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coincident for a relatively short period of time only, a few white "spottings" at a distal end of the leaflets may result; if the time element is lengthened, all the cells of the leaflet may suffer a water loss much below the wilting coefficient, and instead of a "spotted" appearance the entire leaflet will bleach white. This is exactly what happens under both experimental and field conditions. In the intermountain country where a very large number of observations have been made, it has been noted that fields showing a considerable incrustation of alkali when irrigated exhibited white spot in more or less amount, depending upon the other environmental factors above mentioned. Also, a sudden rise of the water table in irrigated districts has brought about the same appearance of the plants in the fields. Some very interesting observations have been made on fields adjacent to each other, with plants of the same age and all conditions the same excepting the application of water. The irrigated fields showed extensive white-spot trouble, while the non-irrigated fields showed none.

It has been noted by eastern pathologists who have made observations on this disease that it occurs mainly in the spring of the year. However, the writer has observed it in the intermountain districts during the early spring, during mid-summer and during the late fall; in short, throughout the entire growing season.

Specimens of artificially produced white spot of alfalfa were submitted to several plant pathologists who reported that these specimens were identical with diseased alfalfa plants which they had themselves collected.

An extended report will be published in due time after the completion of certain experiments which are now in progress.

P. J. O'GARA
AMERICAN SMELTING AND REFINING CO.,
SALT LAKE CITY

THE POLYHEDRAL VIRUS OF INSECTS WITH A THEORETICAL CONSIDERATION OF FIL

TERABLE VIRUSES GENERALLY

In a previous paper1 J. W. Chapman and I called attention to the fact that the wilt or

1 Biol. Bull., Vol. XXX., No. 5, pp. 367-390..

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polyhedral disease affects many different species of insects. We also showed that the disease is not produced by bacteria, but is caused by minute organisms capable of passing through diatomaceous filters, and further attempted to demonstrate that the polyhedral bodies always found associated with the disease are not organisms, as supposed by Bolle, Fischer, Marzocchi and Knoche, but reaction bodies-simply nucleoprotein by-products.

In order to satisfy myself that we were really dealing with an organism and not merely with an enzyme, toxin or other material a large series of passage infections were instituted. Twenty-five gipsy moth caterpillars were infected at a dilution of 1: 1,000 with material obtained from a caterpillar previously dead of wilt. The animal was merely ground up, sterile water added and the whole filtered through a sterile Berkefeld grade "N" candle. All twenty-five caterpillars fed with the filtrate died typically of wilt during the course of three weeks, whereas twenty-five controls fed with the autoclaved filtrate lived, pupated and transformed into moths. One animal dead in this first series was prepared, the material diluted as before (1: 1,000), filtered and fed to another series of twenty-five caterpillars. The experimental animals all succumbed, whereas the controls did not. Third and fourth passage infections were performed and the results were similar with the exception that the period from infection to death was considerably shorter at the fourth passage than at the first three. This shortening of the time between infection and death seems to point towards an increase in virulence with successive passages.

There are certain autocatalytic substances like chromatin that increase progressively, so to the physiologists my passage infections may not necessarily be proof for the contention that I am dealing with parasitic ultra-microscopic organisms. However, if one reviews the field of the filterable viruses and compares all of the results obtained by other workers with my results, one can not but feel inclined to

2 Thirty-two diseases are now known to be caused by filterable viruses.

adopt the view that one is dealing with minute parasitic forms. Some of the filterable viruses (pleuro-pneumonia of cattle, fowl pest, fowl diphtheria, epithelioma contagiosum and Novy's rat disease) have been cultivated, so that the question as to whether we are dealing

realize Osborn's "hypothetical chemical precellular stages"; they lie somewhere in the scheme between simple colloidal and more complex cellular states like bacteria. Some thirty-two or thirty-three disease-producing filterable viruses are now known to exist, so it

TABLE SUMMARIZING CHARACTERS OF POLYHEDRAL VIRUS OF INSECTS Has not been cultivated

1. Cultivation of virus 2. Filtration of virus

3. Examination of virus with ultra-microscope.

4. Effect of heating on virus when suspended in water

5. Effect of dry heat on virus

6. Effect of drying on virus at room temperature. 7. Effect of glycerine on virus

8. Effect of direct sunlight on virus when dry

9. Effect of putrefaction on virus

10. Effect of alcohol on virus

11. Effect of carbolic acid on virus

12. Effect of virus on 1 per cent. sugar solutions 13. Effect of virus on methylene blue and sodium nitrate solutions

14. Effect of virus on gelatin and casein

with organisms or not is solely an academic one. We are justified at present, however, in not classifying such viruses either with the plants or animals.

The table gives a summary of the chief characters of the wilt virus. The virus used in these tests was prepared from diseased gipsy moth, army worm and tent caterpillars. That proteins like gelatin and casein are not affected when treated with the filtrate in which the virus has been concentrated by centrifuging is curious because insect tissue is completely emulsified through the action of the wilt virus. This action is therefore probably a cytolytic one due to the action of toxins and is not caused by the elaboration of a proteolytic enzyme on the part of the virus.

In a physico-chemical explanation of the origin of organisms on our planet the filterable viruses seem to be of considerable interest and I do not understand why they seem to be so persistently neglected by all writers on the evolution of life. The filterable viruses probably

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SCIENCE

FRIDAY, SEPTEMBER 27, 1918

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MAGNETIZATION BY ROTATION1

So far as we know at present, a magnetic substance, that is a substance whose molecules are elementary magnets, can be magnetized in two ways, and only two ways: In the first place it can be be magnetized by creating a magnetic field in it or putting it in a magnetic field, as has been known for very many years; and, in the second place, it can be magnetized by simply setting it into rotation in a region initially neutral magnetically, and both initially and finally neutral electrically. It is chiefly with this latter process that we are concerned at this time.

In this process, as we shall see, the magnetization is produced directly by a sort of molecular gyroscopic action, which distinguishes it sharply from other processes in which magnetic fields are produced by rotation, but in which magnetization may or may not result, according to circumstances. It will be conducive to clearness to consider briefly some of these processes.

Thus if we take a tube of brass, or other non-magnetic substance, electrify it, and rotate it about its axis, a magnetic field will be produced similar in a general way to the field which would be produced by winding the tube with a coil of insulated wire and passing an electric current through it, as Rowland proved over forty years ago. So far, there is no magnetization. But if a rod of iron is introduced into the tube, and either maintained at rest or rotated with it, the rod will become magnetized-not because of its rotation, but be

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cause of the magnetic field, in this case produced by the rotation of the charges. There would be a similar result, and a similar interpretation, if the rod alone were to be given the charge and rotated.

Again, if we take a metal rod and rotate it in a magnetic field, electric currents will in general be induced in it; and the magnetic field due to these currents will, if the rod is made of magnetic material, change its magnetization. Experiments of this kind were made about one hundred years ago by Barlow, Christie and Arago.

In each of these cases, and in others which might be mentioned, a magnetic field is produced by the rotation, and it is this field which produces the magnetization if a magnetic substance is present.

Coming now to the other or gyroscopic process of magnetization, and starting with a neutral rod of iron or other magnetic substance, we can magnetize it directly by mere rotation, and a magnetic field will result from this magnetization.

In order to understand this process it is necessary to consider first, a simple case of the behavior of a gyroscope; and second, the modern interpretation of Ampère's theory of molecular currents.

Here we have a gyroscope whose wheel, pivoted in a light frame, can be rotated rapidly about its axis A. Except for the action of two springs, this frame and the axis A are free to move in altitude about a horizontal axis B, perpendicular to A; and the axis B and the whole instrument can be rotated about a vertical axis C. If the wheel is spun about the axis A, and the instrument then rotated about the vertical C, the wheel tips up or down so as to make the direction of its rotation coincide more nearly with the direction of the impressed rotation about the vertical axis C. If it were not for the springs the wheel would tip until the axes A and C became coincident. The greater the rotary speed about the vertical the greater is the tip of the wheel. When the wheel's speed about the axis A is zero, no tip occurs.

Ampère's hypothesis, each molecule of a magnetic substance has a magnetic moment, or is a magnet, because it consists in part at least of electrons revolving in fixed orbits with constant angular velocities about an oppositely charged nucleus, and producing a minute magnetic field somewhat like that due to a small loop of wire traversed by an electric current.

If these electrons, revolving in the same general direction, have mass, each molecule has therefore angular momentum like the wheel of a gyroscope; and if the body of which it is a part is set into rotation about any axis, the molecule must change its orientation in such a way as to make the direction of revolution of its electrons coincide more nearly with the direction of the impressed rotation.

Only a slight change of orientation can occur on account of the forces due to adjacent molecules, which perform the function of the springs in the experiment with the gyroscope. The rotation thus causes each molecule to contribute a minute angular momentum, and thus also a minute magnetic moment, parallel to the axis of rotation; and thus the body, whose molecular magnets originally pointed in all directions equally, becomes magnetized.

If the revolving electrons are all positive, the body will become magnetized in the direction in which it would be magnetized by an electric current flowing around it in the direction of the angular velocity imparted to it. If they are all negative, or if the effect of the negative electrons is preponderant, it will be magnetized in the opposite direction. This is what actually happens.

For a simple type of molecular magnet a somewhat exact theory of the effect can be developed.

Assume the molecule (Fig. 1) to consist of n (one or more) similar electrons, all positive or all negative, with total charge ne and total mass nm, revolving in a circular orbit of radius r with constant angular velocity (and areal velocity a= r2) about a much more massive, and fixed, nucleus with charge

-ne.

This molecule will have a magnetic moment μnea, a moment of inertia about the axis of

Now according to the modern version of revolution C=nmr, and an angular momen

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The vectors representing the angular momentum and the magnetic moment are thus in the same or opposite directions according as e is positive or negative.

If now the body of which this molecule is a part is set into rotation with angular velocity

about an axis A, the molecule, or the orbital ring, behaving like the wheel of a gyroscope, will strive, as it were, to take up a position with its axis of revolution coincident with that of the impressed rotation; but it will be prevented from turning so far by a torque T due to the action of the rest of the body and brought into existence by the displacement. In a minute time kinetic equilibrium will be reached, and the axis of the orbit will then continuously trace out a cone making a constant angle with a line through its center parallel to the axis of the impressed rotation. When this state has been reached, as is known from dynamics, and as can easily be estab

FIG. 2.

molecule would keep on turning under the action of the field until its axis coincided with H, but is prevented from doing so by the torque T'' upon it due to the action of the rest of the body and brought into existence by the displacement. This torque is well known to be

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2 The expression for T can be found readily from Fig. 2. Let A denote the moment of inertia of the ring about a diameter, and the angle between the vector representing J, the total angular momentum of the ring, and the vector representing w. J can be resolved into two rectangular components, one parallel to the axis of the impressed rotation, viz., J cos(—§), which is constant, and one perpendicular to this axis, viz., J sin (0-6), which has the constant rate of change J sin (0-8). By the second law of motion this is equal to the torque T. Expanding this expression for T, substituting for J cos and J sin ß, the components of J parallel and perpendicular to the axis of the ring, their equals C(w + cos ◊) and An sin e, and noting that AC, we obtain the relation sought.

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