142 Proceedings of the Royal Society of Edinburgh. [Sess. 
definitely established. The only apparent doubt attaching to this assertion 
is that the formic acid undoubtedly obtained might possibly have been 
formed during the distillation process by the action of phosphoric acid on 
some constituent of the cell contents of the stinging hairs. * There does 
not appear to be any method available at present whereby the occurrence 
of this possibility can be proved or disproved. 
The conclusion arrived at here that formic acid is present in the sting- 
ing hairs of the nettle is not to be regarded as affecting the question as to 
whether or not this acid is the main cause of the intense irritation produced 
by nettle stings. This question, discussed at length by Haberlandt.-f is not 
within the scope of the present paper. 
The author wishes to acknowledge with grateful thanks his indebted- 
ness to Professor Bayley Balfour, who kindly propagated for him, at the 
Royal Botanic Garden, a number of specimens of Urera baccifera for 
comparative purposes ; and to Mr Balsillie for the time and care devoted 
to the optical examination of the numerous preparations that were sub- 
mitted to him. 
* Compare Lieben, Monatshefte , vol. xix (1898), 352. 
f Vide ante, p. 138. 
Chemistry Department, 
University of Edinburgh. 
{Issued separately August 5, 1919.) 
