342 ON CHROMIC ACID AS A BLEACHING AGENT. 
is chromic acid, mixed with sulphate of potash and an ex- 
cess of free sulphuric acid, which is found greatly to assist 
in the bleaching. 
The next part of the operation consists in introducing it 
into the tallow or oil, which, previously melted and well 
settled from all extraneous vegetable and animal matters, 
and at about 130° F., is to be put into a vessel of wood 
capable of holding half a ton, leaving sufficient room for 
stirring. So soon as the liquid mixture of chromic acid, as 
before described, is poured into the tallow or oil, it is to be 
kept well stirred until the whole of the colour is removed, and. 
a light pea green has taken its place. The bleaching opera- 
tion is now complete, and about four pailsfull of boiling 
water are to be poured in, and the stirring to be repeated 
for five minutes ; the whole is then left to settle for about 
two hours, when it Vv'ill be found quite white and fit for use. 
It was formerly our custom to add about four or five 
pounds of muriatic acid to the compound ; but Mr. C. Watt, 
Jun., at the large factory of Messrs. Hawes, found that it 
increased the trouble and expense, with no real benefit, and 
therefore he omitted it, and used only sulphuric acid to de- 
compose bichromate of potash. 
The expense of bleachiAg one ton of bad tallow, or any 
deep-coloured oil, is about sGl ; it therefore became neces- 
sary to devise means of saving the chromic acid; and some 
years since I converted the oxide contained in the green 
liquid, left after bleaching, into chromate of lead; but it 
was found that this article would become so extensive in 
quantity, that all who used much chromic acid, would be 
driven into another branch of business quite foreign to their 
usual occupation; and Mr. C. Watt, Jun., therefore de- 
vised the recovery of it into chromate of lime, equally as 
efi"ectual as applied to bleaching, and much less expensive. 
His process is as follows :— 
