1918 - 19 .] Formic Acid in the Stinging Hairs of the Nettle. 141 
is optically negative, and, further, that the crystal axis c is the first mean 
line. On the assumption that the needles in the preparations submitted 
have their elongation parallel to this direction, there is thus an immediate 
explanation of why the faster ray should constantly be polarised in the 
transverse plane. 
“The slides of lead acetate furnished for comparative purposes show 
that this salt is altogether different in its optical properties. The indices 
of refraction are below that of methylene iodide. The symmetry is mono- 
clinic, the faster ray is not always found to vibrate along the length of the 
needles, in which the substance crystallises, as in lead formate : further, 
the salt is optically positive. There is thus no difficulty in distinguish- 
ing between lead acetate and lead formate. The slides carrying the 
material derived from nettles did not show any determinable traces of 
the former salt. 
“ (b) The single crystals were of orthorhombic symmetry, and exhibited 
generally a combination of prisms and domes, with occasional pinacoids 
closely resembling the figures given by Groth* and by Gehlen-Bernhardi. j- 
Occasionally, the development was equidimensional, conferring an octa- 
hedral appearance upon the individuals, and groups that had grown in 
parallel orientation were not uncommon. Like those of the needle-shaped 
crystals, the indices of refraction were here invariably high, and the 
double refraction similarly strong. So far, therefore, as optical comparison 
(without actual measurements, which were out of the question in view of 
the minute size of the crystals) affords sanction for definite assertion, one 
can have no hesitation in saying that lead formate is present in these 
preparations. 
“ The slides of the barium salt were entirely confirmatory of the fore- 
going conclusion, and were of additional interest on account of the fact 
that one preparation showed the substance crystallised in complete — 
though extremely small — bisphenoids. This habit does not seem to have 
been previously noted in barium formate, and, though several attempts 
were made to crystallise the known salt in this form, no success was 
attained. Very probably concentration and conditions of separation have 
a delicate bearing upon the ultimate form of the solid substance.” 
In the light of the foregoing report, it may fairly be asserted that the 
presence of free formic acid in the stinging hairs of the nettle has been 
* Ghemische Krystallographie , iii Teil (1910), p. 15. 
t Schweigger’s Annalen, vol. iv (1812), pp. 36, 38, and figure. (It is to be noted that 
Bernhardi figures barium formate, but that the habit of lead formate is in some cases 
identical with that of barium formate. See Groth, loc. cit., p. 17.) 
