also given of a number of species either identified as British since the appearance of Verrall's "List," or which may ultimately prove to be so, with the result that "characters more or less satisfactory are given for 2526 species," with "localities for 626 of these " which Mr. Wingate has himself collected in the County of Durham during the last ten years. Turning to the actual contents of the volume before es, we find that, after a couple of pages on collecting and preserving, eighteen pages are devoted to a synopsis of the external morphology of Diptera in the form of a description of a "Fly Chart" (Plate i.) or diagram of a hypothetical Dipteron, so arranged as to display all or most of the characters used in descriptions. This is followed by an analytical table of families, which occupies ten pages, and the remainder of the book, with the exception of a few pages of addenda and indices, consists of tables for The determination of genera and species. In addition to the fly chart, which we think would have been clearer had the shading been omitted, characteristic structural details, such as antennæ, wings, legs, &c., are represented in the six following plates. So much care and thoroughness have evidently been expended upon this work that there is little room for criticism of any kind; a few minor emendations may, however, be pointed out. On p. 198 attention should have been directed to the vertical stripes of longer hair on the eyes of the common drone-fly (Eristalis tenax, L.), which are an easily recognised and distinctive feature of the species. Speiser's name 44 Varichata is used (pp. 212, 228) for the preoccupied Erigone. Rob.-Desv., instead of Ernestia, Rob.-Desv., which Bezzi has recently shown to be the correct designation. It should be noted that E. strenua, Mg., is a synonym of E. rudis, Fin. The life-history of Lipara lucens, Mg., is not unknown," as stated 00 p. 361; the larva mines in the heads of reeds. The Phorida (p. 383) are placed in their time-honoured position among the Cyclorrhapha, following the Borborida (as in Verrall's "List" of 1901), though, as shown by Osten Sacken, the true affinities of these very aberrant Diptera would appear to be with the Orthorrhapha, the more primitive of the two main divisions of the order. The bionomic notes on the strange-looking Hippoboscida (pp. 394-5) lack something in precision: Hippobosca equina, L., stated to be "parasitic on quadrupeds, especially horses," is a parasite of horses and cattle; Lipoptena cervi, L., also described as "parasitic on quadrupeds," is found on red and roe deer; Stenopteryx hirundinis, L., so far as the present writer is aware, is met with on young house-martins, not on young "swallows "; and Oxypterum pallidum, Leach, is a parasite of the swift, not of the swallow. As already stated, however, these are details of minor importance. By the publication of this work Mr. Wingate has earned the gratitude of all who are interested in British entomology, and it is to be hoped that, as the result of his labours, he may have the satisfaction of witnessing a considerable accession to the ranks of British dipterists. E. E. A. OUR BOOK shelf. Illogical Geology. The Weakest Point in the Evolution Theory. By George McCready Price. Pp. 96. (Los Angeles, California: The Modern Heretic Company, 1906.) Price 25 cents. THE author of this book proposes to collect the opinions of his readers, and a printed form is appended on which comments may be recorded. There is little doubt that a writer so much in earnest will make some modifications in his next edition as the result of friendly criticism. But geologists who have taken the pains to base their conclusions on hard work and study in the field, and not on the perusal of each other's text-books, will remain unsatisfied with Mr. Price until he also has undertaken a course of geological observation. In his introduction he offers a munificent sum to anyone who will "show [him] how to prove that one kind of fossil is older than another." It is not until we read his book that we perceive the intellectual difficulty of accepting this sporting proFor Mr. Price believes position. (p. 20) that geologists assume "that in the long ago there were no such things as zoological provinces and zones "; he believes that (p. 30) the inversion of stratified deposits is nowhere proved by physical evidence; that (p. 46) there are numerous families" of molluscs and brachiopods which disappear suddenly and completely with early Palæozoic times, and yet are found alive now in our modern world; and that (p. 68) the custom of classifying the Tertiary strata by the relative percentage of living and extinct forms that they contain is "utter nonsense." If Mr. Price would join one of the field-parties from some American university, he would soon find that his quarrel must be with natural phenomena, and not 66 imaginary hierarchy of illogical geologists. with G. A. J. C. an The Religion of Nature. By E. Kay Robinson. Pp. xii+215. (London: Hodder and Stoughton, n.d.) Price 3s. 6d. MR. ROBINSON has written a book that is sure to interest a large number of readers. His object is to show that there is no cruelty in nature, that animals are not self-conscious, and, therefore, that such pain as they feel is not pain of the kind that human beings are familiar with. It is only "the natural bodily protest of a living organism against injury "; and since there is for animals only this painless pain, and since their cruelty is not really cruel, there is in nature nothing antagonistic to the principles of revealed religion. No one who knows anything about animals can fail to realise that their capacity for suffering is much exaggerated by extreme humanitarians. A highlybred pigeon will undergo an operation for hernia apparently without feeling it, and directly it is over will begin quietly to eat his Indian corn. Animals do not suffer in anticipation, they do not brood over the past, and the actual torture, as everyone knows, is often far less than the picture of it that haunts the mind before and after. Let us hope, therefore, that all unreasonable humanitarians will read this book. In the opinion of the present writer, though Mr. Robinson fails to prove his main thesis, yet he makes it clear that the sufferings of animals as compared with those of men are as moonlight to sunlight. In fact, the human race has almost a monopoly of misery as distinguished from short spells of pain. We can only very briefly trace Mr. Robinson's line of argument. A sensitive plant behaves as if it had feeling, but, being a vegetable, it cannot feel. A sea anemone is an undoubted animal, yet it is not on a higher level than the sensitive plant; it only closes when touched. The argument here is a reductio ad absurdum. If you recognise consciousness in the lower animals you must also recognise it in plants. To which we may reply, "Why not?" They may have a very dim rudimentary consciousness. Deny consciousness to all animals except men, and you are confronted with greater difficulties than this. We all believe in evolution. Mr. Robinson himself accepts it. Consciousness must, therefore, have shown itself first in a rudimentary form. Evolution does nothing but develop the powers of which we find suggestions in the simplest forms of life. Nothing, therefore, can have its first beginning in the highest form of life; in the more intelligent animals it is difficult to believe that consciousness is entirely rudimentary. Mr. Romanes's monkey that got possession of the key of a chest and tried hard for two hours to unlock it must have had a fair allowance of self-consciousness. Dogs have dreams. Why may they not even when awake think over their recent fights and adventures? Animals learn by experience; pleasure and pain are the great educators for them as for us. Whittle their pleasure and pain down to nothing, eviscerate both the one and the other of all reality, and how could they steer their course amid the rocks and shoals of life? Mr. Robinson ends by making a damaging admission. Men, he says, can educate animals and elevate them; but education can only bring out what is already present. There is, therefore, in animals, whether wild or domesticated, something beyond that which Mr. Robinson would concede to them. Carboni fossili inglesi. By Dr. Guglielmo Gherardi. Pp. xvi +586. (Milan Ulrico Hoepli, 1906.) Price 5s. so, what are the advantages and disadvantages? Io those interested in these questions, especially the last this book is offered as an attempt to provide a so-|| tion. The primary means of identification are fur-d nished by the month in which the flowers appear and their colour, after which the size and other details or the flower are taken as the most general guide. With what results? Side by side in the same category are found the frogbit, Italian catchfly, floating waterplantain, common feverfew, and the large-flowered winter-green. This series was taken at random, bur is probably one of the most extreme instances. Does a non-technical method of discrimination compensate for this mingling of unallied plants? Will the learner be led to associate them together? WITE not considerable difficulties arise with regard to the exact size, also with regard to the colour? As for comparison, after becoming conversant with a flora according to one system it is not easy to gauge the merits of another; but to those wishing to identif plants, and to whom the natural system offers great difficulty, this book may be recommended as an earnest endeavour to provide a substitute. IN justice to our American colleagues, I think it nec sary to supplement the paragraph which appears in your Notes" of last week (p. 496), and is similar to a stat ment published in the Times of September 6, by the following explanation : When the international movement was inaugurated here last February at the meeting held at the Mansion House America was, of course, invited to participate in the schem which had been adopted at the public meeting, and for the carrying out of which a fund had to be raised. Ir response to our invitation, a meeting of American chemists was convened in New York and a number of proposals considered, the final outcome being the scheme set forth in the paragraph published in your issue of last (September 13). From this it would appear that t Americans had cut themselves adrift from the internationa movement, and had decided to have an independent celebr.tion in honour of Sir Wm. Perkin and his work. From their point of view-the promotion of chemistry in their own country-there was, of course, very good reason f the line of action which they had decided upon taking but it is also obvious that their scheme, as published, purely local, and that if this represented their whole prject there would have been every justification for the vie taken by our executive committee, that the American wished to detach themselves from the European movement It is necessary, therefore, to add that the scheme published last week does not represent the latest phase of the IT is undoubtedly advantageous "to see oursels as ithers see us," and the British coal-miner has an excellent opportunity of doing so by studying this attractive manual on British coals, cokes, and briquettes from the pen of an experienced Italian works-chemist. The book is divided into four parts. In the first the various coals are dealt with from a theoretical point of view, the methods of analysis, both rapid and exact, being described in detail. In the second part the British coals are described under the head of the various coalfields, numerous analyses being given, with full references to the sources from which they were obtained. In the third and fourth parts the manufacture of coke and of briquettes receives attention, the subject being one of special importance for the future development of Italian industries. The manual concludes with five maps of the coalfields, and with various appendices containing useful information for the coal merchant. The book, which is the first of its kind written in Italian, cannot fail to prove of value to Italian manufacturers desirous of securing the greatest heating effect with the least expense, by supplying them with accurate inform-American celebration. In addition to the objects set forth ation regarding the nature of the coals they purchase. It is written with care and impartiality in a concise and lucid style, and, like all the volumes of the wellknown series of " Manuali Hoepli," is produced in a tasteful manner at a modest price. How to Find and Name Wild Flowers. By Thomas DOES the natural system of botanical classification in that scheme, provision is made for receiving contribtions to the Perkin research fund, founded here as part our scheme, and loyally supported by all the contributing nations, the representatives of which at the last gener meeting of the executive committee on July 27 unreserved?t added their contributions to the general fund to administered through our Chemical Society. At that mo ing, as also at the general international meeting at the Royal Institution the previous day, the official representat ve of American chemists, Dr. Leo Baekeland, confirmed in public the cablegram which we had received from th American committee announcing its decision to cooperat with us and to contribute to our scheme. The American programme, therefore, is not so purel ocal as it appears when set forth without qualification. The objects which our Transatlantic colleagues are desirous of carrying out in the name of our distinguished Countryman will, I am sure, have the entire sympathy of all English chemists, and, for my own part, I can only wish that every success may attend their celebrations next month. At the same time, I may point out that research in this country is very poorly endowed as compared with the munificent foundations for this purpose established by wealthy Americans. We may, therefore, while wishing that the American jubilee will be productive of all the results which they themselves desire, appeal with confidence To Americans to support also that other part of their own scheme which provides for the endowment of our Perkin (search fund. R. MELDOLA. Blair Atholl, Perthshire, September 15. Horizontal Pendulums and Earthquake Echoes. I HAVE just finished reading Captain Dutton's book * Earthquakes in the Light of the New Seismology," the preface of which was written in April, 1904, while the date of publication is 1905. I mention these dates because a paper of mine, bearing particularly on chapter vii. of the book, was read before the Physical Society on March 13, 1903, and appeared 'subsequently in the Proc. Phys. Soc., vol. xviii.; it also was published in the Phil. Mag. for October, 1903. I gather, however, that Captain Dutton has not seen the paper, and as I am not certain of his exact address I am addressing you. To be candid, I say that Prof. Milne has not considered the effect of resonance sufficiently, if at all, in computing the tiltings represented by his seismograms. Certain screws are placed in the base-plate of a Milne seismograph partly for levelling purposes and partly to give the base a known tilt. Prof. Milne has argued thus (vide almost any British Association report): if a tilt of 1" given to the base displaces the end of the boom 1 mm., then a seismogram of 10 mm. amplitude indicates a tilt of 10", if it indicates a tilt at all. On this assumption Prof. Milne discusses the tilting theory and discards it (the theory), and also on this view Captain Dutton's seventh chapter is based. That it is a wrong view my diagrams as well as ordinary mathematics show. I I have also given a more prosaic explanation of Prof. Milne's "earthquake echoes" than is to be found in either the British Association reports or in Captain Dutton's book, pp. 235-6. The tilt represented by any seismogram is a function of the boom period, the wave period, the log. decrements of the free vibration of the boom, and of the earth wave, and is, in general, very much smaller than that which is given in the British Association reports. Earthquake echoes may be regarded as interference effects between the free and forced vibrations. notice that in the diagrams in my paper referred to the fifteen-second wave diagram which shows these interference effects best has been turned round with regard to the others. With the exception of this one, the artificial vibrations all start to the right, and are continued regularly until the end of the diagram is reached, when obviously the swing of the boom dies away naturally. C. COLERIDGE FARR. Canterbury College, Christchurch, New Zealand, July 21, Is a report to the British Association in 1899 I directed attention to the fact that as an earthquake dies its seismograms indicate that it does so in a series of more or less rhythmically decreasing impulses. These, I suggested, were more likely to result from reflection than from interference, and therefore they were provisionally called echoes. Four years later (Phil. Mag., October, 1903) Dr. C. Coleridge Farr discussed the terminal wave group as interference effects between the free period of the recording boom and the period of the ground. Theoretical considerations show their existence, whilst an ingenious experiment carried out by Dr. Farr shows that it is not difficult to reproduce wave groups strikingly similar to those shown in many seismograms. On the two sides of a pillar carrying a pendulum with a period of 16.5 seconds Dr. Farr attached two boxes filled with sawdust. Two chains connected by a rope passing over pulleys hung over the centre of each box. This arrangement was worked up and down at a fixed speed, so that while one side of the pillar was loaded the other was unloaded. By working this arrangement, tiltings were given to the column representing groups of waves with periods varying between twelve and twenty seconds. The resulting diagrams gave three results : (1) The amplitudes were greater than those due to steady loading. The inference is that a horizontal pendulum does not correctly measure the amplitudes of the greater portion of a teleseismic disturbance, a conclusion long recognised by most seismologists. Why, therefore, it may be asked, are columns of figures relating to amplitudes continually appearing in earthquake registers? One reason is that they roughly give relative magnitudes for movements recorded at widely separated stations, and are, therefore, of great value in determining origins. When expressed in angular measure, if the corresponding periods of movement exceed two minutes it is likely that the pendulum has closely followed the tilting which has been recorded. Other reasons may be adduced to indicate that "amplitudes have a value, and it is, therefore, desirable that they should be retained in registers. (2) Although the period of the forced vibration was varied in different experiments, the resultant diagrams showed that the pendulum followed the tiltings of the pier. The converse of this is found in the observation that pendulums with different periods will record the same periods for groups of waves in a given earthquake. (3) The diagrams showed marked interference effects. For example, forced vibrations of fifteen seconds acting on a pendulum with a period of 16.5 seconds yield a series of throbbings very similar in appearance to many seen in a seismogram. Although I entirely agree with Dr. Farr as to the existence of interference effects of this description, this by no means excludes the existence of reflection effects or echoes. To commence with, we will consider the first great echo, or Yuri Kaishi. The Yuri Kaishi, or return shaking, which frequently occurs about four minutes after the first shock, was recognised and christened long before the invention of modern seismographs. You feel it, you may see its effects, and, within a meizoseismic area, a seismograph is not required to give evidence of its existence. To regard it as an imaginary reinforcement of earth movement due to a want of synchronism in the period of the same and that of a horizontal pendulum is out of the question. The Yuri Kaishi rattles doors and windows, and causes people to leave their houses. My own view of the phenomena is that it is similar to what is seen when a bullet is dropped into the middle of a large tub of water. Waves travel outwards to the sides of the tub, where they are reflected, after which they converge at the centre from which they started. In nature, the reflecting surface may possibly be represented by the roots of mountain ranges. As these may be at varying distances from the origin of the disturbance, the reflections will give rise to complications at the focus. The transmitting medium I take to be that material beneath the heterogeneous superficial covering of our earth which transmits large waves with a constant velocity of about 3 km. per second. A megaseismic primary and its echoes may be transmitted to very long distances, and as they travel may by reflection at other surfaces be still farther broken up into minor wave groups. Two wave groups within a megaseismic area might at a great distance from an origin be represented by four groups, and so on. This matter has not yet been carefully looked into, but evidence exists that wave groups increase in their number with distance from an origin (British Association Report, 1897, p. 68). In support of the explanation I offer for the Yuri Kaishi it may be mentioned that the time interval between it and its primary is in those cases where we know the distance between an epifocal area and a supposed reflecting surface, such as might be anticipated. As an illustration of this I will take the Californian earthquake of April 18. This originated along lines of great length on the western side of that country. The reflecting surface I take to be the Sierras, 200 miles distant. A wave group would travel to the Sierras and back in about four minutes, and this is approximately the time interval between the two first large wave groups in the few seismograms I have of that disIn offering these suggestions I concur with Dr. Farr that very frequently terminal vibrations of an earthquake present characteristics suggestive of interference effects, but it is premature to suppose that all the peculiarities of a seismogram are to be explained in the same manner. Rhythmical beats at an origin may result in rhythmical beats at a distance. JOHN MILNE. turbance. Shide, Isle of Wight, September 5. Remarkable Rainbow Phenomena. ON Monday, September 3, a very heavy thunder-shower passed from west to east over the parish of Deerness, Orkney, from 5.30 p.m. to 6.25 p.m. When the dark nimbus cloud to the west had lifted its pall, the sun came out in great brilliancy. A rainbow now began to form to the north-east, but instead of the ordinary bow there was one of a bifurcated nature. Two stumps which coalesced on the horizon gradually developed into two magnificent bows, which met on both horizons, viz. north-east and south-west, but were about five or six degrees apart at the apex. All the colours of a radiant bow were present in both, and both had the colours arranged in the order of the primary bow. The secondary bow also appeared with the colours reversed and the same bifurcation, but in this case it extended only to about thirty or thirty-five degrees above the horizon, as secondaries generally do. As I had never seen or heard of anything like this, my first impulse was to find a cause. When the double rainbows were at their best, there was a bar of stratus cloud extending across the middle of the sun, and in breadth about one-sixth of its diameter. The two primary bows remained complete from 6.30 p.m. to 6.35 p.m., and without the arc of the apex for about another five minutes. At first the bow to the south was the more complete, and finally the one to the north. However, after the sun had crept from behind the bar of cloud there were still double stumps clearly visible. If the cause here attributed be correct, then the only explanation of the bifurcated rainbow being visible after the cloud passed is, that from the points of the heavens where the rainbows were the bar might still be dividing the sun's rays. Nothing in my meteorological books indicated that this phenomenon had been previously seen. On inquiring as to what others had seen after the thunder-shower, two friends, one five miles and the other three miles almost directly west of me, saw only a perfect bow and its secondary. Others nearer the position I occupied saw what they called four rainbows, but had observed neither the coloration nor bifurcation clearly enough for descriptive purposes. Deerness, Orkney. M. SPENCE. The Mixed Transformation of Lagrange's Equations. MR. A. B. BASSET, in a letter to NATURE of August 2, states that the theory of the mixed transformation was first given by himself in 1887, and refers to his treatise on Hydrodynamics, vol. i., p. 171. If he will kindly look at my essay for the Adam's prize, pp. 61-4, he will find an elimination similar to that which he speaks of. The resulting modified functions appear to agree term for term. There is also the introduction of a " modified function " by which we can use Lagrange's equations for some of the coordinates and Hamilton's equations for the others. That essay dates from December, 1876, and was published in August, 1877. The method was afterwards explained without much change in all the editions of my "Rigid Dynamics which follow that date, beginning with the fourth edition, 1882. E. J. ROUTH. September 14. THE THE RECENT CONTROVERSY ON RADIUM HE recent correspondence on the subject of radium, started in the Times by Lord Kelvir has, after lasting nearly a month and causi widespread interest, apparently closed without any very definite conclusion being reached. Whatev opinion may be formed of the merits of the o troversy, all must unite in admiration for the bot ness with which Lord Kelvin initiated his campaig ducted, almost single-handed, what appeared to many and the intellectual keenness with which he cofrom the first almost a forlorn hope against the transmutational and evolutionary doctrines framed a account for the properties of radium. The weight o years and the almost unanimous opinion of his younger colleagues against him have not deterred him from leading a lost cause, if not to a victorious termination, at least to one from which no one will grudge him the honours of war. If peace and tranquility now result, and a measure of agreement is arrived at between conflicting views, it will be a result which all concerned will heartily welcome The most ardent believer in the truth of the new doc trines cannot be other than satisfied that every featur and assumption that is admittedly speculative should be clearly recognised as such and separated from that which is not, if thereby the experimental foundations of the science of radio-activity are freed from further There seemn wordy and unprofitable controversy. now to be a reasonable prospect that this has been secured. Lord Kelvin's opening challenge (August 9) w broad and sweeping. He took exception to the statement, made by the writer in opening the discussion on the evolution of the elements at the British Association at York, that the production of helium. from radium has established the fact of the gradu evolution of one element into others, and denied that this discovery affected the atomic doctrine any mere than the original discovery of helium in cleveite. The obvious conclusion was that both cleveite and radium contained helium. He also stated that there was no experimental foundation for the hypothesis that the heat of the sun was due to radium, and ascribed it to gravitation. The challenge was taken up on the other side successively by Sir Oliver Lodge, the Hon. Mr. Strut and other well-known authorities, and it soon becam apparent that for argument at least Lord Kelvin or his side had to rely practically on himself alone Prof. Armstrong, it is true, immediately enrolled under Lord Kelvin's banner, and entered the lists whom, he declared, are strangely innocent workers with an embracing criticism of physicists in genera'. under the all-potent influence of formula and fashion. He made the statement that no one had handled radium in such quantity or in such manner that we can say precisely what it is, and throughout put the word radium in inverted commas. Whether or no his opponents are all as innocent and ignorant as Prof. Armstrong imagines, the fact remains that, except for this ex cathedra utterance and a leading article, argument against the accepted view there was little or none except that contributed by Lord Kelvin himself. Prof. Armstrong's letter merely served to provide Sir Oliver Lodge with justification for his favourite theme, which appears t be that whereas chemists have an instinct of their own for arriving at their results, reason is the monopoly of the physicist, whose results the chemist usually manages to absorb in the end. No better argument against the unfairness of this could be provided than by the history of radio-activity itself, which wes at least as much to the chemist as to the physicist. Prof. Armstrong is almost alone among hemists, as Lord Kelvin is among physicists, in his hostility to the new doctrines. Mr. Strutt in two letters (August 9 and 21) asked what became of the heat generated by the radium admitted to be present in the earth, and recalled the independent evidence of several workers of the con inuous renewal of helium from radium. Sir Oliver Lodge directed attention to the magnetic deflection of the a-particle as evidence that material particles ire expelled from radium, and in his letter laid rhaps undue weight on the evidence, which is still far from complete, that the a-particle is an atom of helium. The vagueness of this argument, and the Lict that the letter raised a doubt whether Lord Kelvin had sufficiently examined the published evidence, a doubt which Lord Kelvin himself promptly dispelled, was the subject of a leading article in the Times of August 18. The writer of that article attacked the evidence for the production of helium from radium, using some well-known arguments. The minute quantity of emanation was graphically likened to a bubble rising through a glass of whiskey and soda, and it was held that the results were vitiated by the well-known changes the spectra of gases undergo under the prolonged action of the current, due to occlusion by the electrodes and selective conduction rather than to any transmutation. It may be here remarked that the same arguments were set forth in full by Himstedt and Meyer as a preliminary to their experimental examination of the question, yet Himstedt and Meyer, as the result of their own experiments, were finally forced to the conclusion that helium is in fact produced from radium. Lord Kelvin in his replies (August 20 and 24) made it clear that he accepted as a fact the continuous evolution of helium from radium, and this admission narrowed very much the issue involved. In reply to a statement of Strutt that if all the helium is removed from radium after an interval a further supply can be extracted, Lord Kelvin remarked simply that the if of the statement was wrong. This point was dealt with by the present writer (August 31), who considered the argument could be definitely answered. For helium is produced from the emanation of radium, about which no question of its being really reproduced can exist. For the removal of the emanation is marked by changes in the radio-activity, notably by the B rays, which vanish when the emanation is removed. The recovery of the radio-activity occurs at a definite rate, and is concomitant to the reproduction of emanation, which can at any time be again extracted as before. As there is no question of the radium creating helium, the only point open for argument is the exact character of the decomposition by which it and the emanation which gives rise to it are formed. As there was no further reply to this criticism, it may be taken that the main point of the disintegration theory, that there is a continuous change in the radio-active matter accompanying the radio-activity, is unanswerable. On the important question as to the character of the decomposition by which the helium is formed, Lord Kelvin in his later letters favoured a view very different from that of mere occlusion, which the original analogy to cleveite suggested. He quoted a statement of Prof. Rutherford in favour of regarding radium as a chemical compound of helium and other elements, and suggested that radium might be made up of one atom of (?) lead and four of helium. In a final letter (September 4) Sir Oliver Lodge pointed out that this was the key of the position. Is radium a compound or an element? It is satisfactory that, after so much fencing with the question, so simple an alternative has been arrived at. Perhaps the most significant thing about the view that radium is a compound is the silence of the chemists. Surely a chemist might fairly be supposed to know whether a change is what is called a chemical change or not, and the fact that it has been left to a physicist to adopt this view seems fair comment. Not even Prof. Armstrong has yet accepted it. On the second point of his challenge, the denial that the heat of the earth is due to radium, Lord Kelvin naturally had an easier task, for matters connected with the interior of the earth must necessarily remain speculative. If radium did not decompose under the conditions prevailing in the interior it would emit no heat, and would not tend to diminish in quantity, accounting perhaps, although with some difficulty in view of the wide distribution of radium in surface rocks, for the continued existence of the substance at the present time. Mr. A. S. Eve, in a vigorous letter (August 28), stated that he had confirmed the estimate of Mr. Strutt of the amount of radium in the earth's crust by a new method, in which the penetrating radiation from the earth's surface was used as the basis of measurement. Although, of course, in view of the evidence of the independence of radio-active changes upon their environment, it is more of an assumption to suppose that in the interior of the earth radium does not decompose than to take the opposite view, yet clearly here, at any rate, there is plenty of room for legitimate differences of opinion. On the other hand, even the opponents of Mr. Strutt's view cannot deny the potentialities of radio-activity, and the part it might play in cosmical processes under favourable conditions. The theory that radium is a compound, waiving the qualification chemical, will no doubt serve sufficiently well for the present as a point of common agreement. As Sir Oliver Lodge remarked, there is no necessity that the question be settled offhand. As a stepping-stone to further conclusions, it offers advantages to the conservative and cautious. It expresses a bare minimum of established fact which even the most sceptical are unable to invalidate. This minimum, briefly stated, is that radium is undergoing a continuous change intimately connected with its radio-activity, and that in this change helium is produced, and an enormous but definite amount of energy liberated. Whether anything more is known about transmutation now than formerly, whether lead could change into gold or gold into silver with an emission of energy similar to that evolved from radium, whether this or similar energy plays the large share that has been attributed to it in cosmical processes, are questions which may be legitimately discussed and left open, if only for the reason that they are far from decided. They are all admittedly steps into the region of hypothesis. But what a miserable fraction, even of the known facts, this minimum is! Ostensibly an explanation of radio-activity, it begins and ends with the fact of the gradual evolution of helium from radium. The numerous other products of radium, the volatile emanation and its non-volatile products, known by their characteristic radio-activity, much as minute quantities of ordinary gases and solids are known by their characteristic spectra, the slower changing later products, of which polonium is one, and is chemically as reminiscent of tellurium as its parent is of barium, remain still to be systematically accounted for. On the important subject of the nature of the a, 8, and y rays, and their origin, the view is silent! The fact |