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called prehistoric times were skilled in stone- Morale Fondée sur les Lois de la Nature. By working, as is shown by the great megalithic M. Deshumbert. Cinquième et Sixième Mille. ruins at Tiahuanaca, described by Sir Clements Pp. 191. (London : Watts and Co., n.d.) Markham in his book, “The Incas of Peru.” Since The Comité International de Propagande pour la the building of this city, geological changes seem Pratique de la Morale fondée sur les Lois de la to have been in action which caused the elevation

Nature has representatives in eighteen countries. of the plateau and the shrinkage of the body of Its Bureau Central is at Dewhurst, Dunheved water now known as Lake Titicaca.

Road West, Thornton Heath, England. It issues These changes brought about the present con- a propagandist volume on the subject of a natural ditions : a very dry atmosphere, with a small per- morality, written by the secretary, M. Deshumbert, centage of oxygen and a high range of tempera- which has been translated into eight languages. ture, inducing nervousness and mental instability, Much has been written on morality "according to a lack of forethought and industry, an over

nature,” since the Stoics invented the idea, but weening contempt of foreigners, and a perfervid this book, partly because the author understands patriotism. Many physiological facts indicate this both physiology and biology, has a freshness of retrogression, resulting in administrative in- appeal. “Good is all that contributes to the concapacity, and neglect of regularised education. servation and increase of life . .." by co-operaThe

poorness of the food supply, ill-cooker tion and mutual aid of individuals each of whom potatoes and maize-flour cakes, promotes physical is thus aided towards complete self-realisation. degeneration and leads to over-indulgence in Evil is all that diminishes life. The end of Nature stimulants. The writer takes, perhaps, too gloomy is life and more life. a view of a people whom he dislikes and despises, These and connected axioms are well illustrated but he appears to write with adequate knowledge, by examples of anti-natural human superstition and his monograph, if his conclusions be accepted, and of the importance in the animal world of furnishes a good example of the action of an intellectual and moral qualities. The way in unfavourable environment upon a race exposed to which, e.g. the tiger depends for existence upon its influence.

observation, judgment, patience, self-control, A Pilgrim's Scrip. By R. Campbell Thompson. decision, and perseverance, is quite a fresh object

Pp. xii + 345 (London: John Lane, 1914.) lesson. A collection of practical rules of personal Price 12s. 6d. net.

hygiene and a detailed list of physiological funcTHESE slight, discursive sketches of the life of a

tions are useful, and might form the nucleus of wandering archæologist in the Nearer East are

a modern scheme of individual morality. Some interesting and instructive. The studied archaism quotations from J. Payot are interesting here. A of the style, a trick which may have been learned

set of parallels between the intelligence of nature from Mr. Doughty's famous “Travels in Arabia

Arabia and of man is interesting, and might be augDeserta, becomes, after a time, a little mono

mented. "Man in many cases is inferior to tonous, but it gives a piquant flavour to his

nature,” but this simply points the truth that man accounts of eastern life and character. The writer is part of nature. is one of the school of scholarly antiquaries, trained by the British Museum, who, in spite of

LETTERS TO THE EDITOR. many hardships and the necessity of making scanty funds go a very long way, have done noble [The Editor does not hold himself responsible for

opinions expressed by his correspondents. Neither service in adding to our national collections.

can he undertake to return, or to correspond with Mr. Campbell Thompson's experience has given the writers of, rejected manuscripts intended for him a considerable insight into the back of the this or any other part of Nature. No notice is oriental mind, and his hints for dealing with these taken of anonymous communications.] races and conducting excavations will be service

Ultra-Violet Excitation of the D Line of Sodium. able to those who may follow his tracks. His

It is well known that the D line of sodium is the wanderings have extended widely: Mosul, Behis

first member of a series of lines, the other members tun, the Sinaitic Peninsula, the Sudan, Angora, being in the ultra-violet region of the spectrum. It is and Carchemish, are some of the stages. Perhaps known also from the investigations of R. W. Wood the most interesting episode is his excursion, in that sodium vapour at a moderate temperature illucompany with Mr. L. W. King, to make a fresh minated by D light gives rise to a secondary emission copy of the famous inscription of Darius at

of D light. This secondary emission is appropriately Behistun, the riddle of which was solved by the

called by him resonance radiation. It has been further genius of Sir H. Rawlinson. Swung from cables investigated by Dunoyer.

The first ultra-violet line of the series is situated suspended over the precipice the explorers collated

at wave-length 3303. It is very probable for a numRawlinson's copies, which proved to be wonder- ber of reasons that this or any other line of the fully accurate, and succeeded in photographing, series would give rise to resonance radiation, though from a five-foot range, the splendid head of the I do not know of any experiment directly establishing warrior king—a fine piece of work told modestly

the fact. and clearly.

A more doubtful question is whether stimulation The book is well illustrated by photographs and,

by the line 3303 would give rise to D light. The

question is not new. It has been proposed by Prof. which is unusual in popular works of travel, is Wood, and he has looked for the effect, but without provided with an excellent index.

The matter seemed important enough, how

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ever, to justify a fresh effort, and within the last time with a telescope free from secondary colour when few days I have been able to get the effect as satis- the sun is as high as ten or fifteen degrees above the factorily as could be wished.

horizon. When, however, a point of sunlight only is. The chief essential is a source giving a sharp line visible, the rest of the disc being hidden beneath the of great intensity at 1 3303. This was found in a horizon, atmospheric dispersion, if it could be persodium vapour lamp in quartz analogous to the mercury

ceived with unaided vision, should produce a complete lamps in general use. Details of the construction and vertical spectrum from blue to red, as in the case of manipulation of these sodium lamps will be published stars when near the horizon. The red end of this. later. The visible light from such a lamp was filtered spectrum should be most evident, since these rays are out by means of a screen consisting of cobalt-blue least absorbed. In the flash, however, the red is uviol glass, combined with nitrosodimethylaniline. completely suppressed, and the vivid green which is The light which came through was photographed with obvious to the naked eye can only be seen at very a quartz spectrograph, and was found to consist of low horizons. Moreover, it is not always seen, as 13303 exclusively.

Mr. Whitmell remarks, when the conditions seem This radiation was concentrated by means of a otherwise favourable. quartz lens on a quartz bulb containing some sodium. It seems to me very probable that the phenomenon The bulb was made nearly red-hot with a bunsen is in some way connected with the abnormal condiburner, which was then extinguished. A patch of tions which at sea produce mirage effects. The layer luminosity could be seen on the wall of the bulb when of dense air in contact with the sea might produce the ultra-violet beam fell upon it. As the bulb cooled total reflection for solar rays refracted from below and the vapour pressure of the sodium diminished, the horizon, but the critical angle of reflection will this patch of light gradually expanded, and filled the depend on wave-length, and it is possible under cerentire bulb; it then faded away, and had disappeared tain conditions that the green rays may be totally when the bulb was cold. This behaviour is exactly

reflected whilst the red are refracted. the same as is seen when D light is excited by the I have one more observation to add to those deincidence of D light, and although in the present case

scribed by Mr. Whitmell, and this will, I think, give the light is much fainter, the conditions of observa- the coup de grâce to the theory of a subjective effect tion are in some respects more favourable, for there due to retinal fatigue. In May, 1900, I happened to is no disturbance from visible light scattered or re- observe the setting of Venus in the sea from my flected by the walls of the vessel.

eclipse camp on the Algerian coast. Observing with Critics of this experiment will naturally concentrate a 3-in. inverting, telescope, I saw the planet when their attention on two questions :

very near the horizon suddenly change in colour from (1) Was the light observed really due to ultra-violet dull red to vivid green, and as I lowered the telescope excitation ?

to the point where the sea horizon about bisected the (2) Was it of the same wave-length as the D line? field of view I was amazed to see two green images

As regards (1), a sheet of plate-glass 1.2 cm. thick of Venus, one, the normal image, ascending from was interposed between the source and the bulb. The below, and the other sloping down from above. This excited light was completely extinguished.

was probably reflected from the sea itself. The setting As regards (2) the light was rather below the in- took place at the moment of meeting of these two tensity which would easily allow of direct spectro- images. The whole apparition, from the moment scopic examination, though with a little further im- when the colour changed from red to green, to the provement of the conditions it might be made strong instantaneous disappearance of the two images, cannot enough. I have, however, proved it to be of approxi- have lasted more than four or five seconds. The sea mately this wave-length by absorption methods. The about this time was found to be excessively cold, luminosity, was seen undiminished through a thick although the air was hot during the daytime, and this cell containing potassium bichromate solution, held state of things would doubtless favour the production before the eyes. . It was absolutely invisible through a of a relatively dense layer of air on the surface of the cell containing praseodymium nitrate. Thus the wave- sea in calm weather.

Join EVERSHED. length must lie in the region from 1 5820 to 16020,

Kodaikanal, April 13. for this is the only region transmitted by bichromate and absorbed by praseodymium. The D line at 1 5890 lies in this narrow region, and I think, therefore, that

The Larger Tons in the Air. there is no reasonable doubt that the emission does In addition to the well-known small ions, which consist of D light. Discussion of the theoretical are of a type common to all gases, two classes of bearing of this result is deferred.

larger ions exist in the air under ordinary conditions.

R. J. STRUTT. One of these consists of the large ions of Langevin Imperial College, South Kensington, May 8. which have a mobility of about 1/3000, while the

other contains ions with a mobility of about 1/50.

As the latter value lies between those of the mobilities The Green Flash.

of the small and large ions, the members of this MANY descriptions of the green flash have been pub- latter class may be called the ions of intermediate lished in letters to NATURE and elsewhere, but I do mobility, or, shortly, the intermediate ions. not remember to have seen a satisfactory, explanation The slow movement of these larger ions in an of this curious phenomenon. Atmospheric dispersion electric field clearly indicates that they are molecular is invoked, but this does not explain the absence of clusters of more or less complexity. Ordinarily the the red end of the spectrum. My observations agree value of the mobility is the only guide to the nature in every particular with those described by Mr. Whit

of the ionic structure, but in the case of the large mell in NATURE of March 11, p. 35. At sea I have

ion, at least, an important deduction is to be made observed a violet or blue tint occasionally, and on one from the outcome of experiments on the formation of occasion a red flash as the lower limb of the sun clouds in closed vessels. emerged from a cloud into a clear space very near the It is well known, since Aitken's notable work on horizon.

the subject, that, in ordinary circumstances, the air Normal atmospheric dispersion will, of course, pro- is crowded with particles, in suspension, on which duce a red fringe to the sun's lower limb, and a blue the water vapour condenses into visible drops if the fringe at the upper limb, as may be seen at any i air becomes slightly supersaturated. These particles,

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a temperature change due to the mere alteration of state is the same as that required to keep the temperature constant when water evaporates, it may be definitely concluded that the molecules in the contained or adsorbed fluid are in the same condition of aggregation as those of water.

In the case of the intermediate ions the determinations of mobility are not accordant enough to allow the value of n' to be found in this way with any accuracy, but the fit of the points to a line is on the whole better if the mobilities are plotted against vapour pressures than when set out against the relative humidities. This, according to the preceding expression, corresponds to the physically extreme case when n is equal to some large number, though, so far as could be inferred from the plot, n might not be greater than some small integer. In any case, here the latent heat of vaporisation of water is sometimes greater than that of the adsorbed fluid.

The result of the preceding line of argument, though not conclusive in the present instance, at least suggests the idea that the intermediate ion consists of a rigid core enveloped by a collection of water

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the number of which varies greatly from time to time, can be removed by filtration of the air through cotton-wool, or, in closed vessels, by settlement with the drops formed by expansions. In general, these nuclei are electrically uncharged, and whatever their nature, are conveniently known as dust particles.

C. T. R. Wilson has shown that in air recently freed from dust, with increasing supersaturations, the first visible condensation takes place on the small ions. It is now known that the circumstances of the condensation remain unchanged during intervals of tine extending to days after the removal of the dust, The intermediate and large ions are eminently suitable nuclei for the condensation of water vapour, as their mobilities are largely affected by changes in the hygrometric condition of the air, so the results just mentioned indicate not only that these ions are removed with dust particles, but also that they are not produced in air once, made dust-free. There is no doubt that the large, ions are present in ordinary saturated air; it appears, then, that filtration removes some rigid nucleus without which at least the large ion cannot be developed.

From the facts which have been stated, the picture of the large ion most readily formed is that of a dust particle round which water molecules are adsorbed io an extent depending on the vapour pressure, the whole being electrified by the attachment of a small ion.

Some idea of the nature of the relation between mobility and vapour pressure which is to be expected in connection with such an ion, may be obtained by comparing, on simple thermodynamic lines, the working of two Carnot's engines, one with unit mass of a mixture of ions and water vapour as the working substance, and the other with unit mass of water and its vapour. The vapours are to be taken as perfect gases, and it is to be assumed that the density of a vapour is small compared with that of the substance in the corresponding denser state. With these assumptions the result is readily obtained that (Pil Pem (P1/P,)"", when only the change of state is being considered. P and P are the values of the vapour pressures in the two engines at the same temperature, and n is the ratio of the latent heat of vaporisation of water to that of the fluid surrounding the nucleus of the ions. It is convenient here to take m as the mass of the denser part of the substance. The expression, which holds for all cases of adsorption, states that at two temperatures the mass adsorbed will be the same if the ratio of the vapour pressures, in equilibrium with the adsorbed fluid, is the nth root of the ratio of the saturated vapour pre sures at those temperatures. It is the formula of reduction for 'adsorption observations taken at different temperatures, and a clue to the condition of the adsorbed moisture is to be obtained from the value of n found necessary to make the observations fall into line. As the mobility of the ions under consideration, at constant temperature and air pressure, is constant if the mass of the adsorbed fluid remains the same, the formula is directly applicable to mobility determinations is m is taken to refer to the mobility reduced to constant air density.

Trouton, and Masson and Richards, find that the mass of contained moisture in the case of flannel and cotton-wool is a function of the relative humidity. This means that n is unity in the preceding expression. n is also unmistakably unity in connection with the large jon, the determinations of mobility only falling into line if plotted against the relative humidities. The result of such a plot is shown in Fig. 1.

No heat change due to a variation of surface energy is involved in the value of n, so in these cases where n=1, as the heat per unit mass necessary to annul

RELATIVE HUMIDITY

20

10.00

30100
MOBILITY- RECIPROCAL
Fig. 1.–The relation between the reciprocal of the mobility of the

large ion and the relative humidity. molecules existing as a dense vapour rather than in the liquid condition.

Trouton, in 1907, made the interesting discovery that there are two modes of condensation of water vapour on rigid surfaces. If special precautions are taken in drying the surfaces, on exposure to water vapour adsorption occurs as a dense atmosphere of water molecules, in a state, perhaps, intermediate between that of a gas and that of a liquid. At any rate, a change to the liquid condition somewhat abruptly takes place in these circumstances when, according to Trouton, the humidity is about 50 per cent. in the case of glass, and about go per cent. in that of shellac.

The fluid surrounding the nucleus of the intermediate ion is, no doubt, in a state corresponding to that of the moisture condensed at low pressures on_carefully dried surfaces in Trouton's experiments.

Further evidence supports the preceding view of the ion. Fig. 2 shows the relation between the reciprocal of the mobility of the intermediate ion and the vapour pressure as deduced from a plot of the determinations.

At a pressure of about fifteen millimetres the mobility decreases very rapidly with increase in the value of the vapour pressure.

Simvítaneous observa

tions of the intermediate and large ions were obtained

Similitude in Periodic Motion. on many occasions, but with vapour pressures exceed- It may interest those of your readers whose attening seventeen millimetres, while the observations of tion has been direction to periodic motion to know the large ion were equally good, all trace of the inter- that by reducing extremely large and extremely small mediate ion disappeared. Disintegration of the ion frequencies to a musical base, and employing the at a critical vapour pressure is unlikely, and it is middle C (256) as a standard the following results much more probable, assuming a rigid nucleus, that are obtained :the adsorbed fluid is in the condition of a dense Green light (frequency 5-6 x 10') corresponds to the vapour, and that at the critical pressure it changes note C in the forty-first octave above the standard. its state to that of a liquid, like the moisture adsorbed The colours-orange, green, and violet-roughly by glass and shellac in Trouton's experience.

correspond to the musical chord ACE. Such a change means a decrease in the energy of Human heart-beats (seventy-five a minute) correthe aggregation, and is to be expected when the mole- spond to the note E (320) in the eighth octave below cules of water vapour around the nucleus become the standard. sufficiently closely packed. The advent of a liquid The earth's daily rotation corresponds to the note surface involves a diminished rate of molecular escape; G (384) in the twenty-fifth octave below the standard. rapid condensation will therefore occur, with a de- Neptune's sidereal period almost corresponds with creasing unit-surface energy, until further increase in E flat (422) in the forty-first octave below the the size of the ion means an increase in the total energy standard.

HERBERT CHATLEY. of the mixture of ions and vapour. The final result Tangshan Engineering College, Tangshan, is no other than the large ion. The assumption of a

North China, March 17. rigid core for the intermediate ion appears, thus, to be justified.

A Simple Direct Method for the Radius Curvature of

Spherical Surfaces. The following device was developed to obtain the radius of curvature of some lens surfaces that were too small for the available spherometers. It has

proved so satisfactory that, not finding it in any of our 15

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1010 MOBILITY - RECIPROCAL Fig. 2.-The relation between the reciprccal of the mobility of the

intermediate ion and the vapour pressure.

To sum up the whole evidence, the large ion consists of a rigid nucleus surrounded by moisture in the liquid condition, the size of the drop at constant temperature depending on the vapour pressure. The intermediate ion is to be considered as a similar nucleus enveloped by a dense atmosphere of water vapour.

The mass of the ion increases with the vapour pressure, until at a critical pressure the adsorbed fluid assumes the liquid state, and the aggregation develops, by the rapid condensation which ensues into the large ion of Langevin.

It is not quite clear how the electrical energy of the ions is related to their diameter. The charge is, however, not essential to the equilibrium of molecular structures such as those just mentioned, and it is not unlikely that the conclusions as to the nature of the ions, only rendered possible by the happy chance of their electrification, may apply with, perhaps, little modification to the far more numerous class of unelectrified nuclei which exists in ordinary air. University of Sydney.

J. A. POLLOCK. 1 Details of these observations will be found in two papers published in the Philosophical Magazine for April and May, 1915.

laboratory manuals, it has been thought to be of possible interest to others.

Two brass strips, A and B (Fig. 1), are connected by a flat spring, C. To B is soldered a brass ring, D, to serve as a bed for the lens, L, the surface of which is to be examined. A is pierced with two triangular holes, P and Q, as indicated in the sketch, the forward one having its vertex over the centre of the ring. A three-legged optical lever, E, is set with its legs on the glass surface, the front leg being as far forward as possible in one of the triangular holes, P (as shown). The other legs straddle the strip A, one being in contact with A. The lever E is not shown in the lower sketch.

If the mirror be lifted from its position in P to a similar one in which the front leg is at the vertex of Q, it will have been given a linear displacement (s) and an angular displacement (). The former of these quantities is the same as the distance between the vertices of P and Q. It is a constant of the instrument, and may be determined by means of a travelling microscope. The angular displacement (0) depends on the lens surface, and may be obtained by telescope and scale in the usual way. The radius of curvature is then written by p=s/e.

The vertex of Q is placed over the centre of the ring, as this is the simplest way to ensure that the displacement lies along a great circle of the surface.

Will C. BAKER. Physical Laboratory, Queen's University,

Kingston, Ont., April 19.

HOUSE-FLIES AS CARRIERS OF DISEASE. out this being obvious, and the microbes are not THE discovery of the rôle of insects in the

removed by perfunctory washing. transmission of human and animal diseases

Although these three means of spread do prois one of the most striking achievements of medical

duce and maintain epidemics, one has but to conscience during the last twenty-five years. Filari

sider the habits of the house-fly to realise that asis, Texas fever, nagana, malaria, sleeping this insect may be an able and willing assistant sickness, yellow fever, dengue, sandfly fever, re

in the distribution of the bacilli which are the lapsing fever, plague, typhus, and many other

cause of cholera, typhoid, dysentery, and diardiseases of the lower animals, have been shown rhæa, and that flies, if in sufficient numbers, and to be transmissible by blood-sucking insects under conditions favourable for their operations, mosquitoes, ticks, tsetse flies, fleas, or lice, as may constitute the principal way in which infection the case may be. The pioneers in this line of

is distributed. In order to appreciate how this inquiry were Manson, Smith and Kilborne, Bruce may happen it is necessary to be in possession of and Ross.

some few points in the life-history and structure In a number of cases the necessity of interven

of the fly. tion by an insect has been established by the dis- These subjects have been submitted to careful covery that a portion of the life cycle of the inquiry during the last few years, particularly in parasite is passed in mosquito, tick, or tsetse fly America and this country, by Newstead, Howard, respectively. In other cases, the evidence rests Griffith, Hewitt, and Graham Smith, and we are upon the correspondence in time and space of the now well acquainted with this insect, intimate incidence of the disease with the presence of some

knowledge of which was, until recently, curiously particular insect which has been experimentally lacking, shown capable of transmitting the infection. In The female fly lays about 120 eggs at each yet other cases, such as plague, the microbe can laying, and may produce four broods. The eggs also pass directly from patient to patient, as are mostly laid on horse manure or other ferhappens in the pneumonic variety of the disease, menting refuse; they are about 1'5 mm. in length but the paramount importance of flea transmission and 0-3 in their greatest diameter, and hatch in in bubonic plague gains in recognition daily. from three days to eight hours, according as the The rich harvest of discovery reaped by the temperature ranges from 50° F. to 80° F.

The investigations into the part played by blood- larva is a little active grub 2 mm. long; and on sucking insects in the spread of the above-men- / hatching out burrows into the manure or other tioned diseases naturally stimulated inquiry into material on which the eggs are laid. The larval the possibilities of insect carriage as a factor in stage lasts five days to three weeks, and pupation outbreaks of cholera, typhoid, dysentery, and

five days to a month, according to temperature. epidemic diarrhæa. These are not diseases in the Thus the whole cycle from laying of the egg to transmission of which a blood-sucking insect is emergence of the fly occupies ten days to two likely to play a part, for in none of them is the months, according as the weather be warm infecting microbe present in the blood-stream in cold. The young female is ready to lay its first sufficient quantity, but the dejecta, fæces, and batch of eggs in about ten days, or even sooner often urine, contain the bacilli in countless num- in warm weather. Owing to this influence of tembers. A small proportion of convalescents con- perature upon the rate of development of egg, tinue to excrete them for weeks, months, and, in larva, pupa, and imago, the number of flies in the case of typhoid, for years afterwards, although August depends on the temperature during June enjoying perfect health. These people are par

and July. ticularly dangerous to the community as they form During winter a few flies survive in warm and an unsuspected reservoir of infection.

secluded places. In the spring these start the next To produce an epidemic of typhoid, cholera, or year's supply. Dr. Howard, of the United States dysentery, the bacilli dejected by persons sick or Department of Agriculture, estimates that in forty convalescent from the disease must find access days the descendants of one fly might number to the alimentary tract of others.

There are,

twelve million, or 800 lb. weight. however, ways in which this may happen inde- It will therefore be obvious that any attempt to pendent of the agency of insects. A water supply overcome the nuisance from flies must, if success

may become contaminated with infected material; is to be achieved, be directed to their breeding the dejecta may dry up and be distributed as haunts, and, as early in the season as possible. dust, and fall upon food materials (a method, The points in the anatomy of the fly of importthe importance of which may easily be exag- ance for our present object are the legs and feet gerated, as these bacilli are readily killed by and the alimentary apparatus. These will be suffidesiccation), or, owing to bacteriologically inade- ciently obvious from the diagrams (Figs. I and 2). quate attention to cleanliness, food-stuffs, in which The feet are covered with minute hairs, which are the microbes can multiply, may be infected with more numerous and finer than in the diagram, bacilli from patients or convalescents. Typhoid and extremely fine hairs are also placed upon the and cholera bacilli are small objects, less than one- pads. A sticky substance is secreted by the surthousandth of an inch in length, so that fingers face of the pads, by means of which the fly grips. may be easily soiled by considerable numbers with- Each leg is like a minute paint brush, which is

or

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