ON CHROMIC ACID AS A BLEACHING AGENT. 343 
The green liquid, left after all the bleached oil is taken 
off, is put into another tub, and more water is added ; then 
lime, made into a thick cream-like consistence, is gradually 
poured in till nearly all the sulphuric acid is saturated ; the 
liquid is then run off into another vessel from the sulphate 
of 1 ime, and into this liquid is to be gradually and carefully 
introduced some more of thecream of lime, till all the green 
oxide (powder) is precipitated, and the liquor is clear and 
colourless; this liquor is to be drained off, and fresh water 
poured in, and, when settled, it is again to be poured off, 
and a fresh quantity put in, in order to wash the precipitate ; 
this is at last to be dried, and then put on an iron slab, 
heated to redness, and kept frequently stirred. From a green 
it will be gradually changed into a yellow powder, which 
is the chromate of lime, and which, by being decomposed 
by sulphuric acid in such quantity as to leave an excess of 
free sulphuric acid, yields chromic acid, quite as well 
adapted for bleaching, as that obtained from the bichromate 
of potash,* By this process the chromic acid can be re- 
covered again and again, ad infinitum; and thus the 
method of bleaching by this agent is at once the most per- 
fect and economical of any yet brought into operation. t It 
is almost needless to remark, that where, as in the great 
manufactories in Lancashire, much chromic acid is em- 
ployed, this easy and cheap mode of recovering it will 
prove highly beneficial. 
It may here be remarked, that several other methods of 
bleaching tallows and oils have since been tried. One con- 
sists in employing what is termed permanganic acid ; but 
this agent so readily parts with its oxygen, that it is un- 
* This process is perfectly identical with that described by M. Jacque- 
lain in our fifth volume, p. 452 — Ed. Chtm, Gaz. 
t The patent for bleaching and purifying dark tallows and deep- 
coloured oils was taken out about twelve years ago by the writer. 
