SINCE

A NEW FLORA OF GREECE. Conspectus Florae Graecae. Auctore E. de Halácsy. 3 vols. Vol. i., pp. 825; vol. ii., pp. 612; vol. iii., pp. 520. (Leipzig: W. Engelmann, 1900-1904.) INCE the publication of Sibthorp and Smith's great work, "Prodromus Flora Graecæ," more than a century ago, a large number of individual workers have published floras of certain parts of Greece, and have described a very considerable number of new species. But no work dealing with the Grecian flora as a whole has since Sibthorp and Smith-been attempted until now. The author of the present work is to be congratulated upon the success he has achieved. His book is most useful to every systematist who has to deal with European plants. He himself had travelled and collected in Greece, and had written on the botany of Greece. To the results of his own observations he has utilised the data furnished by previous authors, whose names and works are duly tabulated at the end of the third volume. The area treated in the " Conspectus

is Greece

(as politically understood), as well as Epirus and Crete. The three volumes contain 825, 612, and 520 pages respectively. The species are accurately described, except in the case of the more well-known plants, of which bibliographical references and synonyms, as well as habitats, only are given. The larger genera have a key at the commencement of each to facilitate the "running down" of the species. Practically the sequence of the genera is that of Bentham and Hooker's" Genera Plantarum," although some of the suborders of those botanists are given independent rank. For instance, Fumariaceæ is separated from Papaveraceæ, Oxalidaceæ from Geraniaceæ, Rosacea (as understood by Bentham and Hooker) is split up into Amygdalaceæ, Rosaceæ, and Pomaceæ. Silenacea (Caryophyllacea of most systematists) has Alsinacea separated from it.

as

It may be of interest to note the relative space occupied by some of the larger natural orders. Compositæ heads the list with 245 pages, Papilionaceæ comes next with 125, Gramineæ and Labiatæ have 120 each, Umbelliferæ 88, and Scrophulariaceæ 74. The largest genera in point of number of species are as follow. To show at a glance the relative proportions of the Greek to the general European flora given in Nyman's "Conspectus Flora Europææ," the number given by Halácsy is quoted first, and then the total number for the whole of Europe from Nyman. Of Centaurea, Greece boasts 71 species, the whole of Europe 171; Trifolium 61 species against 108, Euphorbia 44 against 107, Campanula 43 against 94; Allium has more than half the total number of species possessed by the whole of Europe, 41 against 78; in Verbascum Greece claims a still larger proportion, 39 species against 54. In Carex Greece has 36 species, the European flora altogether 163. Vicia has 35 species; Nyman enumerates 61 for Europe. Astragalus has 33 Greek species against 120 for the whole of Europe, and Hieracium has only 20 species against 185.

It is worthy of mention that the origin of the

horse-chestnut is here definitely settled. In most books Asia is given as the native country of Esculus hippocastanum; in others it is stated with equal certainty that its native country is uncertain or unknown. Sibthorp records it as occurring in a wild state near Pindus. Nyman, in a note in his "Conspectus Floræ Europææ,' says, "Indicatur a Sibthorpio in Pindo, monte illo Graec. bor, sed post eum a nullo alio ibi inventa est." Halácsy, however, quotes Haussknecht as having found it truly wild in this and other localities (see Mitth. thür, bot. Ver. 1886, p. 71). It was, however, Heldreich (in Sitzungsb. bot. Ver. Brandenb., 1879, p. 139, and 1882, p. 20) who first brought forward sufficient evidence to prove that the real home of the horse-chestnut was in the mountains of Northern Greece. N.

SUBTERRANEAN GEOGRAPHY. Höhlenkunde, mit Berucksichtigung der Karstphänomene. By Dr. W. von Knebel. Pp. xvi+222. (Brunswick Vieweg und Sohn, 1906.) Price 5.50 marks.

THIS book is one of the handy monographs in the collection styled "Die Wissenschaft," which corresponds well in range with the English "International Scientific Series." It may be described as a clear introduction to the study of caves; but it is not so inspiring as the subject deserves. We cannot think, for instance, that it would enable anyone to realise the attraction that the hidden depths have had for certain specialists. There is a tendency in the book to classify phenomena, which may be of service to those who fully grasp their meaning; and perhaps we expect too much from an author who is so eminently exact. Somehow we do not quite see before us the great gouffres leading vertically down to unknown waterways; nor, on the surface, the real desolation of the Karstland, the white dust of waterless days. the fantastic rocks standing up in moonlight like ghosts upon the slabs of enormous tombs, the sudden edge of the ravine, and the clear green river sunk half-a-mile below. Well, if we are to study "Höhlenkunde," the emotions are for other moments. Yet what an emotional subject it all is!

Dr. von Knebel's account (p. 57) of the subterranean connection between the Danube at Immen dingen and a tributary of the Rhine in the Hegau leaves, let us admit, nothing to be desired; and there are plenty of local touches here. Of equal interest is the description (p. 107) of the flow of sea-water into the limestone near Argostoli in Kephalonia, whereby two mills are kept going in the stream. A diagram shows us how this may be accounted for by the outflow of lighter brackish water into the sea at another point, this water being the result of the mingling of a freshwater spring with the marine flow underground. We learn also how a fresh-water spring emerging under the sea may draw in sea-water from some point above it, through a cavity partly filled with air.

Among many useful discussions, we note (p. 26) that dolomite is stated to be equally soluble with calcite

in water, and that hence dolomite-masses are capable It is of giving rise to typical karst-phenomena. observed (p. 195) that the air of caves is a remarkable conductor of electricity. The relation of typical karst surfaces to the removal of forests is pointed out, and French areas, cleared after the Revolution, are cited as examples. The French causses, by-the bye, deserve rather longer mention, considering how accessible they now are from Millau, and how finely they illustrate the author's thesis. But we welcome the use made of the "dolinas" and "poljes," names that recall the fascination of the Slavonic east. The author's classificatory instinct introduces us also to marine erosion and to Fingal's Cave; to a glimpse of the fauna of caves; and to caves as the haunt of early man. But it is the treatment of the karst-phenomena that will probably give this book a place among works of reference, although precise references to original papers are rare in it, and although it has, strange to G. A. J. C. say, no index.

OUR BOOK SHELF.

The Outlook to Nature. By L. H. Bailey. Pp. ix+ 296. (New York: The Macmillan Company; London: Macmillan and Co., Ltd., 1905.) Price 58. net.

PROF. BAILEY is well known as one of the most fertile and inspiring of teachers of science as applied to agriculture and particularly to horticulture, who has built up a great school at Cornell and has also been the source of a wave of teaching from nature among

the schools of the United States.

In all Prof. Bailey's work may be seen the qualities of the enthusiast, who is moved, and gets his power to move his followers, by considerations other than those which are the ostensible object of his work. The life of the country-side, farming and gardening, then, are to Prof. Bailey something more than a scientific study or a means of earning a livelihoodthey are the great regenerating influences of modern life. He sees civilised existence getting every day more complex, more noisy, more hurried, more exacting; nor in the interests of efficiency does he expect or desire any wholesale return to a more primitive mode of living. But what he does plead for is the "return to nature" in "our personal and private hours" as a "means of restoring the proper balance and proportion in our lives." The book consists of four lectures, delivered in Boston, on such topics as the relation of country to city, the part that natureteaching should play in school life and the organisation of rural teaching generally, with a final essay on the position of evolutionary conceptions with regard to religion.

We get a vivid and interesting presentment of the opinions and convictions which have made Prof. Bailey a living force in American education; we see that the writer is a passionate lover of nature with a strain of the poet in him, but we do not always find his treatment convincing. The book must be judged as literature, and in literature neither the best of intentions nor the finest of emotions count unless you can express them with something of the freshness and inevitability of a living thing; here we often find the thoughts and arguments of Thoreau, but without his clear-cut and startling intensity of expression. Prof. Bailey is rhetorical, and that means he is some

times more concerned with the decorative value of his periods than with their absolute truth; for instance, he makes a point that we go to a gallery to see a picture of a sunrise when we might see the sunrise itself! forgetting that it is only the awakened eyes which can see at all. "I never see a sunset like that, objected the critic to Turner; "Don't you wish you could," answered the artist.

However, putting aside the question of these "airs and graces," Prof. Bailey's thesis is sound enough; civilisation is dying and will die of its own selfproduced poisons; it is only by the improbus labor on the land that the human race seems able to persist.

A. D. H.

By

Lecture Notes on Chemistry for Dental Students.
Dr. H. Carlton Smith. Pp. viii+273. (New York:
John Wiley and Sons; London: Chapman and Hall,
Ltd., 1906.) Price 10s. 6d. net.

THE Connection between dentistry and chemistry is a two-fold one. The practical dental surgeon is a worker in metals; he has to prepare amalgams for stoppings and carry out a multitude of similar operations; hence his need for a knowledge of inorganic chemistry. No less important is the second link; he must know the composition of the teeth, the action upon them of the reagents and drugs he employs; he must understand the action of ferments, whether they are contained within the micro-organisms of the mouth or in the secretions, like saliva, which come in contact with the teeth; hence his need for a know

ledge of organic, and especially of physiological, chemistry. Dr. Smith has produced a work which supplies such needs, and one is glad to see he has provided an over-supply; for instance, the sections on physiological chemistry do not deal exclusively with saliva, though naturally this subject is treated with special fulness. This is as it should be; the less specialised and narrow a dentist's education, the more is he likely to benefit those under his care.

In the analyses given of the different parts of the teeth, Dr. Smith states that enamel contains 3 per cent. of organic matter. He does not allude to the work of Tomes, in which it was shown that enamel contains no organic matter at all, and what was formerly given as organic matter (by difference) is really due to water. It is not a very important point, and possibly the author was not aware of Tomes's research on the question.

By Prof. Herbert A. Howe. A Study of the Sky. Pp. xii+340. (London: Macmillan and Co., Ltd., 1906.) Price 2s. 6d. THIS is a cheap edition of a book that appeared originally several years ago. Written in attractive, simple language, Prof. Howe's volume is just the work for those readers who, knowing little or nothing of the oldest of sciences, wish to become personally acquainted with the wonders of the sky.

A very pleasing feature of this book is the way in which the reader is forced to observe and experiment for himself. Chapter i. gives a brief historical sketch of astronomy, and is followed by five chapters dealing with the constellations observable at various seasons, and their apparent diurnal and annual motions. Then come three chapters dealing with astronomers in general and particular, and their tools. A chapter on time and the method of keeping it is followed by five (xi.-xv.) chapters dealing seriatim with the members of the solar system. The concluding chapters discuss in a simple but instructive fashion comets and meteors, the fixed stars, and the nebulæ.

LETTERS TO THE EDITOR.

[The Editor does not hold himself responsible for opinions expressed by his correspondents. Neither can he undertake to return, or to correspond with the writers of, rejected manuscripts intended for this or any other part of NATURE. No notice is taken of anonymous communications.]

The Positive Charge carried by the a Particle. SOME time ago I made the suggestion in NATURE (March 9, 1905) that the a particle was initially uncharged on expulsion, and that it gained its charge subsequently by collision with atoms in its path. I need only now repeat that the suggestion was based on the brilliant work o Bragg in Australia, who showed that the a particle passes through, rather than collides with, the atoms of solid or gaseous matter in its path, and that whether uncharged or not initially, it must, equally with the atom struck, become charged positively after the encounter by the detachment of a negative electron.

Recently P. Ewers (Physikalische Zeitschrift, March 1), using the a particles from polonium, attempted to put the view to an experimental test with negative results, and concluded against the probability of the hypothesis. Bragg (Phys. Zeit., July 1) has pointed out that Ewers's experiments by no means settle the question, and, indeed, he evidently considers it a question which cannot be settled experimentally. Certainly the requisite conditions to be fulfilled for a positive result are so rigorous that no one could be certain they had been fulfilled, and it is impossible to disprove the view by a negative result. But it is obvious that a positive result, that is, the actual isolation of the a particle in an uncharged state, would settle the question. This I have been fortunate enough to do, although only after a long experience of negative results where it might reasonably have been concluded the requisite conditions had been realised. "The best laid schemes of mice and men gang aft a-gley." A determining factor in the problem conditioning whether a positive or negative result is obtained could not possibly have been foreseen, and it was only when all hope of getting anything but a negative result had been abandoned, and what was intended to be a final experiment was being performed, that a slight change in one of the factors happened to eliminate the disturbing cause, and I obtained the coveted positive result. The precise nature of this disturbing influence is, perhaps, not yet fully demonstrated, although personally I think I now hold the clue. But there is not the slightest doubt that the a particle initially expelled is not charged as the experiments given prove.

The essential conditions are two. In the first place, the a particle must be examined in a vacuum such that during its path it does not encounter a single gas molecule. Secondly, the layer of radio-active matter from which it is expelled must not be more than one molecule thick, and must not be mixed with or overlaid by inactive matter. These conditions being fulfilled, the a particle will not traverse a single atom after expulsion, and if uncharged initially must remain so. As a third condition, it is desirable that the test for the charge shall be made on the particle during its flight. It is at least conceivable that an uncharged a particle striking a plate will convey to it a positive charge if the electron detached from the uncharged a particle on impact has sufficient energy to escape the plate.

The second condition is, as may be imagined, the difficult one to make sure of. I hoped to secure it by using radium C as the source of the rays. The rate of its disintegration is so rapid that there is only just the necessary time for an experiment to be carried out. Hence the actual number of atoms of the radio-active substance is for radium C the minimum it is possible to employ. Moreover, this number can readily be calculated, and since it is deposited from a gas uniformly on the exposed surface, not only can an experiment be devised so that the thickness of the deposited layer fulfils the monomolecular condition, but, what is equally important, it can be assumed with reasonable certainty that the radio-active layer is not overlaid or mixed with inactive matter.

With regard to the first condition, all the factors are known, and the necessary conditions can readily be calculated by two independent methods, which, as it proved, are strikingly verified by the actual results obtained. The only pitfall is in the altogether exaggerated impression which is abroad as to the ease with which a high degree of vacuum can be obtained by modern methods.

The third condition was realised by using the magnetic deviation of the a rays as a test for their charge. The rays passed out of the capillary tube from a deposit of radium C at the far end. This was obtained by the use of the emanation from 30 mg. of radium. Conditions were arranged so that the rays were completely deviated under ordinary conditions, and with the magnetic field on did not succeed in escaping from the tube, and the experiment consisted simply in re-examining the deviation in the highest vacuum that could be produced.

Long series of negative results led to the refinement of each essential condition until it seemed no further improve. ment was possible, and a wide margin of probability that the essential conditions had been realised had been secured. A most unmistakable negative result was obtained. But the next experiment intended to confirm this finally was as unmistakable a positive result as the other had been a negative one. In a partial vacuum the rays were completely deviated. In the highest vacuum the field made no perceptible difference. Between the two experiments there were two slight differences of conditions: (1) In the second experiment the radio-active deposit had been heated in vacuo after removal of the emanation and disappearance of radium A in order to remove a possible overlying film of condensed gas. (2) In the first experiment the emanation had been left in the capillary 2 hours 25 minutes, in the second 1 hour 30 minutes, the volume occupied by the emanation being less in the latter case.

In a third experiment the heating of the radio-active surface was omitted, and the emanation was allowed to act for only 45 minutes. The result was unequivocally positive.

In a fourth experiment the film was heated, and the emanation left in 1 hour 20 minutes, reproducing prac tically the conditions of the second experiment. Again the result was positive, and the magnetic field produced no But this experiment appreciable effect in a high vacuum. was continued for nearly two hours after the start, and at the end of the time the radiation, although, of course, much enfeebled, was quite intense enough for the purpose. As time elapsed a change came over the experiment. Little by little, the rays began to be affected by the field. This change was hastened by heating the active film in place in the high vacuum. At the end the result was as unequivocally negative, all the rays being deviated by the field in the highest vacua, as at the start it had been positive.

The clue, I think, is the change of the glass surface of the capillary, which it experiences under the excessive bombardment to which it is exposed, and which is indicated by the blackening of the glass. In the lead glass used it was remarked independently that the darkening appeared to commence somewhat suddenly. At the conclusion of the experiment it was always marked. But on cutting down the capillary before the commencement in the three final experiments with relatively short exposure to the emanation the darkening 'had not commenced, whereas in the last experiment, when the pole pieces were removed to allow the deposit to be heated in place it was noted that the darkening had begun. It can be imagined that the slightest roughening of the surface is all that is necessary to cause a negative result. The whole series of experiments from start to finish is explained if accompanying the darkening of the glass there is also a slight roughening. Whether this will prove sufficient to be within the range of the microscope remains to be seen.

I hope to examine the hypothesis that the blackening of the glass is accompanied by the roughening of the sur face more in detail later. But whether this or some other explanation proves correct there can, I think, be no doubt about the conclusion that the a particle has been isolated under conditions in which it is not deviated by a magnetic field, and, therefore, is not charged. The theoretical conse quences of the discovery need not here be dealt with. Cer

a

THE important question whether there is any mechanical stress in an iron rod or ring when magnetised, and, if so, whether the stress is compressive or tensile, was discussed in NATURE ten years ago (vol. liii., pp. 269, 316, 365, 462, 533), but has not yet, so far as I know, received any generally accepted answer. That a magnetised rod must necessarily be in the same condition as if under mechanically applied compressive stress tending to shorten the iron, was, I believe, first suggested by myself (Phil. Trans., vol. clxxix., p. 216, 1888). Those who support this view generally speak of the stress as "Maxwell's stress," and assume its value to be B2/8. The stress in question seems, however, to be quite unconnected with the

stress

in the medium "proposed by Maxwell, and its value is not in general exactly B2/87, but (B2-H2)/8π. I have lately had occasion to consider the problem again, and perhaps I may be allowed to re-state my argument in a slightly altered form, and illustrate it by means of an imaginary model.

The

If a uniformly magnetised rod is divided transversely, and the cut faces are brought close together, the magnetic force inside the narrow gap will be B-H+4′′I. force acting on the magnetism of one of the faces, and urging this face towards the other, will be less than B by 271, the part of the total force due to the first face itself; hence the force per unit of area with which the faces would press against each other if in contact is P=(B-2I)I = 2πI2+HI = (B2-H3)/8. (In the case of an endless permanent magnet, H=o, and P= B2/8.) The width of the gap may be diminished until it is no greater than the distance between two neighbouring molecules, when it will cease to be distinguishable; but, assuming the molecular theory of magnetism to be true, the above statement will still hold good for the intermolecular gap. The same pressure P will be exerted across any imaginary section of a magnetised rod, the stress being sustained by the intermolecular springs, whatever their physical nature may be, to which the elasticity of the metal is due. The whole of the rod, therefore, will be subject to a compressive longitudinal stress P, the resulting contraction, expressed as a fraction of the original length, being P/M, where M is Young's modulus for the metal.

Let a magnetic molecule of iron be represented by a rigid steel sphere, uniformly magnetised and covered with a closely fitting shell of india-rubber, to play the part of the intermolecular springs." Imagine a straight row of these spheres in contact with one another, and kept in place by a force analogous to cohesion, which, while binding the spheres together, leaves them free to turn on their centres. This arrangement would, for present purposes, serve as a model of a filament of iron one molecule in diameter. If the magnetic axes of the spheres pointed indifferently in all directions, the attractions would be balanced by the repulsions, and the length of the filament would be the same as if the spheres were unmagnetised. If, however, the magnetic axis of every sphere pointed in the same direction along the filament, as would be the case when the filament was magnetised, the india-rubber between all the pairs of unlike poles would be compressed and the filament would be shortened. Let F be the compressive stress across the rubber between a single pair of poles, and s the amount, expressed as a fraction of a centimetre, by which the rubber is contracted; then, if there are n spheres, the total contraction will be ns (n being assumed so great that it is sensibly equal to n+1), which is the same as would be caused by an equal compressive stress F applied at the two ends of the unmagnetised filament. The whole filament when magnetised may therefore be regarded as under compressive stress due to the magnetic forces, and since Young's modulus M = Fl/ns, where 1 is the length of the unmagnetised filament, the contraction expressed as a fraction of the length is, as

originally stated, F/M, the value of F in an actual piece of iron being 2πI2+HI.

Sometimes there may presumably also be a longitudinal tension, as in the case of an iron rod placed along the lines of force in a uniform field, when the tension would be HI. In a ring electromagnet this would not exist.

As to what effect would be produced in magnetised iron by Maxwell's distribution of stress in the ether, I cannot venture an opinion. But if there is a tension, it can hardly have the familiar value B2/87, which is possible only when B is equal to H, and there is no magnetisation ("Electricity and Magnetism," § 643). My point is that an important component of the stress in magnetised iron is a compression which can be calculated and allowed for. The question whether or not this view is tenable is of the highest interest in connection with the possible correlation of magnetic phenomena, and urgently needs an answer. SHELFORD BIDWELL.

The Mixed Transformation of Lagrange's Equations. I SHOULD fancy from the review by "G. H. B." in NATURE of July 19 (p. 265) that the papers of Prof. Levi the mixed transformation of Civita relate largely to Lagrange's equations, the complete theory (Proc. Camb. Phil. Soc., vol. vi., p. 117; Hydrodynamics," vol. i., p. 171) of which was first given by myself so far back as 1887. But what I wish to point out is this, that this theory depends no any so-called theory of ignored coordinates (or kinosthenic coordinates Prof. J. J. Thomson [Phil. Trans., 1885, part ii.] calls them) than it does on the existence of the hypothetical personage known as the Man in the Moon.

as

more on

as

The theory is merely the result of a piece of elimination, and is follows:-Let the coordinates of a dynamical system be divided into two groups and x; let and be the momenta of types and x; and let T be the Lagrangean expression for the kinetic energy. Then it can be shown that