1918-19.] Formic Acid in the Stinging Hairs of the Nettle. 139 
In view of the uncertainty thus attaching to the matter, it seemed 
desirable to seek for some quite conclusive evidence as to the presence 
or absence of formic acid in the stinging hairs of the nettle. The attempt 
to procure such evidence necessitated, first, the collection and fixing of 
the acid contents of these hairs in sufficient quantity and wholly uncon- 
taminated with cell contents or juices from any other part of the plant ; 
and, secondly, the conversion of any formic acid which might exist in the 
contents of these hairs into a formate that could be identified as such 
beyond all doubt. 
With a view to attaining the first of these ends, strips of the purest 
stout filter paper were impregnated with finely-divided barium carbonate 
by first soaking them in a 2 per cent, solution of barium hydroxide and 
then exposing them to the air for a sufficiently prolonged period. Two 
strips of this prepared paper, each measuring about 15 by 7 cm., were 
taken in gloved hands, on a fine day, when the foliage was dry, and were 
somewhat firmly pressed against the upper and under surfaces of a large 
number of the leaves of growing nettles (the species being exclusively 
U. dioica). In this manner, by dealing with some hundreds of leaves, 
the liquid expelled from many thousands of the stinging hairs could 
be collected in a comparatively short time and under conditions which 
ensured the fixing of the free acid contents of the hairs while they pre- 
cluded the contamination of these contents with extraneous matters. The 
quantity of liquid taken up by the filter papers during this operation was 
sufficient to impart to these a distinctly damp feel. 
A rough estimate of the number of hairs whose contents had been 
collected was rendered possible by carrying out a collection under similar 
conditions when the papers employed in the operation had been prepared 
by soaking them in a solution of sodium carbonate instead of barium 
hydroxide, since such papers showed, after drying, a pale olive-green stain 
coinciding with the spot at which each stinging hair had discharged its 
contents. One estimate of the number of these stains indicated that about 
fifteen minutes’ work had resulted in the collection of part, at least, of 
their contents from between 10,000 and 12,000 stinging hairs. Whilst 
this number seems large, the total quantity of acid collected in this 
instance could not amount to 1 milligram if the estimate of Haberlandt * 
be correct that there is expelled from each hair at most 0'00006 milligram 
of formic acid. 
The filter papers containing any new barium salt produced by the 
interaction of the acid from the stinging hairs with the barium carbonate 
* Loc. cit., p. 132. 
