Imagens das páginas
PDF
ePub

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 time 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 to 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 (Pr/p2m (P1/P2), 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 pressures 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 if 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 ion, 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

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

[blocks in formation]

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 90 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. Simultaneous observa

VAPOUR

tions of the intermediate and large ions were obtained on many occasions, but with vapour pressures exceeding seventeen millimetres, while the observations of the large ion were equally good, all trace of the intermediate ion disappeared. Disintegration of the ion at a critical vapour pressure is unlikely, and it is much more probable, assuming a rigid nucleus, that the adsorbed fluid is in the condition of a dense vapour, and that at the critical pressure it changes its state to that of a liquid, like the moisture adsorbed by glass and shellac in Trouton's experience.

Such a change means a decrease in the energy of the aggregation, and is to be expected when the molecules of water vapour around the nucleus become sufficiently closely packed. The advent of a liquid surface involves a diminished rate of molecular escape; rapid condensation will therefore occur, with a decreasing unit-surface energy, until further increase in the size of the ion means an increase in the total energy of the mixture of ions and vapour. The final result is no other than the large ion. The assumption of a rigid core for the intermediate ion appears, thus, to be justified.

Similitude in Periodic Motion.

Ir may interest those of your readers whose attention has been direction to periodic motion to know that by reducing extremely large and extremely small frequencies to a musical base, and employing the middle C (256) as a standard the following results are obtained :

Green light (frequency 5.6 × 1014) corresponds to the note C in the forty-first octave above the standard. The colours-orange, green, and violet-roughly correspond to the musical chord ACE.

Human heart-beats (seventy-five a minute) correspond to the note E (320) in the eighth octave below the standard.

The earth's daily rotation corresponds to the note
G (384) in the twenty-fifth octave below the standard.
Neptune's sidereal period almost corresponds with
E flat (422) in the forty-first octave below the
standard.
HERBERT CHATLEY.

Tangshan Engineering College, Tangshan,
North China, March 17.

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

PRESSURE

10

15

E

[ocr errors][ocr errors][merged small][merged small]
[merged small][ocr errors][merged small][merged small]

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.

FIG. 1.

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 (6). 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/0.

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.

B

THE

HOUSE-FLIES AS CARRIERS OF DISEASE. HE discovery of the rôle of insects in the transmission of human and animal diseases is one of the most striking achievements of medical science during the last twenty-five years. Filariasis, Texas fever, nagana, malaria, sleeping sickness, yellow fever, dengue, sandfly fever, relapsing fever, plague, typhus, and many other diseases of the lower animals, have been shown to be transmissible by blood-sucking insectsmosquitoes, ticks, tsetse flies, fleas, or lice, as the case may be. The pioneers in this line of inquiry were Manson, Smith and Kilborne, Bruce and Ross.

In a number of cases the necessity of intervention by an insect has been established by the discovery that a portion of the life cycle of the parasite is passed in mosquito, tick, or tsetse fly respectively. In other cases, the evidence rests upon the correspondence in time and space of the incidence of the disease with the presence of some particular insect which has been experimentally shown capable of transmitting the infection. yet other cases, such as plague, the microbe can also pass directly from patient to patient, as happens in the pneumonic variety of the disease, but the paramount importance of flea transmission in bubonic plague gains in recognition daily.

In

The rich harvest of discovery reaped by the investigations into the part played by bloodsucking insects in the spread of the above-mentioned diseases naturally stimulated inquiry into the possibilities of insect carriage as a factor in outbreaks of cholera, typhoid, dysentery, and epidemic diarrhoea. These are not diseases in the transmission of which a blood-sucking insect is likely to play a part, for in none of them is the infecting microbe present in the blood-stream in sufficient quantity, but the dejecta, fæces, and often urine, contain the bacilli in countless numbers. A small proportion of convalescents continue to excrete them for weeks, months, and, in the case of typhoid, for years afterwards, although enjoying perfect health. These people are particularly dangerous to the community as they form an unsuspected reservoir of infection.

To produce an epidemic of typhoid, cholera, or dysentery, the bacilli dejected by persons sick or convalescent from the disease must find access to the alimentary tract of others. There are, however, ways in which this may happen independent of the agency of insects. A water supply may become contaminated with infected material; the dejecta may dry up and be distributed as dust, and fall upon food materials (a method, the importance of which may easily be exaggerated, as these bacilli are readily killed by desiccation), or, owing to bacteriologically inadequate attention to cleanliness, food-stuffs, in which the microbes can multiply, may be infected with bacilli from patients or convalescents. Typhoid and cholera bacilli are small objects, less than onethousandth of an inch in length, so that fingers may be easily soiled by considerable numbers with

out this being obvious, and the microbes are not removed by perfunctory washing.

Although these three means of spread do produce and maintain epidemics, one has but to consider the habits of the house-fly to realise that this insect may be an able and willing assistant

in the distribution of the bacilli which are the cause of cholera, typhoid, dysentery, and diarrhoea, and that flies, if in sufficient numbers, and under conditions favourable for their operations, may constitute the principal way in which infection is distributed. In order to appreciate how this may happen it is necessary to be in possession of some few points in the life-history and structure of the fly.

These subjects have been submitted to careful inquiry during the last few years, particularly in America and this country, by Newstead, Howard, Griffith, Hewitt, and Graham Smith, and we are now well acquainted with this insect, intimate. knowledge of which was, until recently, curiously lacking.

The female fly lays about 120 eggs at each laying, and may produce four broods. The eggs are mostly laid on horse manure or other fermenting refuse; they are about 15 mm. in length and o'3 in their greatest diameter, and hatch in from three days to eight hours, according as the temperature ranges from 50° F. to 80° F. The larva is a little active grub 2 mm. long; and on hatching out burrows into the manure or other material on which the eggs are laid. The larval stage lasts five days to three weeks, and pupation five days to a month, according to temperature. Thus the whole cycle from laying of the egg to emergence of the fly occupies ten days to two months, according as the weather be warm or cold. The young female is ready to lay its first batch of eggs in about ten days, or even sooner in warm weather. Owing to this influence of temperature upon the rate of development of egg, larva, pupa, and imago, the number of flies in August depends on the temperature during June and July.

During winter a few flies survive in warm and secluded places. In the spring these start the next year's supply. Dr. Howard, of the United States Department of Agriculture, estimates that in forty days the descendants of one fly might number twelve million, or 800 lb. weight.

It will therefore be obvious that any attempt to overcome the nuisance from flies must, if success is to be achieved, be directed to their breeding haunts, and as early in the season as possible.

The points in the anatomy of the fly of importance for our present object are the legs and feet and the alimentary apparatus. These will be sufficiently obvious from the diagrams (Figs. 1 and 2). The feet are covered with minute hairs, which are more numerous and finer than in the diagram, and extremely fine hairs are also placed upon the pads. A sticky substance is secreted by the surface of the pads, by means of which the fly grips. Each leg is like a minute paint brush, which is

applied to the surface of whatever it rests upon, excrement or food-stuff, as opportunity offers.

The alimentary canal comprises a gullet, stomach, crop, intestine, and rectum (see Fig. 2). The gullet is prolonged forwards to a minute. opening between the flaps of the proboscis, halfway down which it is joined by the salivary duct (S D). At the entrance to the stomach (S) it is bifurcated, and one limb of the bifurcation is extended backwards to the bilobed crop (C). By a valvular apparatus at the entrance to the stomach, the insect can direct the liquid driven by the pump in its trunk into either the stomach or crop. The proboscis is a highly elastic muscular organ with universal movement. At the end are two flaps

FIG. 1.-Leg of a house-fly.

or labella (only one of which is shown), which it can open out like the leaves of a book, and apply the medial surfaces to the material it feeds upon. From the middle line or hinge, minute chitinous channels pass outwards to the margin. At the base of the trunk a number of muscle fibres are attached to the gullet by the peristaltic contraction of which fluid is pumped up from the mouth and propelled into the stomach or crop. The structural arrangement of the channelled flaps of the trunk acts as a filter, through which solid objects larger than 1/4000th in. seldom pass. When feeding on a liquid, the fly applies the labella to the surface, and sucks the liquid through the

MUSCLES

FIG. 2.-Alimentary system of a house-fly.

"strainer" first of all into the crop. When this is full, a further quantity is admitted into the stomach. In the case of solid material, such as sugar, the insect must first dissolve the material. This is done by pouring saliva upon it, or by regurgitating some of the contents of the crop.

A well-fed fly deposits fæces abundantly, and also the contents of its crop upon sugar and other solid objects.

It is clear, therefore, that there are a priori reasons for suspecting the fly of carrying bacterial infection. Born in a dunghill, it spends its days flitting between the sugar basin, milk pan, and any fæcal matter available. Its hairy, probably sticky, feet and the habit of regurgitating the

contents of the crop and defæcating at frequent intervals, suggest it as an excellent inoculating agent for any bacteria it may pick up in the satisfaction of its catholic tastes. That it does, indeed, operate in this way has been abundantly demonstrated. Flies which have wandered over cultures of organisms and afterwards been allowed to walk upon gelatin plates leave a rich crop of germs in their footprints, which can be demonstrated by subsequent incubation.

Flies fed in the laboratory upon material containing easily identifiable pathogenic microbes have been shown to harbour them in their crops for days, and to deposit them in their fæces and the regurgitations from their crops. Internal carriage is probably more important than soiling of the exterior of the insect, as many pathogenic bacteria soon die from desiccation on the appendages of the insect.

In addition to these laboratory experiments, there are numerous recorded instances in which the pathogenic organisms of cholera, typhoid, phthisis, anthrax, and plague have been recovered from the interior or dejections of flies which have been captured in the immediate neighbourhood of cases of the disease, or, in the last two cases, of carcases of animals dead of the disease.

Although, however, flies may be discovered with the infection of a number of diseases in or upon them, and by their habits may not unlikely serve as agents in transferring infection, it by no means follows that they are the determining factor of epidemicity in the case of cholera, typhoid, dysentery, etc. In the case of fulminating epidemics of typhoid and cholera associated with an infected water supply, this is obviously not so.

It is in temporary encampments of troops or pilgrims, when the disposal of excreta must necessarily be of a primitive character, that the conditions obtain which are most favourable to the breeding of flies and the distribution of infection by them, if cholera or typhoid appear. Even in these circumstances it is difficult to assess the relative importance of fly carriage and other means of spread, but the conclusion that fly transmission is the principal means of spread of typhoid in military encampments and stations has been arrived at by a number of competent observers, amongst them the commission to inquire into the origin and spread of typhoid fever in the United States military camps during the Spanish war of 1898, and by a number of medical officers concerned with the severe outbreaks of enteric which occurred during the Boer war.

The sanitary arrangements of a military camp are not exactly those of the Ritz Hotel, and the prevalence of flies in late summer can scarcely be appreciated by those who have not had camp experience. The conditions are most favourable for transmission of disease by flies, and the circumstantial evidence against them is so strong as to have left no doubt in the minds of the American Commission that these insects play a large part in disseminating infection, for on page 28 of their general statement and conclusions we read:

"Flies undoubtedly served as carriers of infection."

An estimate of the fly population and its relation to admissions for enteric fever was made by Ainsworth in Poona, where enteric has a very definite season. A definite number of fly traps was set, and the daily catch taken as a measure of the fly population. The observations showed that the abundance of flies increased earlier than the admissions for enteric, and, speaking generally, the rise in fly population ante-dated the rise in enteric cases by about one month.

Taking into account the incubation period for the disease, this fact is in agreement with the view of a causal relation between cases and flies in Poona.

15

14

13

In considering the possible influence of flies in the spread of typhoid in a well-sewered city, it must be remembered that the opportunities for them to pick up the infection are vastly fewer than under the conditions of a military encampment, or even in rural surroundings. In large cities with modern. sewerage, dejecta and urine from patients may be left available to flies, but the bulk goes promptly into the main drain, and similar observations to those above-mentioned have shown no close relationship, in point point of time, between cases of typhoid and prevalence of flies in London, Washington, or Manchester.

1

6

5

4

2

2

124

[ocr errors]

11

A. Mean atmospheric temp. for the week

ing dejecta in the case of an outbreak of cholera amongst a limited population in the Puri jail. These were attended with immediate good results. There are the same general reasons for assuming that fly transmission plays an important part in epidemics of summer diarrhoea of infants as in the case of typhoid and cholera. Anyone familiar with the domestic ménage of the average working man on a hot summer day, with the baby sick with diarrhoea, and other small children to care for, must realise that the opportunities afforded for flies to transport the infective agent from the dejecta of one child to the food supply of another are more than adequate.

Epidemic diarrhoea of children does not occur except during that season of the year when flies are abundant and active, and, as will be seen from the accompanying chart, the relation between fly population and diarrhoea cases is so

8. No.of flies caught per day + 9000

$ per week +20

C.Diarrhoea deaths

13 June 20 27 July 4 11 18 25 Aug1 8 15 22 29 Sept.5 12 19 26 Oct.3 10 17 24 31

As with typhoid, the case against flies as agents in the distribution of the infection of cholera is circumstantial, as other means of spread cannot be excluded. Take, for instance, the case of an accumulation of 300,000 pilgrims in Puri, India, in July, 1912, which was studied by Greig. The sanitary accommodation of the town was inadequate for such an accession to the population. Some of the pilgrims imported the infection of cholera, and an outbreak occurred. Flies in Puri "amounted almost to a plague," and a bacteriological a bacteriological examination of the legs and the contents of the alimentary tracts of flies caught in the neighbourhood of cholera cases demonstrated the presence of cholera vibrios.

[blocks in formation]

FIG. 3.-Dr. Hamer's observations on relation in point of time between prevalence of flies and diarrhoea mortality in London, 1908. (141 fly-collecting centres.) The deaths from diarrhoea have been antedated 10 days

Knowing the habits of flies, it is impossible to forgo the conclusion, arrived at by Greig, that some amount of distribution of the infection of cholera was due to their activity. But to what extent they were contributing could only be ascertained by the result of measures directed either to the diminution of their numbers, or to depriving them of access to infectious material.

Greig could not supervise the private latrines of the native inhabitants, but was able to carry out practical measures to prevent flies from visit

striking as to suggest something more than a mere accidental dependence upon the same phenomena.

The chart is constructed from Dr. Hamer's observations on the numbers of flies caught daily in the same number of traps in 141 localities in London during 1908. An important point brought out by these observations is the dependence of both the number of flies and the epidemic upon the cumulative effect of previous warm weather -as, for instance, is indicated by the earth temperature four feet below the surface, a fact to which attention was directed by Ballard in 1889. Similar observations in Manchester, by Dr. Niven, in 1904 to 1906, showed the same relationship.

The reason why the number of flies should be dependent upon this factor is obviously that the generation time (cycle from egg to egg) is

« AnteriorContinuar »