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fluorescence at all, while the inside which had cooled more slowly, and was found to be crystalline, gave a very brilliant green glow.

Calcium metaborate can by very sudden cooling be obtained in a vitreous state showing no fluorescence, while the crystalline variety gives a moderate blue glow. The presence of manganese induces no fluorescence in the vitreous form, but a very brilliant green in the crystalline modification. Evidence has been obtained that many substances which, in their crystalline form, fluoresce brightly, exhibit no glow in the amorphous or vitreous state.

From numerous experiments it is concluded that the green fluorescence of X-ray tubes is associated with the presence of a notable quantity of calcium and a relatively small amount of manganese, that a truly vitreous glass exhibits little, if any, fluorescence, and that a glass containing manganese can only be kept in this condition by extremely sudden cooling. Without suggesting that ordinary X-ray glass has any definitely crystalline structure, the evidence would indicate that something akin to this is more readily obtained when manganese is present.

A tendency to crystallisation may be so slight in a glass as not to interfere with its use under ordinary working conditions, but there is no advantage in fostering this tendency. The presence of manganese in any appreciable amount introduces other defects of minor importance. I mention them only to emphasise the point made before, that if the intensity of green fluorescence seen in many foreign X-ray tubes is considered imperative it can only be obtained by sacrificing some of the good working qualities of the new and purer English glass. HERBERT JACKSON.

University of London, King's College, June 23.

The Magnetic Storm and Solar Disturbance of
June 17, 1915.

THE magnetic storm described in NATURE of June 24 by the Rev. A. L. Cortie seems to have been larger at Stonyhurst than at Kew. The extreme westerly position of the declination needle at Kew occurred about 1.30 p.m., and the extreme easterly position about 5.37 p.m., the total range being about 72. Between 5 and 6 p.m. the movements had a range of 61'. I am not clear which of the two corresponds to the 91.5' mentioned by Father Cortie, but either is substantially less, even allowing for the fact that the strength of the horizontal field is about 6 per cent. higher at Kew than at Stonyhurst. This is, of course, quite in accordance with the usual tendency for disturbance to be greater in higher latitudes, but it helps to illustrate the fact that whatever the ultimate source may be, terrestrial position counts for a good deal. The total range shown by the horizontal force at Kew was about 4607 (17=1× 10-5 C.G.S.), the maximum occurring about 5.42 p.m., and the minimum about 9.30 a.m.

Father Cortie's remarks on the absence of the rapid oscillations sometimes characteristic of magnetic storms refer, I presume, to the time 4 to 6 p.m., when the largest movements occurred. Earlier in the day, for instance, near 6 a.m., the oscillatory character was fairly prominent at Kew. One would, in fact, have expected to hear of telegraphic interruptions.

Father Cortie seems to associate the magnetic storm with a particular spot or group of spots on the sun. That at least is what one would naturally infer from his remark: "Such a close approximation of the position of the spot and the earth referred to the sun's central meridian during a magnetic storm is very unusual." Whether magnetic storms are directly due to the emission of electrons from the sun, and, if

so, whether the emission is localised in spots are questions on which there is a diversity of spices The present case seems a good example of the 25culties in the way of a final decision. Farber Come tells us that "on June 17-18 there were no fewer th seven groups of spots visible." On the other boss, the disturbance of June 17, though much the largest was by no means the only magnetic about the time. There was, as Father Corde mentions, what is usually termed a "sudden coce ment " about 1.50 a.m.-I make it a minute or later on June 17, but there was another-st larger about I p.m. on June 16. The subs disturbance on June 16 was not large, and after the conditions were fairly normal. We should ally regard the disturbances on June 16 and distinct. Then on June 21, about 3.10 p... there yet another sudden commencement," the la the three, and this was followed by a consienl disturbance lasting to the end of June 22. -S. commencements," even when smail, are usually reci nisable over at least the greater part of the w Of all types of disturbance they seem to have the best claim to a cosmic origin. The subsequent is turbances, when there are any worth which is not always the case-show much more rapid local variation.

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We have here, then, three disturbances with sudian commencements in the course of about five days, so that even accepting the sun-spot emission theory, in the absence of special identification marks, the association of one particular disturbance with one particular spot or group of spots would seem to be arbitrary.

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Another aspect of the case is that magnetic disturb ances sometimes occur when sun-spots are spicuously few or wholly absent. For instance, there was a "sudden commencement" of considerable size on June 7, at a time when Father Cortie tells us the sun was almost free of spots.

The fact that magnetic disturbances occur at intervals of from twenty-six to twenty-eight days more frequently than is accountable for by pure chance is obviously consistent with the sun-spot emission theory; but it does not necessarily favour it. Quiet magnett conditions show the "twenty-seven-day" period to practically the same extent as disturbed conditions. Sometimes disturbance is the rule, and quiet condtions the exception, and it is not clear that the one phenomenon is more fundamental than the other. There seems a difficulty in associating quiet conditions with some limited area, some "anti-spot," on the sun. C. CHREE. June 26.

The Names of Physical Units.

MAY I point out that Dr. Guillaume is wrong in suggesting, in his letter in NATURE of June 17, that the adjective "specific," employed in connection with physical magnitudes, has no constant and defne meaning? "Specific" is the adjective of species," and the "specific resistance of iron" is that functier of the resistance of a piece of iron and the oth physical magnitudes characteristic of the piece which is the same for all pieces which belong to the species "iron." The statement made in the last sentence is true if for "resistance" be substituted any other magnitude to which "specific" is attached, and for “iren.” any other form of matter which is recognised as a "species."

I am not urging that the retention of the term "specific" is desirable; on that matter I offer no opinion. I am only urging that the word has a perfectly definite and constant meaning. Teddington, June 22.

NORMAN R. CAMPBELL.

THE USE OF COTTON FOR THE PRODUCTION OF EXPLOSIVES.

THE

The

HE history of the laboratory production of various forms of nitro-cellulose has been well stated by many chemists, and everything essential can be found either in their own researches or in the ordinary text-books. practical outcome of such work has been the establishment of modes of manufacture for many purposes, but in the present instance it is proposed to deal entirely with the use of cellulose in one shape or another for explosives of any practicable kind. It is almost unnecessary to state here that every kind of propulsive explosive now used has cellulose as its basis, but it may not be superfluous to say that all military propulsive explosives have cotton for their basis as distinct from cellulose.

The word cellulose must not be understood in the strict chemical sense, but rather as including all those materials which are chiefly cellulose, and this definition will include materials like woodpulp. Now one may clear the ground on this subject at once by saying that, for military purposes, wood-pulp and other impure forms of cellulose are useless. Very good sporting powder can be made from nitrated wood-pulp, but the artillerist would be in great difficulty if he were provided with such a propellant, because in order to obtain any sort of regularity the nitration of the wood-pulp has to be kept at a low point, and the ballistics on which the artillerist depends would be quite thrown out. The modern gun is really a machine of precision; the artillerist knows that and expects it to throw one shot after another to reach the same point within a fraction of its possible range as computed from its elevation of sighting, and his whole attention has been based on this. If he were supplied with a material, however good, which on explosion developed a lower pressure, he would be relatively helpless and his rivals, using their own standard material, would have him at a sore disadvantage.

In modern practice, the raw material used is cotton waste, which is, as its name implies, merely the rejected stuff in the manufacture of cotton goods; and although linters, which is the technical term for the short fibre material adhering to the cotton seed, may be used, yet its employment presents serious difficulties in that the seed with which it is associated has to be removed by chemical treatment. There have been many experiments made to use such substances as jute, ramie, kapok fibre, and in short everything from sulphite pulp to spun cotton, but as workable substances these have been rejected in favour of the staple material-cotton waste.

The method of producing a satisfactory form of nitro-cellulose from cotton waste is as follows:The waste is hand-picked so as to remove the grosser impurities. The product is teased, picked once more, and then dried. After that, the nitration process is carried out, and this has been much modified in the light of experience, but in essence

still consists in the immersion of the purified waste in a mixture of nitric and sulphuric acids of the following composition: H2SO4, 71 per cent.; HNO3, 21 per cent.; HO, 8 per cent. The amount of water in this mixture is important, and the acids as they are written are as their formulæ represent and do not refer to the commercial products. The strict relationship of

the water to the two active materials should be preserved. It is of course easy now to obtain sulphuric anhydride (SO) and make an anhydrous mixture, but this gives a nitro-cellulose with too high a nitrogen content. After the mixed acids have acted for the required time, they are removed and the gun-cotton is washed to remove as much of the acid as possible, and purified by several boilings with water. The boiling with water is of great importance, as in this part of the process the unstable bodies produced during nitration are dissolved or decomposed, leaving the nitrocellulose in a stable condition. The only thing now remaining is to pulp the cotton, which is again washed and then partly dried and moulded into the required shape by pressure.

The old idea that something as nearly as possible to the so-called hexa-nitrate of cellulose should be aimed at has been fairly well exploded, and the manufacturer seeks to regularise his output so that he may obtain a nitro-cellulose with approximately 11 molecules of NO to the quadruple molecule, as shown in the formula C24H2909(NO3)11. This formula must not, however, be taken as any more than a convenient way of expressing the degree of nitration, which is really better stated in terms of content of nitrogen which ranges between 12'93 and 1305. This is merely a parenthesis, but is necessary as showing how delicate and complicated a matter it is to obtain a uniform and trustworthy material for propulsive explosives, and as it has been found in practice that even what is apparently such a simple matter as the preparation of a mixture of acids of known composition is really one requiring some care and skill. It will be readily understood that the difficulty is trifling compared with that of providing an equally regular form of cellulose. So well is this recognised that different consignments of cotton waste, all of approved quality, all picked, teased and re-picked, are mixed so that the cellulosic raw material may be as nearly the same grade as possible.

With this fact in front of us, let us consider what the condition of a factory would be which had to use any kind of raw material, clean or dirty, lignified or not, and had to try to make that into a trustworthy propulsive explosive of standard quality. This question has only to be asked for the answer to provide itself. In the present case a great deal too much has been assumed as to the capability of our enemies for making trustworthy nitro-cellulose without cotton waste. Because any competent chemist in his laboratory could make some form of nitro-cellulose from his own shirt cuffs if he pleases, people have jumped to the conclusion that that will be of some use

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process of an explosive like this is of the most delicate kind and has to be conducted with military precision, has been constantly overlooked; and at the present moment it is not too much to say that there is only one material available for modern gunnery, and that is cotton.

The fact that the manufacturing | temperature weigh one pound, so that a lifting
unit displacing that volume would lift one pound
minus its own weight.
minus its own weight. Consequently, if the lift-
ing unit consisted of "nothing shut up in a
box" as the schoolboy's definition of a vacuum
runs, only the weight of the box would have to
be deducted from the gross lift obtainable. As
no light vacuum-container could maintain its
shape against atmospheric pressure, however, a
gas must be used to keep the displacement body
distended by its expansive properties. The gas
universally used for airships is hydrogen. This
weighs about one-fifteenth of unit volume of air,
so that only 1/15 gross lift is lost by its use. The
possibilities of getting wonderfully enhanced lift
by new gases, lighter than hydrogen, are thus
seen to be illusory.

PROBLEMS OF AIRSHIP DESIGN AND
CONSTRUCTION.

THE
HE problem of the airship falls naturally into
three parts, concerned with flotation, pro-
pulsion and steering respectively. The best
results in any of these three branches are to a
great extent antagonistic to similar success in
one or both of the other two. For instance,
flotation, which is purely a displacement problem
at bottom, demands that the displacement body
should have the greatest volume for the least
superficies, i.e., that it should be spherical.
Propulsion, on the other hand, demands that the
body be of the shape having least head-resistance,
i.e., of long fish-shape. Steering, with which is
linked dynamic stability, demands that large fins
and control surfaces be affixed to the body, which
otherwise would set itself broadside on to the
relative current caused by its forward movement.
These auxiliary surfaces add to the weight, that
is, oppose flotation and add to the head-
resistance, thus opposing propulsion. Again,
the displacement body must of necessity consist
mainly of a gas lighter than air. All the light
gases are highly inflammable (or if not have some
other disadvantage), and consequently are danger-
ous in proximity to an internal-combustion motor,
such as is universally used for propulsion, as
being the only motor with a good ratio of power
to weight. Therefore the motor must not be
placed too close to the gas-container, and in
consequence it is difficult to enclose all the parts
of the airship in a single "streamline" body of
least resistance, and the head-resistance and
weight are thus both increased considerably,
opposing propulsion and flotation.

The above list of incompatibilities might be extended considerably, as every airship designer knows to his cost. It is not to be wondered at, therefore, that airship design is in so fluid and embryo a condition that the future of the airship is looked upon as extremely dubious in many cuarters. The fact, however, that so much progress has been made in face of stupendous difficulties is a happy augury for the future of the arship, especially as many of the difficulties met with are due mainly to the fact that airships are at present small, and they will disappear as soon as experience and growing confidence enable large and larger vessels to be built.

To deal with the displacement body, or lifting unit, first. The lift obtainable is, of course, directly proportional to the weight of air displaced and inversely proportional to the weight of the displacement body in in itself. Roughly, thirteen cubic feet of air at sea-level and normal

Coal gas was long used (and still is) for ordinary spherical balloons, as being cheaper and more available than hydrogen, but being about ten times as heavy as hydrogen, is comparatively useless for airships. Ammonia vapour has been suggested for airships, as being non-inflammable, but is about eight times as heavy as hydrogen and of a destructive character to metal, etc. The provision of a stable non-inflammable light gaseous mixture would solve so many practical difficulties in the construction of airships that many thousands of pounds could profitably be expended in research on this problem. Failing this provision, all precautions must be taken to prevent fire, or to minimise its effects on board airships.

Hydrogen being non-explosive apart from oxygen, can be isolated in containers jacketed with an inert gas and thus rendered harmless. The division of the displacement body of an airship into compartments is desirable from this and other points of view. For example, a large volume of gas in a thin fabric container is prone to surge about and strain the container when in motion. Compartments prevent this and also localise leakage due to rupture of any part of the container.

The type of airship in which this principle is carried farthest is the rigid type (Zeppelin) in which the displacement body consists of seventeen or eighteen separate gas-containers, set in a rigid cylindrical framework, like peas in a pod. The chief advantages of the rigid framework are (i) that the actual gas-containers are relieved of strain and are (ii) protected from the influence of weather. The disadvantages are (i) the loss of gross lift due to the weight of the framework, and (ii) the fact that the airship cannot be folded up for transport or storage, and must consequently be housed in a large and expensive shed.

The gross lift of a large Zeppelin is about twenty-five tons, of which about twenty tons are absorbed by the framework, engines, etc. This gives a net lift of only about one-fifth of the gross lift, a figure that could be much improved upon by making the vessel larger. This net lift has to account for crew, etc., so that not more

than two tons of explosives could be carried, and this only at a low altitude. Naturally, other things being equal, the weight of the framework, etc., of a small airship is a larger proportion of the gross lift than the corresponding weight of a large airship.

In that type of airship in which the walls of the gas-container are themselves the "framework" of the displacement body (the "non-rigid " type), much weight is saved, but disadvantages come in that strains on the fabric affect its gas-tightness, which is also much affected by action of sun and other influences.

Again, the attachment of the car (containing the engines, etc.) by wire ropes to the container is worked out on the assumption that the gascontainer will retain its shape. This end is attained in single gas-containers by having a bag of air (the "ballonet") inside the container, into which is pumped air under pressure, to maintain the full volume and shape of the envelope. If, however, compartments are to be used in the container, some means of equalising their pressures even if one be ruptured must be devised, otherwise the shape will be distorted. This is no easy task.

The non-rigid type has the great advantage of being quickly deflatable for transport packed up. Examples of this type are the Parseval and Astra-Torres, in which latter ship an ingenious system of suspension greatly strengthens the gascontainer.

The "semi-rigid type has some of the advantages and the disadvantages of both the other types. Examples are the Forlanini (Italian) and Astra XIII. (Russian).

are

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The material of which gas-containers usually constructed is made of layers of cotton fabric cemented to layers of rubber. In order to intercept the blue (actinic) rays of light that "rot" the rubber very quickly and make it porous to the gas, the fabric is coloured yellow. beater's skin makes a very gas-tight container, but untreated is affected by rain, which is absorbed, and by its weight decreases the net lift. This disadvantage applies to untreated fabrics, which are therefore usually varnished with an aluminium varnish, thus preventing water absorption and promoting gas-tightness. Fabric impregnated with gelatine, rendered flexible by added glycerine, and insoluble by formaldehyde, has given promising results. Oiled silk is very gas-tight but seams are troublesome. Very much research is still required into the question of fabrics.

Propulsion demands a power plant and means for obtaining a reaction from the air. The ratio

h.p. As an indication of the performances that may be expected from airships in years to come, we may note the proportion of power to weight lifted in the last vessel as one horse-power to every So lb. lifted. The speed attained is fifty miles an hour. In the case of an aeroplane doing ninety miles an hour or so, the weight lifted is only about 15 lb. per horse-power.

Screw propellers are universally used for airships, and are often of wood. They are usually placed at the sides of the gas-container in rigid vessels and below it in non-rigid vessels. Much research is needed as to the best position for propellers relatively to the body to which they are attached.

A strong reason for increasing the power of airships is that by so doing a large amount of lift can be obtained by the dynamic action of the large control surfaces, which, by directing the airship's nose up, are able to give it a very fast rate of rise, much quicker than that of aeroplanes.

The maximum height attained by airships is somewhat more than 10,000 feet (Zeppelin and Italian). Aeroplanes have ascended twice as high and ordinary balloons three times as high. To attain 10,000 feet high an airship must sacrifice much ballast and gas, so that it cannot voyage for its longest period at a great height. Zeppelins are claimed to be capable of holding the air for three days, but not at full speed or height. There is no advantage in going very high (except for military reasons), and under 3000 feet would be a usual zone in which to operate were it not for anti-aircraft measures. Some day, when the airship is better developed, it may pay to go to great heights in order to obtain the advantage of lessened resistance to advancement due to the tenuity of the air.

As regards steering and stability, it may be said at once that most airships steer clumsily and require large spaces in which to manœuvre. Our little non-rigid vessels have been specially developed for handiness in our much wooded country, but Zeppelins are craft for vast open spaces. The dynamic stability of an airship is a complicated matter to work out. Besides ordinary pitching and rolling there are added effects due to surging of the gas and distortion of the gas-container. Propellers also complicate the stability question. Large control surfaces are essential, sticking well out from the body, to avoid its "wash."

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of power installed to weight lifted has been
steadily rising both in airships and aeroplanes.
The first Zeppelin airship (1900) weighed 10,200
kilograms and the motors were two, totalling
32 horse-power. Zeppelin III. (1906) lifted
12,575 kg., and the motors (2) totalled 130
effective h.p. The "LI" (marine) of 1913 lifted
about 28,000 kg., and the motors totalled 720❘ 1911-15.)

"A Biological Survey of Clare Island in the County of Mayo, Ireland, and of the Adjoining District.' Section I. (cmprising Parts 1 to 16), Introduction, Archæology, Irish Names, Agriculture, Climatology, Geology, Botany. Section II. (comprising Parts 17 to 47). Zoology (Vertebrata, Mollusca, Arthropoda, Polychata). Section III. (comprising Parts 48 to 68), Zoology (Oligochata to Protozoa), Marine Ecology, Summary. (Dublin: Hodges, Figgis, and Co., Ltd.; London: Williams and Norgate,

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Such are the biological questions which, as Mr. Lloyd Praeger remarks, have led many naturalists to study islands. He recalls Alphonse de Candolle, Edward Forbes, Charles Darwin, Alfred Russel Wallace, and Sir J. D. Hooker; and many other names might be cited. The same old questions led a number of naturalists in 1909 to plan and inaugurate the survey of Clare Island, which has now been completed to the great credit of all concerned. The island was chosen because of its suitable size and position, because of its unusual elevation as compared with most of the islands lying off the west coast of Ireland, and for various practical reasons.

Clare Island lies across the entrance to Clew Bay, at about the middle of the great projecting buttress of ancient rocks which forms west Galway and west Mayo. It is almost cliff-bound, the cliffs varying from 50-100 ft. in the east and south to 1000 ft. in the north-west. The dominating feature is the high ridge of Croaghmore (1520 ft.) on the north-western shore. "On the inland (southern) side Croaghmore presents a steep heathery slope, and on the seaward face plunges down a magnificent precipice into the Atlantic." Its scarp is the home of a very interesting Alpine flora, and affords a sanctuary of wildness to many animals which could not survive the close grazing of other parts of the island. The adjoining islands of Inishturk and Inishbofin, which are included in the survey, are in a general way similar to Clare Island, but with no such lofty elevations.

The survey had two main objects in view: (1) The study of the fauna and flora of this extreme verge of the European continent, and (2) the study of an insular area with reference to the special problems of island life and of the dispersal of organisms. As regards the first object, the results have far exceeded expectations, but as regards the second there was some disappoint

ment.

A geological study of the area showed that it was not only probable, but almost certain, that a landconnection between the island and the mainland existed long after the Glacial period, which would have permitted of the immigration of much of the present fauna and flora after normal climatic conditions were resumed.

In short, the investigators soon found that they had to do with an assemblage of animals and plants that had not crossed even a few miles of sea.

As regards the fauna and flora in general, excluding winged animals and spore-plants, there is practical unanimity of opinion, resting on varied evidence from many different groups, that the narrow strait of sea which separates Clare Island from the mainland represents a very serious barrier to migration, and one across which the existing fauna and flora of the island, taken as a whole, could not have passed.

If the study of Clare Island as an island was rather negative, the other aim of the survey was realised in a manner positive enough to delight everyone, and some indication must be given of the number of additions made to the fauna and

flora of Ireland and of the British Isles, and of the number of species discovered which were new to science. No fewer than 3219 plants were recorded, of which 585 were new to Ireland, fifty-five new to the British Isles, and eleven new to science. No fewer than 5269 animals were recorded, of which 1253 were new to Ireland, 343 new to the British Isles, and 109 new to science. This is a very gratifying result, and shows how many new forms of life still lie to be discovered not very far from our doors. It is pleasant to read that "almost the whole survey was carried out by volunteers, whose field-work had to be done in

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-Signal Tower Head, Clare Island. Silurian cliffs, 700 ft. high. Looking north. From "Clare Island Survey."

their own time, and, to a great extent, with their own money."

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There are sixty-eight reports altogether, so that it would take considerable space even to mention subjects and authors. We are tempted to remark on the Foraminifera dealt with by Messrs. Heron-Allen and Earland (recording 287 species, thirteen new); on the Rhizopods by Messrs. Wailes and Penard (recording 129 species, five new); on the Flagellates and Ciliates dealt with by Mr. Dunkerly (recording ninety-eight species, many of them very interesting forms); on the marine sponges described by Miss Stephens (sixty-five species, including the new and interest

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