1918-19.] Formic Acid in the Stinging Hairs of the Nettle. 137 
XI. — On the Presence of Formic Acid in the Stinging Hairs 
of the Nettle. By Leonard Dobbin, Ph.D. 
(MS. received May 16, 1919. Read June 2, 1919.) 
It is well known that when the stinging hairs of the common nettle 
( Urtica dioica or U. urens ) are caused to discharge their contents upon 
blue litmus paper, intensely red spots are produced. On the subsequent 
exposure to the air of the paper thus spotted, the red colour gradually 
diminishes in intensity, and in a day or two is scarcely distinguishable, 
although it does not entirely disappear even after several weeks’ exposure. 
This behaviour indicates that the reddening is due, in the main at least, 
to a volatile acid, and the range of acids probably present is thereby very 
strictly limited. 
Although the statement is made quite definitely in many text-books 
and elsewhere that formic acid occurs in the stinging hairs of the nettle, 
an examination of the original literature bearing upon the subject shows 
that the evidence upon which the statement seems to be based is not at 
all convincing in light of our present-day knowledge. It appears that 
the earliest evidence for the statement is contained in a paper by Gorup- 
Besanez,* who distilled finely-cut and crushed nettles with four times 
their weight of water, with and without the addition of sulphuric acid, 
and obtained slightly acid distillates. He submitted these distillates to 
subsequent treatment designed to collect the acid, or the calcium salt 
prepared from the acid, into a small volume of liquid, and in the solutions 
so obtained he satisfied himself, by the application of a series of tests, as 
to the presence of formic acid or of calcium formate. In view, however, 
of the facts, first, that a number of observers have reported the presence 
of formic acid in the distillates obtained either by boiling various plant 
parts with water (with or without the addition of sulphuric or other acid) 
or by passing a current of steam through tubes packed with such material, f 
and, secondly, that distillates obtained by these methods are known frequently 
to afford reactions resembling some of those of formic acid (reduction of 
silver and mercury salts, for example), although it was not found possible 
to separate from them and to identify this acid,J the question of formic 
* J. Prakt. Ghem ., vol. xlviii (1849), p. 191. 
f See, in particular, investigations by Bergmann ( Bot . Zeit., vol. xl (1882), p. 731, etc.), 
who gives a review of the earlier literature. 
f Compare Shannon, Journ. Indust. Engin. Ghem., vol. iv (1912), p. 526. 
