chromic acid of half its potash. When this process began 
the salt was sold at a guinea an ounce ; but now when the 
price is as low as twoj shillings a pound it r is necessary to 
prepare it by a cheaper method. 
It has been found that common potash of commerce may 
be substituted for saltpetre ; and Dr. Thomson believes the 
manufacturers now contrive to form the bichromate at once, 
without requiring the use of an acid,which would nearly dou- 
ble the expense. It is stated, that all the bichromate used 
by the calico-printers is made here and in Liverpool. In the 
same manufactory may be seen a beautiful product, tartaric 
acid, which is used by the calico-printers to a large amount, 
chiefly to disengage the chlorous acid from bleaching powder, 
and enable it to destroy the color on particular parts of the 
cloth, either that these parts may remain white, or that some 
