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tain that the principle is altogether erroneous, depending as it 
_ does on the assumption that the pressure of the atmosphere is 
_ purely a function of its specific gravity or density. This is not 
true, for pressure may vary within wide limits, whilst the density 
remains unchanged. Experimentally this might be shown by 
putting, say, an aneroid and a balance, such as I have been 
speaking of, in a large glass vessel, which can be made air-tight 
when closed. Under normal conditions the two will at first 
register the same pressure ; but if the temperature is sufficiently 
increased or diminished, the increase or diminution of elastic 
force will manifest itself by the aneroid ; but as the density re- 
mains unaltered, the balance will show no change. Does such 
an experiment at all correspond with any natural observations? 
I think so, in, of course, a limited degree. If the lower part of 
a column of air is heated, its expansive force will push the ad- 
jacent air outwards and upwards ; but as it does so, it has to 
overcome a certain amount of inertia ; to do this requires time, 
during which, as the volume of the heated air does not increase 
in proportion to the temperature, the elastic force does. This 
ought to be shown by the barometer ; I think it often is, but the 
barometer is a sluggish instrument at best, and its indications are 
undoubtedly wanting in quickness, and therefore in exactness. 
Still its principle is correct ; so is the principle of the aneroid, or 
of Bourdon’s barometer (on which there is an interesting paper 
in the Quarterly Journal of the Meteorological Society for 
April 1872), though practical difficulties stand in the way of 
their use becoming general. But the tangent balance is not 
capable of measuring atmospheric tension, except when that ten- 
sion depends on density alone; and this is frequently not the 
case—perhaps never. J. K. Laucuton 
- April 23 
Acquired Habits in Plants 
Av p. 446 of Naturg, J. G, records a ‘‘dog violet” which he 
thinks has assumed anunusualform. As there are several plants 
called ‘‘dog violet,” and as one of them does in favourable 
_ situations attain a very considerable height, it would be interesting 
to know what was the species observed by the river Aled. The 
Viola canina (V. riviniana Reich.), in one of its forms which is 
probably a distinct species, has flowering shoots which some- 
times attain a foot in length, and if supported by the surrounding 
yegetation do sometimes stand nearly upright, If this was the 
plant {observed, J. G. only found a more than usually strong 
form, Maite 2 C. C. BABINGTON 
The Zodiacal Light 
Mr. BAcKHOoUuSsE asks if the observations given in vol. iii. 
p- 203, afford any proof that the Zodiacal Light is not a lens- 
shaped disc of light enveloping the Sun; if this theory were 
correct, and the sun enveloped in a continuous mass of light- 
reflecting matter, whenever the light is seen in the evening 
after sunset, it ought to be also seen in the morning before sun- 
rise, of the same brilliancy at the same angular distances from 
the sun, especially when those distances are small, for then the 
effect of an elliptical form in the section of the envelope by the 
plane of the ecliptic would be almost entirely eliminated. 
The results of observation given in most of our hand-books of 
astronomy are therefore directly at variance with this theory, 
and I did not consider it necessary to allude to it before. 
Jamaica, April 6 MAXWELL HALL 
ON VENOMOUS CATERPILLARS * 
OISON and venom are often used as convertible 
terms. I do not understand them to beso. Poison 
properly means something which injures the system by 
introduction through the stomach. Venom, something 
which injures by introduction into the vascular system 
through lesion of the tissues. Most poisons are also 
venoms ; whatever injures, if introduced into the stomach, 
will most probably also injure ifintroduced directly into the 
blood. But the converse is not true : most venoms are 
not poisons, that is, itis not by digestion and assimilation 
that they work, but by entering the vascular system from 
without. It is said that you may swallow the venom of 
the rattlesnake with impunity ; and I imagine you may, 
if it does get absorbed through the mucous membrane ; 
_ but Dr, Fayrer’s experience, lately published, of the effects 
___* A paper read at the opening of the Kensington Entomological Society. 
NATURE 
of the semi-swallowing, which occurs in extracting the 
venom from a poisoned wound by sucking, would rather 
seem to show that such extremely virulent venom would 
penetrate the mucous membrane, and act as if actually 
introduced by a wound, his throat having become dange- 
rously ulcerated from sucking the poison from the wound 
of aman bitten by a cobra, There is yet another way 
than swallowing or being wounded, by which venom may 
injure, and that is through the nervous system, by appli- 
cation to the skin. This is the way in which the nettle 
must sting. In that case there is not the smallest lesion in 
the skin, and if anettle were artistically made to touch the 
open surface of a gaping wound, it would not sting at all ; 
neither is it by mechanical irritation that the pain is 
caused. The nettle has a venom gland, as well as the 
rattlesnake, and it is the application of this venom to the 
delicate termination of the nerves in the skin which pro- 
duced the pain felt. 
The subject to which I invite the consideration of the 
Society this evening is whether any insects possess simi- 
lar power of injury to that of the nettle. In ordinary 
cases the venom of insects is applied by a puncture in 
the skin, into which the venom is introduced by an appa- 
ratus provided for the purpose, But for along time it 
has been said that certain caterpillars sting like the 
nettle, although the authorities have for the most part 
been too vague to allow us to be very sure as to the fact ; 
and supposing the fact to be true, it has been argued that 
the pain or annoyance was merely the result of mechani- 
cal irritation of a similar nature to that which medical 
men sometimes meet with in hairdressers, or rather hair- 
cutters, where minute portions of the cut hair of their cus- 
tomers work their way into the skin below the shirt-sleeve 
and give rise to a painful and irritating sore on the wrist. 
Two passages which I shall take leave to quote, will bring 
the question, as it at present stands, pretty fairly before 
the meeting. The first is from a paper by myself on the 
geographical relations of the chief Coleopterous Faunas, 
which was published in the Linnaean Society’s Journal 
for 1870 (p. 55) :— 
“ A very remarkable African affinity in the Lepidoptera 
has been mentioned to me by Dr. Welwitsch. It is plain 
that an affinity to any genus endowed with peculiar pro- 
perties is rendered doubly certain if the supposed allied 
species possesses the same properties. There is a Lepidop- 
terous insect in Australia, the larva of which possesses re- 
markable poisonous powers. It has been named Doraéo- 
phora vulnerans. Such insects also occur in South 
Africa, Livingstone speaks of a caterpillar called Rzgura 
as producing fearful agony if a sore is touched with its 
entrails. Mr. Baynes, in his “ Explorations in South-west 
Africa,” speaks of another, or perhaps the same, which 
he calls the Aaa, and which is used as a poison for their 
arrows by the Bushmen ; and Dr. Welwitsch had a per- 
sonal experience of the severe swelling and pain in every 
part of his body which he touched with his hand after 
collecting specimens of a caterpillar against which he had 
been warned as poisonous. He had in consequence of 
the warning carefully avoided touching them, shoving 
them into a phial with a straw ; but whether he had inad- 
vertently touched them or fingered the leaves on which 
they had been feeding (which he collected for examina- 
tion), he and his servant were both laid up helpless for 
two or three days. His specimens of the caterpillar 
were lost ; but among his Lepidoptera Dr, Fendler of 
Vienna, who has undertaken a description of them, finds 
no less than four species of Doratophora, and these, 
doubtless, are the per.ect insects of species of the cater- 
pillar, from one of which he suffered.” 
The second passage which I wish to quote is from a 
paper by Mr. Roland Trimen, Notes on the above paper, 
and also published in the Linnean Society’s journal, It 
is as follows :— 
“ At p. 55 Mr. Murray notes what he considers ‘ a very 
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