Dr. Stenhouse on Pyrogallic Acid, Sc. 279 
posure of a moment is quite sufficient to convert the surface 
which has undergone a change, into an iodide, chloride, 
bromite, or nitrite of silver. I certainly cannot see the ne- 
cessity of going so far out of our way for an explanation of 
this effect as Dr. Draper has done. I fear I have already 
occupied too much of your valuable space, or I might be in- 
clined to trespass further. I shall however drop my pen for 
the present, again assuring you that I only desire to keep the 
image of Truth which is just shadowing our path, as free as 
possible from mists which might in any way obscure it. 
I remain, Gentlemen, 
Yours most obediently, 
Falmouth, March 8, 1843. Roserr Hunt. 
L. On Pyrogallic Acid, and some of the Astringent Substances 
which yield it. By Dr. Jonn STENHOUSE*. 
HE usual method of procuring pyrogallic acid is by cau- 
tiously distilling either gallic or tannic acids. ‘The py- 
rogallic acid is obtained partly as a crystalline sublimate and 
partly dissolved in the empyreumatic liquor which passes into 
the receiver. Pyrogallic acid thus procured is very seldom 
free from empyreumatic oil, from which it can only be puri- 
fied by repeated distillations, in each of which much acid is 
unavoidably lost. The usual method, therefore, of procuring 
pyrogallic acid by distillation is troublesome and unproduc- 
tive. The process which I have found most advantageous for 
preparing it in quantity, is the following :— 
Finely pounded gall-nuts are to be treated with successive 
portions of cold water till they are exhausted, These extracts 
are then to be evaporated and strongly dried, till all their hy- 
grometric water is driven off. In this state they form a spongy 
deliquescent mass, in taste and colour very much resembling 
catechu. Instead of distilling this dried extract in a retort, 
it is much better to employ Dr. Mohr’s apparatus for subli- 
ming benzoic acid. It consists of a cast iron pan from three 
to four inches deep, and from eighteen inches to two feet wide. 
The dried extract is coarsely pounded and spread equally over 
its bottom to the depth of about half an inch. The top of the 
pan is then coyered with a diaphragm of bibulous paper, fit- 
ting closely over it by being pasted round its rim. ‘The dia- 
phragm may be pierced by a few small pin-holes, which 
greatly facilitates the sublimation. The pan is surmounted with 
a paper cap twelve or eighteen inches high, fitted closely to its 
top, and fastened by means of a cord passed two or three 
* Communicated by the Chemical Society ; having been read Novem- 
ber 1, 1842. 
