science is particularly disappointing. It consists almost wholly of disconnected excerpts from letters to or from correspondents, interesting enough in themselves, but embodying no connected review of his relations to science, and leaving the reader very much in the dark as to what these relations really were. The truth is that, what with politics on the one side and the management of his estates on the other, the Duke had but little time for other occupations. Science was to him not so much a serious study as a refreshing relaxation. Even had he undergone the training and possessed the special mental gifts which go to make the successful man of science, he could hardly have found room for their exercise in his busy life. His mind, however, was so active, that such intervals of leisure as he could secure sufficed to enable him to keep himself informed of what was being done in various important lines of investigation. And it was this course of interrupted reading and the thoughtful reflection that accompanied and followed it, rather than any original inquiry of his own, that blossomed out into the lectures, addresses, articles, and books which came in such a crowded procession from his pen. His death left a blank in society which has been filled by no one of his contemporaries. Few men of his class were endowed with so remarkable a mental versatility and took such an eager interest in all kinds of intellectual pursuits. He will be remembered as an illustrious example of a type too rare among us, wherein the grand seigneur, the statesman, the man of letters, and the lover of nature and of science are blended in one noble

character.

CHEMISTRY AND THE DETECTION OF CRIME.

Lehrbuch der gerichtlichen Chemie. Zweite gänzlich umgearbeitete Auflage, bearbeitet von Dr. Georg Baumert, Dr. M. Dennstedt, und Dr. F. Voigtländer. Vol. ii. Pp. x+248. (Brunswick : F. Vieweg and Son, 1906.) Price 9 marks. IN addition to cases of alleged poisoning, there

exist a number of crimes in the detection of which chemical and physical science can render special aid to the dispensation of justice. Thus, in

About three-fifths of the volume is devoted to the methods of discovering and demonstrating fraudulent alterations of documents. The treatment is very com plete, embracing as it does not only the microscopical examination of the written characters, the chemical testing of the ink and paper, and the indications of erased or altered letters brought out by photographic enlargement, but also the consideration of pencil marks and "secret" inks.

Some fifty pages are assigned to the examination of blood-stains, and include a careful description of the conditions which should be employed in carrying out the "biological" test for the characterisation of human blood. The authors think, in opposition to Uhlenhuth, that, given the requisite knowledge of bacteriology and physiology, the analyst rather than the medical man should be entrusted with this experiment. A good plate shows the absorption spectra of hæmoglobin and its congeners, and, indeed, a word of praise is due to the excellent photographic reproductions with which the book generally is furnished. Next follows a short chapter on the examination of suspected articles for the presence of human spermatozoa, whilst the last thirty pages deal with the evidence of incendiary origin which the chemist may find on closely scrutinising such objects as may have been left undestroyed where a fire has broken out.

Throughout the book careful directions are given for conducting the various operations, and numerous pitfalls which beset the unwary are indicated. As is befitting where serious charges are concerned, clear distinctions are drawn between the results which constitute proof and those which, however strongly confirmatory, are not in themselves decisive. The general impression left by a perusal of the volume is that in the solution of the crime-problems dealt with the guidance afforded is admirably practical and safe.

NERVOUS DISEASE.

C. S.

The Management of a Nerve Patient. By Dr. A. T. Schofield. Pp. ix + 267. (London: J. and A Churchill, 1906.) Price 5s. net.

detective as well.

WE cannot congratulate Dr. Schofield on the title he has selected, for a book written, as the author tells us, for the use of students and practitioners requires no such popular designation as "The Treatment of a Nerve Patient." Further, the writer does himself an injustice, for many medical men would not trouble to read a book the title of which suggests some words of advice for a nurse of layman.

Now we consider this little manual well worthy of a careful perusal, for although we do not agree by any means with all that the writer tells us, nevertheless it is a book full of valuable suggestions and advice. We agree with the statement that "many physicians do not sufficiently recognise the influence of mind over body," but Dr. Schofield, in his desire

to emphasise his point, is apt at times to state his case too strongly.

We cannot by taking thought dispel disease; the influence of the patient's mind over his body is powerful, but is it "almost all-powerful "? The consumptive patient is usually full of hopefulness to the last, but unless other means are taken to promote recovery his light-heartedness is of little avail. The writer later greatly modifies his original statement regarding the influence of mind over body by stating (p. 29) that "there are many diseases not cured by the mind alone"; in fact, he might add that quite few maladies can be so treated. Nevertheless, we agree with the statement that "in every case of disease the condition of the mind is an important

factor."

We cannot concur with the writer's distinction between "madness" and "hysteria" (p. 21). Hysteria is a disease with definite physical symptoms, and, in addition, the patient exhibits some mental aberration. Now if this mental disturbance becomes more marked, the patient is usually considered to have passed from the realm of physical disease into a state commonly spoken of as "madness," and yet the disease is the same in both cases, only in one instance the physical symptoms are the more prominent and in the other the psychical. Dr. Schofield writes that “a person whose conscious mind is unsound is suffering from madness: one whose unconscious mind alone has gone astray suffers from neuromimesis or hysteria; and the distinction is good." Now a few pages previously the writer tells us that "there is but one mind." Clearly, then, the mind is either sound or unsound, for the whole cannot be what a part of it is not. Further, we are told that "the recovery of the patient from disease depends more upon the efficiency of the vis medicatrix naturae, in other words, unconscious mind, than upon any other agent." Therefore it would appear that in hysteria the apparatus which is all-powerful in cure is itself diseased; thus if this statement is true it is a factor which must greatly influence the prospect of recovery.

Dr. Schofield speaks in no uncertain manner concerning the tendency of some persons of the present day to mix up a "very exaggerated psycho-therapy with a pseudo-Christianity." We entirely agree with his remarks, and consider that he has stated the case none too strongly. In the chapter entitled "The Diagnosis," we would specially commend to the student the advice the writer gives of the "importance of cultivating tact." There is probably no attribute of greater value to a physician, and no opportunity should be lost for developing it. The writer makes some very sound remarks concerning the personality of the "doctor." Some persons may consider that too much detail is given, and that some advice is almost too trivial to be recorded; but with this view we should disagree, for undoubtedly the strength of this book lies in the attention which is bestowed on detail.

This book supplies a want, and certainly deserves a place on the bookshelf of the young physician.

OUR BOOK SHELF.

Traité d'Exploitation commerciale des Bois. By A. Mathey. Tome i. Pp. xviii+488. (Paris: Lucien Lavens, 1906.) Price 15 francs.

THIS volume will rank high among the many excellent Continental books which deal with forest utilisation. The author gives a great amount of important and practical information concerning the commercial exploitation of timber from every possible point of view. The work is profusely illustrated by well drawn and excellently reproduced figures, numbering no fewer than 377, and to these must be added eight beautiful chromo-lithographic plates. The volume is divided into five parts.

Part i. deals with the general properties of wood. The anatomical features are also described. The macroscopic characteristics of the various home and exotic deciduous and coniferous species are gone into, and the diagnostic features are brought out very clearly by an excellent series of figures, which show specimen blocks of the various woods cut in transverse and longitudinal sections. The numerous chemical and physical properties of timber are treated in detail. This part finishes with an excellent account of the effect of soil and climate on the growth and texture of the wood.

Part ii. deals principally with defects in timber, such as abnormality of growth, knots and wounds of all kinds, which may be caused by physical agencies. The different kinds of rot arising from the attack of fungi are exhaustively dealt with. This part is extremely well illustrated by means of the coloured plates already referred to, which should greatly facilitate the recognition of these maladies that are only too frequently ignored in this country. The various forms of white and red rot being due to specific organisms greatly increases the danger of sound timber being contaminated by diseased timber; hence the importance of recognising those diseases in order, if possible, to prevent their future occurrence and spread.

Part iii. of the work deals fully with the important subject of seasoning and storing timber, and the different artificial methods of rendering wood antiseptic by means of immersion in, and injection with, the various kinds of preservatives. The artificial methods of seasoning and preserving timber are now receiving considerable attention as the price of wood increases and the supply diminishes, so that this part of the book should be of the greatest interest to all concerned in the production and use of wood.

In part iv. the felling and conversion of timber is adequately considered. The different instruments used are also fully described and figured. In the last part is given an exhaustive account of almost every possible means of timber carriage and transport. On the whole, the author is to be congratulated on the production of this excellent work. Illustrations of British Blood-sucking Flies. With Notes by Ernest Edward Austen, Assistant, Department of Zoology, British Museum (N.H.). Pp. 74: 34 plates. (Printed by Order of the Trustees, 1906.) Price 258.

GNATS and other blood-sucking flies have always been a great pest in most countries, but it is only within the last few years that their active agency in the dissemination of many of the most serious diseases which afflict both men and the higher animals has been fully recognised. In England, however, modern drainage and sanitary regulations have so far diminished their numbers that whenever gnats are exceptionally troublesome many people jump to the conclusion that there has been an invasion of mos

quitoes" (not knowing that the terms gnats and mosquitoes are applied indiscriminately to any biting species of Culicidae), and, what is more important, the gnats belonging to the genus Anopheles, though far from extinct in England, have ceased to disseminate ague as formerly.

Mr. Austen informs us that there are practically only six families of blood-sucking flies in England, Chironomidae (midges), Culicidæ (gnats or mosquitoes), Simulidæ, Tabanidæ (horse-flies), Muscidæ, and Hippoboscidæ. In Chironomidæ and Muscidæ the habit is exceptional, occurring in a few species only, and, except in the Muscidæ (and perhaps the Hippoboscidae), the habit is confined to the females. Mosquitoes, however, are also capable of subsisting on the juices of plants.

The illustrations in the present work are considerably enlarged, and with few exceptions represent only females. The originals have been prepared for exhibition in the north hall of the Natural History Museum. The letterpress consists of a brief general account of each family, and a notice of the chief characteristics, habits, and localities of the various species figured, technical descriptions, however, being omitted. Little has been done in England to popularise the study of Diptera, and there are very few illustrations of the species; so we welcome this excellently arranged and illustrated book as a useful contribution to our knowledge of the British Diptera. W. F. K.

Bielschowsky for the staining of neurofibrils. Doubtless these omissions will be rectified in a future edition. An excellent description is given of the microscopic characters of the various regions of the cerebral cortex, the basal ganglia, the cerebellum, pons, medulla, and spinal cord. The cranial nerves are discussed with remarkable clearness, the diagrams illus trating this part of the book being particularly good. concise account of the main Finally, there is a sensory, motor, and association systems of fibres in The book is well indexed. the central nervous organ.

Dr. Villiger is to be congratulated on having produced an excellent book. Not only does it amply fulfil its avowed scope of serving as an introductory guide to the student, but it will be read with pleasure and profit by many neurologists.

Naturkonstanten in alphabetischer Anordnung. By

Prof. Dr. H. Erdmann and Dr. P. Köthner. Pp. 192. (Berlin: Julius Springer, 1905.) Price

6 marks.

THIS handy little work is a book of constants intended for the use of chemists and physicists. It differs from others of its kind chiefly in the fact that the information in it is arranged alphabetically, with a marginal thumb index for rapid reference.

The work of the compilers has on the whole been very well done. Only one value of each constant is given, and usually no reference is made to the source or author. The work of the last ten years has, however, been incorporated to a much greater extent than is usual in books of this kind, and even data only published during the past twelve months are included. The plan adopted by the compilers should conduce to a considerable saving of time in looking up inform ation. We think the book should be of especial value to chemists, as the data necessary in quantitative analysis are dealt with in a specially complete manner. There are also tables giving for each element and its most important compounds the atomic or molecular weight, density, melting point, boiling point, thermochemical constants, &c., together with a five-figure

medical student, whether he be undergraduate or post-graduate, who is desirous of acquiring that thorough grasp of nervous anatomy on which the successful solution of diagnostic problems must of necessity depend. To such students as are able to read German we can cordially recommend Dr. Villiger's book. Within the compass of 177 pages the author discusses in lucid style the main facts of the morphology of the brain and spinal cord, and describes all the more important tracts of nerve-fibres. An excellent series of illustrations, many of them original, illuminate the text, whilst we are glad to observe that the author evidently describes the gross anatomy as if demonstrating the actual brain, using the diagrams as accessories. In this way the practical value of the book is undoubtedly enhanced.

Commencing with an account of the embryological development of the nervous system, the author proceeds to discuss in detail the naked-eye anatomy of the brain and spinal cord, with their surrounding membranes. An interesting historical account is given of the successive stages in the methods of neuro-histology, but we are surprised to find no reference to Marchi's well-known osmic acid method of staining recently-degenerated nerve-fibres, a method which since its introduction more than ten years ago has done more than any other to clear up our knowledge of nerve-tracts. Nor is any reference made to the still more recent methods of Cajal and of

to the most important spectroscopic features of each substance are given in a very handy form, the conditions as to the particular spectrum being clearly specified. Another very useful table containing data not often easily accessible is that of the electrochemical equivalents of the metals.

It is difficult in the time possible for a reviewer to spend on a book of this kind to detect many of the errors nearly inevitable in a first edition. The plan adopted by the writer has been to put the work for a while on his reference shelf, and turn to it frequently when looking up constants, verifying from other sources the data thus obtained.

Obvious slips are the value of 1, given on p. 114 ten times too small, the E.M. F. of the Clark cell, given on p. 40 as 0.60735 volt, and several misprints among the tables of English weights and measures, where the gallon is included under measures of surface.

Other inaccuracies are the value for the melting point of palladium, given as 1950° C. instead of 1525° C. 25, of nickel, given as 1500° C. instead of 1427° C., and of wrought iron, given as 1600° C. instead of 1500° C.

One rather unfortunate tendency of the work is to deal in a multiplicity of units. There is, for example no need to speak of "hektowatts," and it is certain that some of the subdivisions of the millimetre dealt with in the chapter on units are only confusing and rarely met with in practical work. Then, also, the units other than metric given in the book as at pre

sent in use in various countries are not always those ordinarily adopted. In Japan, for example, the present standard of mass is the "Kwan," prototypes of which were recently standardised at Sèvres. We can, however, cordially recommend the book, which should prove very useful. J. A. H.

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.]

Thermodynamic Reasoning.

Is the address delivered by Principal Griffiths at York, which is printed in your issue of August 9, I read: "Prof. Armstrong remarks that it is unfair to cloak the inquiry by restricting it to thermodynamic reasoning, a favourite manoeuvre with the mathematically minded.' He adds that such a course may satisfy the physicist but is repulsive to the chemist.' The inquiry, Why is the application of thermodynamic reasoning repulsive to the chemist?' naturally suggests itself."

This statement shows a strange misapprehension of my position. I have taken exception to the restriction of the inquiry to thermodynamic reasoning, not in any way to the mere application of thermodynamic reasoning. My objection was to formula worship. I still and shall ever object to it, for it is the bane of progress. As I said at York, physicists too nearly resemble the visitors to London who walk along the Strand and Shaftesbury Avenue and are content to look at the theatres from outside; they resemble those who admire the British Museum building but have no desire to examine the treasures within it.

If I did not misunderstand him, Mr. Whetham implied at York that it was enough for him that a certain thermodynamic expression was valid: what the condition termed osmotic pressure really is-whether a true pressure or whether, as I suggested, a negative pressure or thirstmattered not a jot. A certain mathematical thermodynamic picture being painted, no other artist need apply. This does not seem to me to be the attitude a scientific inquirer should adopt. Whether I represent the opinion of chemists matters little personally I am not willing to remain outside the Museum: I shall go inside, if possible, trusting that in some faint degree I may be able to appreciate the

wonders within it.

At present, progress is not a little hampered by the fact that chemists and physicists cannot wander through the museums of nature looking eye to eye in complete sympathy with one another: surely we are destined to be the closest of friends; more should be done to cultivate an understanding; a confusion of tongues has arisen which keeps us apart: we must both strive to speak a simpler language. Together

"Tet us inspect the lyre and weigh the stress
Of every chord and see what may be gain'd
By ear industrious and attention meet."

HENRY E. ARMSTRONG.

IT is the strength and weakness of thermodynamical reasoning that it connects different phenomena without the aid of theories about the mechanism by which the connection is effected.

In the discussion at York, Prof. Armstrong put forward certain arguments in favour of the view that solution is a chemical phenomenon, and osmotic pressure due to an attraction of the nature of chemical affinity. He used these arguments in an attempt to invalidate van 't Hoff's thermodynamic theory, which shows that, from the observed solubility phenomena of volatile substances, it follows that the ideal osmotic pressure of a number of particles of such substances in a dilute solution must be equivalent to the pressure which the same number of particles would exert as a gás occupying the same space.

In my reply to Prof. Armstrong I pointed out that the

thermodynamic theory is quite independent of the particular view we may adopt as to the fundamental nature of soluOsmotic tion, and the modus operandi of osmotic pressure. pressure may, as van 't Hoff himself supposed, be due to the impacts of the dissolved molecules; it may, as Prof. Armstrong believes, be caused by chemical affinity; it may The be produced by some other undiscovered cause. thermodynamic reasoning avoids all such hypotheses, and connects directly the experimental facts of the solubility of gases with the osmotic pressure they would exert against a perfect semipermeable membrane in dilute solution.

I have never suggested that the ultimate nature of solution was a matter of no interest. It is the question of most supreme importance now outstanding in these subjects; We must but let us clear the issue before attacking it. recognise clearly that the relations indicated by thermodynamics and confirmed abundantly by experiment are among the established facts to be explained by a theory of

the nature of solution.

It is for this recognition of the true position of the problem that I contend. The thermodynamic reasoning which connects the ideal osmotic pressure with experimental phenomena is not in question. That reasoning is confirmed by measurements of actual osmotic pressures and of freezing points. It can only be invalidated by a general attack on thermodynamic theory, such as that which foreshadowed in Mr. Campbell's recent reconnaisance-in-force. I do not think any such attack has much chance of success. Osmotic phenomena seem to me to be entrenched in the strongest part of the vast lines occupied by the science of thermodynamics.

was

Cannot Prof. Armstrong agree to accept the thermodynamic reasoning as confirmed by experiment, and pass on to the further problem? Personally, I think that the evidence at present available is on the whole in favour of the chemical theory of solution and osmotic pressure -the theory which Prof. Armstrong supports; but there is work to be done before such a conclusion can be taken as established. May we not agree that it is better both for physicists and chemists to do such work than to waste their energies in attacking with inadequate artillery the well-fortified citadel of thermodynamics?

W. C. D. WHETHAM.

High Borrans, Westmorland, August 21.

The Iron Arc.

WHILE carrying on some experiments with the electric arc between iron electrodes, one of my students, Mr. H. D. Arnold, noticed that there was a certain critical P.D. at which an abrupt change took place in the conditions of the arc. Subsequent investigation has shown that the effect " of the carbon is closely analogous to the "hissing point arc. How close the analogy is may be seen from the following remarks. If the iron arc is started with a large external resistance and maintained at such a length that the current is well below one ampere, it burns with little or no sound, and its appearance in the neighbourhood of the anode is very diffuse and ill-defined. As the external resistance is gradually decreased, the P.D. falls and the current rises until a certain critical value, depending on the length of arc and size of electrodes, is reached. At this point a very small decrease in external resistance suffices to cause a sudden increase in current and drop in At the same time P.D., precisely as with the carbon arc. the arc contracts, a bright spot appears on the anode, and a characteristic hissing sound begins. Further increase of current is accompanied by a continued decrease in P.D. The hissing stage, in fact, begins at quite a different point on the P.D.-current diagram from that in the case of the carbon arc. If the experiment is carried out in the reverse order, starting with a large current, the discontinuity is encountered again, but not until the current has been diminished beyond the value that it had at the beginning of the hissing stage. Indeed, with arcs of 6 mm. and more, the current on the hissing stage can with care be decreased until it is smaller than its previous largest value on the quiet stage. Thus there are two possible values of P.D. for the same current and length of arc, corresponding to the quiet, the other to the hissing stage.

one

How closely the physical cause of this discontinuity resembles that in the case of a carbon arc is still in doubt, though investigations bearing on this question are under way. With the iron arc there seems to be no sharply defined crater, for each electrode terminates in a viscous, incandescent globule of what seems to be magnetic oxide of iron, from which the discharge takes place. Thus we have to do, properly speaking, not with an arc between iron electrodes, but with one between electrodes of Fe,O,. Even when the arc is hissing strongly, the discharge seems to take place from only a small area on the surface of the globule. Moreover, a large increase in diameter of electrodes is accompanied by only a small increase in the value of the critical current, which varies between 0.8 ampere and 1.5 ampere over a wide range of values of length of arc and thickness of electrodes. On the other hand, I have found no positive evidence that the discontinuity is not due to the presence of oxygen around the anode. A test with an exploring electrode showed that the effect is confined mainly, if not entirely, to the anode. Given an are burning on the quiet stage in the neighbourhood of the hissing point, the hissing can be precipitated by shortening the arc, just as in the case of the carbon

arc.

After the current has been increased somewhat beyond the hissing point, the arc begins to rotate rapidly, so that on the anode a ring instead of a spot of light appears. This is accompanied by a high-pitched squeak or whistle, which, as the current is still further increased, degenerates into a sputter, and this in turn into a steady, strong hiss, the ring meanwhile having disappeared. At the beginning of the " whistling stage the arc has a curious tendency to jump back into the quiet stage, so that for an instant the hissing ceases, the current falls abruptly, and the P.D. rises several volts. If one begins to diminish the current immediately after one of these abrupt changes, the quiet stage can sometimes be maintained steadily, even though the current is far greater than that at which hissing normally occurs. It is not impossible that slight irregularities in the supply E.M.F. may in certain circumstances serve to precipitate the change from the one stage to the other, even though the current be not that at which the change normally takes place.

In conclusion, the question may be raised whether Lecher's observation of the discontinuous nature of the arc discharge between iron electrodes was not made on the hissing stage alone, and whether, as with the carbon arc, the discharge may not be perfectly continuous when the current is made sufficiently small. It is planned to repeat Lecher's experiment, making tests on both the quiet and the hissing stages of the iron arc. Middletown, Conn., August 9.

W. G. CADY.

Volcanoes and Radio-activity.

IN the Popular Science Monthly for June Major Dutton has an interesting article on the above subject, which was noticed in a recent issue of NATURE. Having been occupied lately with the study of volcanoes in connection with a more general inquiry into the cause of earthquakes, it occurs to me to point out that Major Dutton has overlooked the recognised distribution of volcanoes about the sea coast, which seems completely to invalidate his theory. If radium, which the researches of the Hon. R. J. Strutt have shown to be so abundant in typical rocks of the earth's crust, such as granite, were an exciting cause of volcanic activity, we should expect to find an abundance of active volcanoes in the interior of continents, such as the United States, Europe, Asia, Africa, Australia, and Brazil, which is contrary to observation.

T. J. J. SEE.

Naval Observatory, Mare Island, California, August 10.

The Radio-activity of the Chemical Elements. IN connection with the emission, from the radio-active elements, of corpuscles with velocities below the critical velocity necessary for the ionisation of gases, it has occurred to me that such a form of radiation is possibly a fairly general property of the chemical elements. It is, I think,

usually accepted that "y" radiation always accompanies. the projection of " B" particles, and the extreme penetration of the γ rays seems to be directly due to the very high velocity of the average "B" particle. As the efficiency of the X" rays is due to the sudden negative acceleration of the unit electrical charges (Le. the corpuscles) as they strike the anti-kathode, it appears quite possible that corpuscles, moving with comparatively low velocities, may yet be capable of causing a form of "y" radiation of feeble penetrating power. The fact that the kathode stream, which can hardly penetrate the glass of the tube, is still able to set up very penetrating X radiation when given a sudden negative acceleration by impact with the platinum anti-kathode may perhaps be given as an instance in support of this idea. It seems probable that the photographic action of a beam of corpuscles (deviated away from the "y" radiation by a magnetic field) may be chiefly due to a form of "y ray set up on contact with the plate itself. The several mysterious instances of the fogging of photographic plates left in certain conditions for considerable periods may be caused by a very feeble form of "y" radiation set up by the impact of slowmoving corpuscles on the surrounding matter. evidence of these slow-moving corpuscles may be somewhat meagre and doubtful, but I think that, so far as the ordinary chemical elements are concerned, the emission of such corpuscles may be very much greater than the measured activities would lead us to suppose. C. W. RAFFETY.

Streatham Common, August 25.

Such

THE OXIDATION OF ATMOSPHERIC NITROGEN IN THE ELECTRIC ARC.

N the year 1775 Priestley published his "Experiments and Observations on Various Kinds of Air." in which he showed that when a series of sparks was passed through air, the air became acid. The experiment was carried out by means of a glass tube, having one end closed with wax through which a wire was fixed, the open end being placed over a solution of blue litmus. Sparks were passed between the solution and the wire, and in a short time the blue litmus turned red. He further noticed the important fact that the water gradually rose up towards the wire. The observations of Priestley were shortly afterwards substantiated by Cavendish, and in 1893 Lord Rayleigh, with better apparatus and appliances, repeated the experiments which ultimately led him to the discovery of argon. Priestley attributed the acidity to the formation of carbon dioxide, but Cavendish, on repeating the work, proved it to be due to the forma tion of nitric and nitrous acids.

After the successful experimental work of Lord Rayleigh, attention was turned towards the production of nitric acid from atmospheric nitrogen. But it was undoubtedly due to Sir William Crookes, who as president of the British Association in 1898 directed attention to the gradual depletion of the world's stor of nitrogenous products, that the importance of the fixation of atmospheric nitrogen was recognised by the scientific and commercial world. At the present time about 15 million tons of Chili saltpetre are annually exported, but those who have studied the question consider that at this rate of exportation the Chilian beds will be, at the latest, depleted by 1940 But as the population of the world increases, the quantity of nitrogenous material required for fertilising purposes advances in equal ratio. Sir Willia Crookes pointed out in 1898 that the world's growth of wheat was about 163,000,000 acres, which at the average of 12.7 bushels per acre gave 2,070,000 009 bushels. "But thirty years hence the demand will be 3,260,000,000 bushels. . . . By increasing the present yield per acre to twenty bushels, we should with our present acreage secure a crop of the requisite