a 
THE STINGING HAIRS OF URTICA DIOICA. 93 
tested first of all with individual hairs, carefully removed 
so as to avoid injury to the hair, with a solution of nitrate 
of silver in ammonium hydrate. On slightly warming, a 
granular precipitate appeared in some of the hairs, which, 
with reflected light, gave a bright metallic lustre, pointing 
to the deposition of metallic silver. From the rapidity of 
this reaction we were inclined to suspect the presence of 
tartaric rather than of formic acid. We then tested hairs 
with lime-water, with the result that a dense opaque white 
precipitate made its appearance in some hairs, which preci- 
pitate proved to be soluble in acetic acid. This confirmed 
our suspicions of the presence of tartaric acid. At the same 
time we found no evidence of the presence of formic acid 
in the hair itself. We therefore at first felt inclined to 
consider tartaric acid as the irritant sought for. Tartaric 
acid injected hypodermically produces symptoms similar 
to those produced by normal contact with fresh nettle 
stings. 
We next endeavoured to ascertain why the free acid 
reaction was forthcoming in some hairs and not in others. 
The only explanation we can suggest is, that in excess of 
lime, calcium tartarate is formed, which however would 
be soluble in free tartaric acid when the latter was in 
excess. This may explain the presence normally, especially 
in old hairs, of a granular opaque white precipitate soluble 
in acetic and other acids. The wall of the hair cell is 
strongly cuticular, the apical fourth and the bulb however 
resist boiling in sulphuric acid, and, even after being 
submitted to red heat, leave an incombustible residue 
retaining the appearance of the unaffected tip. 
By a simple mechanical arrangement we then watched 
the effect of normal contact between the skin and the 
stinging hair beneath the magnifying power of 50 diameters. 
The bulb at the end of the hair springs at almost a right 
