240 Mr. Donovan on the nature and combinations 
boiled for half an hour with 2.3 times its weight of sul- 
phuric acid, s. g. 1090, supplying water as fast as it eva- 
porates, and taking care to keep the mixture constantly 
stirred with a glass rod. The clear liquor is to be filtered off, 
aud poured into a tall glass jar of small diameter. While 
still hot, a stream of sulphuretted hydrogen is to be trans- 
mitted through it, and when all the lead has been precipitated, 
the fluid is to be filtered off, and boiled in an open basin, 
until the discharged vapour no longer blacken paper impreg- 
nated with acetate of lead. 
The theory of the process is obvious. When acetate of 
lead is added to the juice, malate of lead and the combination 
of the new acid with lead precipitate ; the latter is decom- 
posed by boiling water into a super and a sub-salt ; the super- 
salt is held in solution, but as the liquor cools, the neutral 
compound deposits itself in crystals, and the first washings 
contain most free acid. When boiling water is no longer 
able to overcome the attraction of the latter portions of acid 
to oxide of lead, no more crystals can be formed. We then 
apply the stronger power of sulphuric acid, we obtain the free 
vegetable acid, and proceed as at first. When all the crys- 
tals are collected, such a quantity of sulphuric acid is added 
as will be nearly sufficient to decompose them : this is so 
done in order completely to exclude the sulphuric acid, which 
without this precaution would be exceedingly difficult to 
effect. The undecomposed portion of the crystals dissolves 
in the vegetable acid newly extricated : but, if in the boiling, 
the fluid were not continually stirred, a mass would be formed 
in the bottom so hard as to resist decomposition. If the 
liquor after filtering were allowed to cool, the neutral salt 
