280 Dr. Stenhouse on Pyrogallic Acid, 
times round it. The apparatus is to be cautiously heated for ten 
or twelve hours on a sand, or still better on a metallic bath. 
The temperature is to be kept as nearly as possible at about 
400° F., though towards the end of the sublimation it may be 
raised a few degrees higher. The crystals of pyrogallic acid 
pass up through the bibulous paper, which absorbs the empy- 
reumatic oil by which they are always accompanied. Should 
the heat have been carefully regulated, the crystals, which 
are either large scales or needles, are perfectly white ; should 
they be slightly coloured, which sometimes happens, they may 
be easily purified by a second sublimation. This method 
possesses the great advantage that a pound or more of the ex- 
tract can be operated on at once; the apparatus is extremely 
cheap, and as it is not liable to break it may be used for any 
number of times. On one trial 1380 grains of dried extract 
yielded 69 grs. of perfectly pure crystals, and 74 grs. which 
were slightly coloured, in all 143 grs., or 10°3 per cent. Now 
as galls yield rather more than half of their weight of soluble 
matter, the quantity of pyrogallic acid obtainable from them 
by this process is very considerable. I think it right to men- 
tion, however, that on a previous trial, when the sublimation 
was not so carefully conducted, I did not obtain more than 
half this quantity. 
The following are some of the leading characters of pyro- 
gallic acid. It has a very bitter taste, resembling that of sali- 
cine. When pure it does not redden litmus paper ; but if it 
has been sublimed at too high a heat, it is accompanied with 
a little of some volatile acid, which causes it to redden litmus 
slightly. It gives a deep indigo-blue colour with solutions of 
protosulphate of iron, but no precipitate falls. Ifthe proto- 
salt contains any peroxide, this colour soon changes to dark 
green; but if the salt is pure the deep blue colour remains 
for a considerable time. With persulphate of iron it gives a 
yellowish red; with perchloride a much brighter red, but in 
neither case any precipitate. When pyrogallic acid is dropt 
into milk of lime, a beautiful reddish purple colour appears, 
which however speedily changes to a dark brown. Caustic 
barytes produces a dark brown colour which quickly becomes 
black. Its reactions with salts of iron and milk of lime are 
the best tests for pyrogallic acid, and by these its presence 
even in very small quantity can be easily ascertained. It is 
very soluble in water, but its aqueous solution, if exposed to 
the air, speedily blackens. It is also very soluble in alcohol, 
though not so much so as in water. The taste of its alcoholic 
solution very much resembles laudanum. Dilute sulphuric 
acid first reddens pyrogallic acid and then blackens it. Iodine 
has no effect upon it, Drychlorine instantly colours the crystals 
