Marcu 25. 1897 | 
NATURE 
499 
of aluminium, about Io cms. long and 14 ems. wide, cut from 
the same sheet as the surrounding aluminium tube, was sup- 
ported at one end by a small piece of paraffin so placed as to 
be out of reach of the action of the Rontgen lamp. The rays 
from the lamp were allowed to pass from a lead cylinder sur- 
rounding it by a small hole about -3 of a square cm. in area. 
They fell on the aluminium sheath transparent to them, and 
rendered the air between it and the insulated aluminium 
within conductive. 
We tried various differences of potential, ranging from a few 
volts to 2200 volts. In one series of experiments we charged 
the insulated metal to —97°5 volts, and then disconnected the 
battery electrodes. The lamp was then set a-going, and the 
electrometer deviation taken each half-minute for a minute and 
a half with one pair of quadrants insulated. The rays were 
then stopped, the quadrants metallically connected, and metallic 
zero again found. Then the reading during another period of 
one and a half minutes, with the rays acting, was observed, and 
so on until no deviation from the metallic zero of the electro- 
meter was found with one pair of quadrants insulated, and the 
rays falling on the aluminium outside coating of the Leyden A. 
The sensibly complete discharge thus observed took place in 
about a quarter of an hour. We found that the rate of de- 
viation from the metallic zero was the same as the difference of 
potential fell from —97°5 volts to about —4 volts. With 
differences of potential of —930,-—1750, and — 2000 volts the 
rate of deviation was not appreciably greater than with + 20 
volts. 
This confirms and extends, through a very wide range of 
voltage, the interesting and important discovery announced by 
J. J. Thomson and McClelland, in their paper in the Cambridge 
Philosophical Society Proceedings of March 1896, to the effect 
that the conduction of electricity through air under influence of 
the Rontgen rays is almost independent of the electric pressure 
when it exceeds a few volts per centimetre. 
In the experiments on paraffin, the outside coating of the 
Leyden A consisted of an aluminium cylinder 27 cms. long, 
4 cms. diameter, connected to sheaths. A metal bar about 1°75 
ems. in diameter, and 30 cms. long, supported centrally on 
paraffin filling the whole space between it and the aluminium 
sheath, constituted the inside coating. With this arrange- 
ment we made experiments with differences of potential of 
+ 94, + 119, + 238, — 2000, + 2500, and — 2400 volts. At 
none of these potentials did we find any perceptible increase of 
conductance produced by the Roéntgen rays above the natural 
conductance of the paraffin when undisturbed by them. 
In the experiments with glass, the Leyden A consisted of a 
glass tube silvered on the inside. The inside silvering was 
placed in metallic connection with the inside coating of B. 
That part of the glass tube which projected beyond the lead 
sheath was covered with wet blotting-paper connected to the 
sheaths. We observed the behaviour of glass under the 
Rontgen rays at differences of potential of +800, + 1500, 
+ 2000 volts. We found no indication of increased conductance 
due to the rays at these voltages. 
We are forced to conclude that the experiments described by 
J. J. Thomson and McClelland do not prove any conductance 
to be induced in paraffin or glass by the R6ntgen rays. . It 
seems to us probable that the results described in their paper 
—pages 7 and 8—are to be explained by electrifications induced 
on surfaces of glass or of paraffin in contact with air rendered 
temporarily conductive by the Roéntgen rays. 
THE INTRODUCTION OF BENEFICIAL 
INSECTS INTO THE HAWAIIAN ISLANDS.) 
FEW countries have been more plagued by the importation of 
insect pests than the Elawaiian Islands ; in none have such 
extraordinary results followed the introduction of beneficial 
species to destroy them. By far the most conspicuous of the 
former class, and hitherto the most injurious, have been the 
scale-insects. The number of species of this group, which have 
spread throughout the islands, is remarkable, and not less so the 
enormous multiplication of individuals of many or most of these 
species. 
1 Notes on the result of introducing predatory and parasitic insects into 
the Hawaiian Islands for beneficial purposes. Communicated by the Secre- 
tary of the Committee, appointed by the Royal Society and British Associa- 
tion, for investigating the Fauna of the Sandwich Islands. 
NO. 1430, VOL. 55] 
The first importation of Coccineléde to destroy these hordes 
was made in 1890, when Vedalia cardinalis, Muls., a native of 
Australia, was sent over by Mr. Albert Koebele. At that time 
many trees were in a deplorable condition from the attacks of 
Icerya, monkey-pod trees being particularly badly infested—so 
much so that they were being largely cut down, as the only 
resource. The Vedaléa was a complete success ; it became per- 
fectly naturalised, increased prodigiously for a time, practically 
cleared the trees, and then, as the Zcerya became comparatively 
scarce, decreased in numbers; while at the present time it is 
evident that the number of the scale and its destroyer has arrived 
ata fixed proportion. Previously to its introduction here the 
same ladybird had done excellent service in the fruit orchards 
of Lower California. 
The complete success of this first experiment was followed by 
the engagement of Mr. Koebele by the Hawaiian Government 
and planters for a term of years, to contend against other plagues 
no less serious than the /cerya. The wisdom of this course 
cannot be too highly commended, when compared with the in- 
difference shown by the countries similarly circumstanced, and 
1S a set-off against the reckless importation of infected plants 
which had been allowed in former years. Mr. Koebele, after 
seeing the wants of the country, with his unrivalled knowledge 
of the habits of Coccéned/éde, introduced numbers of other species 
in 1894, many of which, no doubt, failed to establish them- 
selves, while a considerable number (how many is yet uncertain) 
have become completely naturalised, and done splendid service. 
Before mentioning these, it may be said that the two chief 
products of the islands are sugar (which until lately has been far 
the most important export) and coffee, the cultivation of which 
has lately enormously increased. There is also a considerable 
amount of fruit grown ; and this, too, is lately increasing. All 
these industries have been continually threatened with destruc- 
tion from imported insects. The Rey. T. Blackburn, who 
studied the insects of the islands during six years—now nearly 
twenty years ago—wrote that the fruit trees were afflicted with 
incurable blight. Coffee plants were introduced in 1825. Its 
cultivation formed quite an industry in the middle of the century 
on Kauai, where only it was systematically cultivated; its growth 
was finally abandoned there in 1856, owing to the ravages of 
blight, said to have been importedin 1850. The sugar-cane has 
been, and is, attacked not only by scale-insects and Aphzdes, 
but by several other creatures of quite different orders. 
To return now to the ladybirds : one of the most useful has 
been Coccénella repanda, Thun. (from Ceylon, Australia, China, 
&c.) which feeds on Aphzdes. The services of this species can- 
not be over-estimated. On Kauai recently the cane was so 
much attacked by an dffzs as to cause considerable alarm, On 
visiting the locality the Coccze//a was found to be already 
present, breeding in such numbers as to leave little doubt that 
the plants would be soon cleared. On the same island, on 
another occasion, I saw the fruit trees (especially orange and 
lime) in a beautiful garden in a most deplorable condition from 
the attacks of Afhzs and scales. Very few ladybirds could be 
found after a careful search. The owner was for spraying the 
trees, but, seeing their condition could not be much worse, I 
advised him to wait and give the beetle$ a chance. In a few 
weeks these were swarming; and when I returned, after six 
months, the infested trees were all in perfect condition, full of 
fruit and flower. Not less numerous than the preceding is a 
Cryptolemus (C. montrouzter?) introduced from Australia, and 
thoroughly naturalised. It attacks the highly injurious species 
of Pulvinaréa. When I visited the Kona district of Hawaii in 
1892, many of the trees were literally festooned with the masses 
of this pest, and appeared on the point of being totally de- 
stroyed. In 1894 the ladybirds were sent there, and very soon 
had entirely changed the condition of things, and the affected 
trees speedily recovered. To show the vast increase of this 
species of ladybird, I may state that in June of the present year, 
many large trees in the city of Honolulu had several square feet 
of their bark entirely hidden by the larvze, which formed great 
white masses, presenting such an extraordinary appearance that 
I much regret not having obtained photographs of some of the 
trees. At the present time this species and Coccenella repanda 
are far the most conspicuous and abundant of the introduced 
Coccinellide, either of them far outnumbering even the most 
abundant native insects. Their wide distribution is remarkable, 
for not only are they all over the lowlands, but throughout the 
mountain forests as high as four or five thousand feet above sea- 
level ; indeed, the Coccene//a is still higher up beyond the limits of 
