Marcu 4, 1897 | 
INA TURE 
415 
type of the diphtheria bacillus, he must be a bold man who 
ventures to say off-hand that this bacillus is or is not a diphtheria 
bacillus.” Yet the same difficulty does not exist in diagnosing 
cholera or diphtheria. 
I do not suppose that recognition-marks are wanting in many 
groups of higher animals, even higher invertebrates. The fresh- 
water bivalves can hardly be supposed to present them, and 
hence their separation into species becomes exceedingly difficult. 
But it appears that recognition-marks need not be in colour or 
markings, but may be, and often are, in odour or voice, which 
are not observable:in dead specimens. Thus the nocturnal 
lepidoptera, the species of which are often perplexingly similar, 
undoubtedly many of them emit subtle odours—too subtle 
usually for us to appreciate. So also, some species of birds are 
known, which are almost exactly alike in the preserved skins, 
but are readily distinguished in life by the song or voice. 
T. D. A. COCKERELL. 
Mesilla, New Mexico, U.S.A., February 7. 
The Force of a Ton. 
Your readers will notice that Prof. Greenhill (p. 365) 
uses symbols as mere numbers, and that, so long as he 
does this, it can be of no possible interest or importance 
whether he writes #2 pounds or w pounds, or z or a or 4 or = 
pounds. 
If he intends anything definite by his hint that ‘‘ Dr. Lodge 
can testify to the treacherousness of y,” will he kindly give a 
reference? Perhaps he is thinking of Narurg, 1891, vol. sliii. 
Fo Slish 
ra is a little surprising that the label ‘‘ 5000 tons-weight” on 
a hydraulic press capable of exerting that thrust, should be con- 
sidered liable to mislead a practical man into supposing that the 
piece of metal itself was so extremely heavy ; but, though the 
addition of the syllable ‘‘ weight” in that connection would 
have been both cumbersome and needless, and J should never 
have thought of suggesting it, I cannot see that it makes the 
slightest difference to his argument either way. Nor, I am 
almost glad to say, do I appreciate any of his other difficulties ; 
especially not the difficulty said to be caused by “tossing 
standard weights in the air!” It reads like the popular method 
of studying geology ‘‘upon the Stanislaw.” Why do they 
then weigh more? Is it because they come down with a 
bang ? 
Prof. Greenhill is very persistent about this question of a force- 
unit ; but his justification lies in the fact that he is really tilting 
against the whole idea of adso/ute measwre—that truly practical 
and most useful conception which this century owes to Gauss 
and Weber and to Thomson and Tait. All new ideas must pass 
through their era of attack, and should emerge the better for 
the process. The idea of absolute measure is still not finally 
and restfully settled down in the minds of all physicists ; it is 
still too much mixed up with the comparatively trivial question 
of the particular kind of unit that shall be most commonly 
employed for numerical specification. Prof. Greenhill is doing 
indirect service to the better method by his resolute insistence 
on conservative traditions. 
I rather regret Prof. Fitzgerald's letter (p. 389), because, 
although containing many statements which are manifestly true, 
it tends to confuse the issues. 
Does he really maintain that the English words mass and | 
massive should never be used in an accurate physical sense? Is 
he prepared to object to the expressions ‘‘ quantity of heat” 
and ‘* quantity of electricity ” as well as to ‘quantity of matter” ; 
or does he think that whereas those ether quantities may be 
measured in various recognised ways, the quantity called 
“* matter ” cannot be legitimately measured by any of its inalien- 
able properties ? Does he hold that the conservation of matter, 
as ascertained by the constancy of its inertia and of its weight 
under various conditions, is a wholly metaphysical and confusing 
idea ? 
How would he wish us to express the gravitational attraction 
between two masses, yrzv4/7*; the 7's do not stand for inertia 
there? The physical factor g, which turns mass into weight 
without necessarily altering the numerical specification in any 
way, may be regarded as an abbreviation for yE/R®, with a 
correction for the shape of the earth and an allowance for centri- 
fugal force, and is not a thing to be lightly ignored or introduced 
NO. 1427, VOL. 55] 
for the sake of some entirely imaginary convenience about units = 
not even for the sake of complicating mechanics, after all these 
years, by trying to express mass in something else than mere 
grammes or pounds or tons. Let the British student say so- 
many pounds when he interprets 7, and let him say so many 
pounds-werght when he interprets 7g, and there is no difficulty 
whatever. 
Lastly, does Prof. Fitzgerald seriously propose to introduce a 
new and impractical inertia unit, based upon the intensity of 
gravity near London, for general scientific purposes, or only for 
engineering-students’ consumption ; and, if the latter, does he 
hope thereby to heal the supposed breach between science and 
practice ? O. J. L. 
Immunity from Snake-Bite. 
THAT a relative immunity is acquired after a certain number 
of bee stings, as mentioned by Mr. R. C. T. Evans (NATURE, 
February 18, p. 367), is, I believe, admitted by most bee-masters. 
But from the few inquiries I have been able to make, the degree 
of immunity varies very much in different individuals, though 
when acquired it would seem to be permanent, or at least long- 
lasting. 
A certain degree of immunity is acquired also by most persons 
against the stings of those varieties of insects which in Norway 
are commonly called AZyg, and in East Anglia, to the great indig- 
nation of those who really suffer from them, Gnat. The reaction 
of different individuals to the stings of these mosquito-like in- 
sects is very different in degree, but on the whole the resident 
suffers less than anew comer. A curious fact is that in many 
susceptible persons there is a distinct periodicity in the pheno- 
mena which follow a sting. The immediate result is a small 
flattened wheal, 3 to 4 mm. in diameter, of a pale colour, but 
surrounded by a zone of pink injection. This is attended by 
itching, but both wheal and itching have gone in less than an 
hour, About twenty-four hours later the part begins to itch 
again, and in a few minutes a hard, rounded, deep-red papule, 
about 10 mm. in diameter, appears, and is quickly surrounded by 
an area of cedematous skin. The formication is intense, and in 
the affected area, while ordinary tactile sensations are dulled, 
those for temperature and painful sensations are exaggerated. 
In two or three hours the itching diminishes, and the cedema 
disappears, leaving a small red papule which itches little, if at 
all. After another interval of twenty-four hours, or more often 
rather less, all the phenomena recur, but with diminished 
intensity ; a third, a fourth, and even a fifth recurrence usually 
takes place, but on each succeeding occasion the itching and 
swelling are less severe. After the periodic exacerbations have- 
ceased, a small indolent papule persists for weeks, sometimes for 
months. This periodicity is not observed in all persons, and: 
is certainly most marked in those who suffer most severely. In 
the same individual the reaction is very much greater after some 
bites than after others. 
Whether the ‘‘ mosquito” injects a toxin, or whether it is 
merely in some instances the carrier of a pathogenic microbe, 
might be worth ascertaining. Dawson WILLIAMS. 
February 19. 
Copper and Oysters. 
IN my previous letter (p. 366) I had not gone into details, 
but Prof. Herdman’s remarks on it induce me to do so. 
The oysters referred to were brought to me by Mr. G. I. 
Wells, F.I.C., who had already examined some of them, and 
found copper to be present in such quantity that it could be 
readily dissolved out, direct from the oyster, with cold dilute 
nitric acid. 
| These observations I fully confirmed. The oysters were, no 
doubt, very exceptional ones, and they were believed to have 
caused diarrhoea in persons eating them. Most of them were 
free from colour, and from these no copper could be detected 
| by direct treatment with dilute nitric acid; whilst from the 
coloured ones, sufficient could be obtained to easily prove the 
presence of that metal. j 
Some of the oysters were dark green, and others a bright sky- 
blue, the colour being in patches, and in one oyster almost 
entirely concentrated in the large muscle for closing the shell. 
Assay Office, Chester, February 23. W. F. Lowe. 
