62 REPORT— 1840. 
residue, after this combustion is lixiviated with water, and when the 
solution is sufficiently concentrated, the prussiate of potash crystal- 
lizes. Connected with this manufactory of prussiate of potash is 
another, of Prussian blue. It is made by mixing sulphate of iron, 
alum, and prussiate of potash, and precipitating the whole by 
an alkali. The precipitate is at first light blue. But it is washed 
with new portions of water every day for several weeks. At every 
washing the colour deepens, and when it has acquired the requisite 
shade, the Prussian blue is allowed to subside, the water is drawn off, 
and the powder allowed to dry. The colour varies according to the 
proportion of alum employed; and it has the finest colour of all, with 
the coppery lustre which is so much admired, when no alumina what- 
ever is mixed in it. 
7. Another beautiful chemical product may be seen at Shawfield, 
near Rutherglen, about two miles from Glasgow, in the manufactory 
of Mr. White. Thisis bichromate of potash, a salt very much used by 
calico-printers, and forming the finest and most indelible yellows, 
oranges, and greens. Its introduction constituted quite an era in 
calico-printing. This salt was originally made by heating chromiron 
ore with saltpetre, dissolving out the chromate of potash, 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 two shillings a pound, 
it 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 double 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 
colour on particular parts of the cloth, either that these parts may 
remain white, or that some other colour may be superadded. Tartaric 
acid is obtained from cream of tartar, by throwing down the tartaric 
acid by means of lime, and afterwards decomposing the tartrate of 
lime by means of sulphuric acid, and crystallizing the tartaric. At the 
same manufactory may be seen a pretty and simple process, by which 
the carbonate of soda is converted into the sesquicarbonate. By 
simply exposing it dry, and in powder, in an atmosphere of carbonic 
acid gas, it absorbs the requisite quantity to be converted into sesqui- 
carbonate. And this sesquicarbonate is chiefly used by the makers of 
soda water. 
8. It is hardly proper to mention the manufactory of acetie acid 
from wood, which has been carried on for many years by Mr. Turn- 
bull, because the first part of the process is carried on at a distance, 
the distillation of the wood. To free the acetic acid from the tar, 
which destroys its flavour and taste, the acid is combined with lime, 
and the acetate of lime exposed to a heat sufficiently high to char the 
