technic a higher department, providing for the more specialized wants of each locality. This will be a work which no body is so well fitted to undertake as the great Institute which has been a pioneer in higher technical instruction. Such, it appears to us, is the true solution of the question of the relations between the Charity Commissioners' scheme and the City and Guilds of London. One word of caution in conclusion. The new institutes should be allowed to grow, and not be started on too ambitious a scale at first. Local wants change, and the institutes should develop in harmony with their changes. This is the lesson of the old Mechanics' Institutes and Athenæums. The lesson is repeated in the newer experiments of Mr. Hogg's Polytechnic, and the People's Palace. We do not want to begin with erecting huge shells of bricks and mortar, hoping that life will somehow come into them after a time. The life first, then the buildings, to grow as it expands and deepens-that surely is the law of nature. "Several architectural white elephants" is the dismal but suggestive forecast of a writer in the Charity Organization Review, on the supposition that this law is violated. If these warnings are neglected, the promoters of the movement will be merely courting failure, however good their intentions may be. And they will have failed because "they were not poets enough to understand that life develops from within.” ASSAYING. Text-book of Assaying. By C. Beringer and J. J. Beringer. (London: Griffin and Co., 1889.) THI HIS text-book marks an important departure in the literature of assaying. The authors abandon the dreary details of traditionary methods, and attempt with success to rationalize the art of the assayer, rather than to follow the usual course of reproducing "dry" assay methods and elaborate classifications of processes the interest of which is only historical. Assaying is here treated, in a broad sense, as the determination, by analytical methods, of components of ores and of intermediate or finished metallurgical products. Such compounds may be either of value in themselves, or important from being valuable or injurious in the operations of smelting, or in adapting the metals for use. The methods of the authors, and the measure of success which they have attained, may be fairly judged by their treatment of copper, lead, and iron. Copper ores and furnace materials are still sold in the English market by the "Cornish" assay. This antiquated method of assaying has really no claim to retention, now that more trustworthy methods are well known, and the authors give it but little prominence. They, however, repeat the fallacious argument of its apologists by stating that "it gives the purchaser an idea of the quantity and quality of the metal that can be got by smelting." The Cornish assay does not deserve even this modified approval, as the results it affords neither represent the actual amount of copper contained in the ore, nor the proportion of metal which can be produced by smelting, and several expert assayers, working on portions of the same samples, will obtain results which vary in the most erratic way. Fortunately for those who may be guided by this text-book, its authors proceed to describe assaying processes which are really well calculated to give trustworthy indications as to the quantity and quality of metal obtainable from ores. These are to be found in well proved "wet" methods of determining actual copper contained in ores as well as the components that interfere with the extraction and the quality of the metal. In describing these methods, ample information is given for the guidance of the smelter under the varying conditions of the metal's occurrence. While passing shortly over the Cornish assay, the authors judiciously omit such clumsy "wet" methods of assay as the direct titration by cyanide of potassium, which is retained in some recent books of standing, although it has been abandoned by most skilful assayers. On the other hand, titration by cyanide of potassium after separation of the copper from interfering metals, and the assay by electrolysis, leave little to be desired in rapidity and accuracy, and to.these due prominence is given. Failing reasonable manipulative skill, no assay can be accurate, and the expertness demanded by those who conduct the "dry" or Cornish assay is not more easily acquired than is the analytical skill needed for better "wet" methods. In an assay method giving accurately the amount of metal actually present in the ore, the metallurgist has a sure basis for calculation, the results of which can be brought under the control of his experience as to the losses of metal in operations on a large scale. The results of the Cornish assay, with all its inherent uncertainty, have equally to be judged in the light of the smelter's experience as to what the final "out-turn" will be. In lead, again, the dry assay is usually treated in books on assaying with much elaboration, which is no longer useful, if it ever was. It gives results that indicate neither the actual amount of metal contained in the ore, nor the amount which will be produced by smelting, and like the Cornish assay for copper is most unsatisfactory for guidance in smelting. The wet methods of lead assaying which are described are convenient and trustworthy, while the only practically useful methods of dry lead assay are given in sufficient detail. In the assay of iron ores we find dry methods entirely omitted. The wisdom of this cannot be doubted, for the want of exactitude which is characteristic of the dry assay of copper and lead is still more marked in the dry assay of iron. Processes of wet assay capable of giving prompt and strictly accurate results are available, and these are fully described. The plan of subordinating or ignoring unsatisfactory methods of assay, while giving prominence to those which have proved to be trustworthy, runs through the treatment of methods of assaying the other metals, as well as estimating the components of ores which are not usually dealt with in books on assaying. Among the latter are silica, the earths, sulphur, arsenic, and phosphorus. These demand study by the metallurgist, to whom, under either the necessity of "fluxing" them away, or of minimizing their interference with the purity of the metals, their ready and accurate determination is a matter of the greatest importance. The details of assaying the precious metals, though hardly sufficient for adoption in the assay of bullion in a mint, are all that is needed in a works. The authors have clearly not been content to merely record published processes, but in order to add to the completeness of their work have given unpublished results of the experience acquired by themselves and BREWING MICROSCOPY. The Microscope in the Brewery and Malt House. By THE HERE are certainly few industries the growth and development of which have been more influenced by the progress of pure scientific discovery than those of the brewer and distiller. These industries, formerly carried on upon purely empirical lines, handed down from father to son through countless generations, have in recent years, through the advances in chemical and biological science, been so transformed that their successful conduct at the present time requires a most thorough acquaintance with the leading principles of these sciences. As a consequence of this change, we find an increasing tendency for these industries to become concentrated in a smaller number of hands each producing on a larger and larger scale. The small brewer himself lacking the necessary scientific training, and not able to afford the requisite skilled assistance, gives way before the larger breweries employing a complete scientific staff and provided with the latest improvements. The present work is, we understand, intended to bring before those connected with brewing a concise account of the assistance which may be derived in the conduct of their business from the use of the microscope. We are of opinion that the authors have been unfortunate already in the choice of their title, as one of the most conspicuous results of modern scientific research in this direction is that the use of the microscope alone is of comparatively little value in the study of micro-organisms in general, whether connected with fermentation or other processes. This inadequacy of microscopic study per se the authors in various parts of their work indeed frankly admit. Modern | students of these low forms of life have, in fact, become more and more aware of the fallacious results yielded by mere microscopical observation when unaccompanied and uncontrolled by those processes of cultivation which have been developed during the past ten years. Even the work performed under the auspices of the masterly genius and supreme experimental skill of Pasteur has had to be revised and brought up to date by Hansen, with the aid of the more recent methods of research. Now, although the authors appear fully aware of the great change which has taken place since the earlier work of Pasteur, Reess, Fitz, and others, they have not sufficiently distinguished between observations which rest upon the surest foundation and fulfilling the most modern requirements, and those which, though possibly correct, require repetition and confirmation. The absence of sharp differentiation in this matter cannot fail, we believe, to occasion much confusion in the mind of the ordinary practical student who depends upon text-books and manuals for his guidance and information, and it is, in our opinion, quite unnecessary that he should be burdened with the microscopic descriptions of the various forms of yeast given by the older observers, who were almost certainly dealing with impure cultures, but on the contrary he should rather devote his whole attention to the characters of such undoubtedly pure forms of yeast as have been obtained by the most recent methods. Moreover, unless the necessity of resorting to these cultivation experiments for obtaining accurate information is duly impressed upon the student, he will naturally be inclined to shirk these far more laborious and difficult observations, and place undue reliance upon microscopic features. These remarks apply, perhaps, with even greater force to the manner in which the authors have dealt with the schizomycetes; in this part of the book we find much space devoted to microscopic descriptions of bacteria of uncertain purity, whilst there is little or nothing said about the methods by which these organisms can be really identified, and their characters defined. We also miss any adequate account of the staining-processes which are so invaluable in obtaining a correct idea of the microscopic forms and dimensions of bacteria. As an instance of the unsatisfactory present condition of brewing microscopy, we may quote the following sentence: "Bact. lactis, as seen in beers, is generally in the form of small rods, 2 to 3 μ in length, and sometimes in threads containing from 2 to 5 individuals; it is not certain, however, that this form is B. lactis." Thus, in respect of the bacterium which is perhaps of most consequence to the brewer, as being "the most commonly occurring disease-organism encountered in the brewing process' there is this absolute lack of all precise information. What may be called the more purely scientific part of the work is succeeded by a chapter of "general remarks on the brewing process," which, embodying as it does some of the practical experience of the authors themselves, we would have gladly seen enlarged. The book, which is printed on excellent paper and elegantly got up, is illustrated with a number of admirably executed plates, many of the best of which are original. A full index and glossary are appended. THIS is a capital first book of botany, intended for small children. The style, however, is really more elementary than the matter, and a child who has mastered this book will have made a very good start in the science. There is a good deal of information given about the internal structure and function, as well as the external form, of the organs of plants, and this information is given correctly, as well as clearly. The book is illustrated by 177 woodcuts, most of which are well suited to their purpose. D. H. S. Five Months Fine Weather in Canada, Western U.S., and Mexico. By Mrs. E. H. Carbutt. (London: Sampson Low and Co., 1889.) IN this book Mrs. Carbutt records her experiences during a remarkably pleasant journey made by herself and her husband in the New World. The scenes she describes have often been described before, but she writes so brightly about what she saw that even readers to whom she has nothing new to tell will find a good deal to interest them in her narrative. They will be particularly pleased with her account of " sunny Mexico, and its merry, courteous people." 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 Duke of Argyll and the Neo-Darwinians. IT has a curious and not uninstructive effect to see the pages of this journal invaded by the methods of discussion which are characteristic of political warfare. The letter of the Duke of Argyll, published in NATURE for December 26, 1889 (p. 173) is a clever debating speech. But it rather obscures than illuminates the questions really at issue. And, after the fashion of the political orator, it attributes to those who disagree with the writer motives which, in so far as they differ from reasoned conviction, are essentially insincere. In politics, the personal rivalry which is bound up inextricably with the solution of great problems may make it a necessary part of the game to endeavour to belittle one's opponents. But in science it is not so. The newer problems which have been raised by Darwinism depend for their solution upon the discussion of evidence, and no competent biologists will, in the long run, be influenced in the opinions they form about them by anything else. There is nothing in the Duke's letter which has not been worn threadbare by discussion. Still, there are, no doubt, many readers of NATURE who, while taking a general interest in the matter, have not followed all that has been written about it. I am disposed to think, therefore, that it may not be without its use to go over the ground which the letter covers. Now we First, as to acquired characters. Let us take a simple case. It is admitted that a blacksmith, by the constant use of his arms, may stimulate their abnormal muscular development; that is an acquired character. But a working man, whose arms are of perfectly average dimensions, may nevertheless have a son with arms which would seem to mark him out for the blacksmith's profession; that would be a congenital variation. know that a congenital variation is likely to be inherited; that is a matter of observation. What is the case as to the acquired character? The answer must be, I take it, that there is no probability that the arms of a blacksmith's son will differ in any respect from those of the average inhabitant in the locality where he was born. The Duke of Argyll, however, suggests that there is "no necessary antagonism between congenital variation and the transmission of acquired characters." This is perfectly reasonable; theoretically, there is none. But this does not make the transmission of acquired characters less doubtful. The Duke has no doubt about it, however. "So far from its being unproved, it is consistent with all observation and all experience. It lies at the foundation of all organic development." Very possibly, but where is the observation and where is the experience? These are the biological desiderata of the day. Imagine the fate at the Duke's hands of any scientific writer who put forward statements such as these unsupported by a shred of a fact. "This being so," however, the question then arises, Why do extreme Darwinians so fiercely oppose the idea of the transmission of acquired characters? Well, it is obvious that they do so because they think the evidence in its favour insufficient, and it is clearly the duty of a scientific man, whether an extreme Darwinian or not, to oppose the acceptance of that which experience does not support. But the Duke of Argyll attributes their opposition to two causes: first, jealousy of associating the names of Lamarck and Darwin; and, secondly, the dethronement of their idol Fortuity. The first of these reasons is almost too preposterous to discuss. No serious naturalist would speak with other than respect of Lamarck's position in scientific history; this cannot be effaced however much that of Darwin may be magnified. And no serious naturalist would adhere to any theory Darwin had propounded a moment longer than the evidence seemed to carry conviction. The charge in this particular matter is, however, the more grotesque, because, although Darwin did not esteem as of much value Lamarck's doctrine of development and progression, we know that his own mind became more and more fluid on the question of the "direct action of conditions." The idea is in fact so plausible that the difficulty is not in accepting it, but in shaking oneself free from it. What were probably the last words which Darwin wrote on the subject are contained in a letter to Prof. Semper, dated July 19, 1881. I quote a passage which appears to me to pretty accurately define the present position of the question :-- It "No doubt I originally attributed too little weight to the direct action of conditions, but Hoffmann's paper has staggered me. Perhaps hundreds of generations of exposure are necessary. is a most perplexing subject. I wish I was not so old, and had more strength, for I see lines of research to follow. Hoffmann even doubts whether plants vary more under cultivation than in their native home and under their natural conditions ("Life and Letters," vol. iii. p. 345). Darwin's difficulty, in point of fact, was exactly that of everyone else. The evidence, instead of being "consistent with all observation and all experience," failed to be forthcoming. And as The second reason is equally baseless. Fortuity is no idol of the neo-Darwinians; if it is an idol at all, it is an "idol of the market," imposed upon their understanding by the Duke. But at any rate he does not attribute any blame to Darwin. this is a rather important matter, on which I admit that persons who ought to know better have gone astray, I will quote a passage on the subject from Prof. Huxley's admirable biography (Proc. Roy. Soc., No. 269) : "Those, again, who compare the operation of the natural causes which bring about variation and selection with what they are pleased to call 'chance,' can hardly have read the opening paragraph of the fifth chapter of the 'Origin' (ed. 1, p. 131): 'I have sometimes spoken as if the variations had been due to chance. This is of course a wholly incorrect expression, but it seems to acknowledge plainly our ignorance of the cause of each particular variation."" It is obvious that the use of accidental in the guarded sense in which it is employed by Darwin is widely different from fortuitous as employed by the Duke of Argyll. Darwin took variation as a fact of experience. Its causes and laws have still to be worked out. One of the latter, due to Quetelet, was explained by Prof. George Darwin in this journal (vol. viii., 1873, p. 505). He says: "One may assume, with come confidence, that under normal conditions, the variation of any organ in the same species may be symmetrically grouped about a centre of greatest density.' And this is quite in accord with the remark of Weismann that variability is not something independent of and in some way added to the organism, but is a mere expression for the fluctuations in its type. Variation is therefore not unlimited, and we must admit with Weismann that its limits are determined by "the underlying physical nature of the organism;" or as he again puts it, "under the most favourable circumstances a bird 66 can never be transformed into a mammal." There is something more therefore than blind chance at work here. But within the limits, it is a matter of experience that every possible variation may occur. If anyone will take the trouble to examine the leaves of the ribbon-grass so commonly cultivated in gardens, he will find it impossible to obtain any pair in which the green and white striping is exactly alike. If it were possible to raise to maturity all the progeny of some prolific organism, the same diversity (in different degree, of course) would manifest itself; but the whole group of variations in respect of any one organ would obey Quetelet's law. When we attempt to give some physical explanation of this fact, we know from the objective facts which have been made out about fertilization that, although the protoplasmic content of the fertilized ovum is, in a general sense, uniform, its actual structure and physiological components must be combined in as endless variety as the green and white stripes of the leaves of the ribbon-grass. If, with Prof. Lankester, we say that the combinations are kaleidoscopic, I do not see that we go beyond the facts. And it appears to me quite permissible to correlate the ascertained variable constitution of the ovum arising from this cause with the equally ascertained varying structure of the organism developed from it. Of the varied progeny, we know that some survive and others do not. And what Darwin has taught us is, that the reason of survival is the possession of favourable variations. The surviving race necessarily differs somewhat from its progenitors, and Darwin has further stated that it is probable that by the continued repetition of the process all the diversity of organic nature has been brought about. The area of fortuity is narrowed down therefore, on this point of view, to the variable constitution of the individual ovum. And it is upon the recognition of this fact, for which there seems to be good scientific evidence, that the Duke of Argyll founds his charge that the neo-Darwinians make fortuity their idol. The reason appears to be that it comes into collision with teleological views. But such collisions are no new event in the history of the biological sciences. And teleology, like a wise damsel, has generally, though temporarily ruffled, managed to gather up her skirts with dignity and make the best of it. For some element of fortuity is inseparable from life as we see it. It is at the bottom one of the most pathetic things about it. Nowhere is this more vividly portrayed perhaps than by Addison in the "Vision of Mirzah.' Yet I do not remember that anyone was ever so unwise as to taunt Addison with making fortuity his idol. But, philosophically considered, what is gained by this tenacity about out-works? I reply, exactly as much as was gained by the tenacity of the Church in respect to the geocentric theory of the planetary system. Scientific men cannot be stopped in the application of their best ability to the investigation of Nature. If their conclusions are false, they will detect the falsity; if true, they will not be deterred from accepting them by some a priori conception of the order of the universe. It is not justifiable to say that this is due to any devotion to such an empty abstraction as fortuity. No scientific man is, I hope, so foolish as to suppose that, however completely mechanical may be his conception of Nature, he is in any way competent to account for its existence. The real problem of all is only pushed further back. And the Duke of Argyil's difficulty resolves itself into the old question, whether it is more orthodox to conceive of the universe as an¦ automatically self-regulating machine, or as one which requires tinkering at every moment of its action. It may be replied that this is all very well, but that it is not the way the neo-Darwinians state their case. I may be, therefore, excused for quoting some passages to the contrary from Weismann's "Studies in the Theory of Descent" : "This conception represents very precisely the well-known decision of Kant: Since we cannot in any case know a priori to what extent the mechanism of Nature serves as a means to every final purpose in the latter, or how far the mechanical explanation possible to us reaches,' natural science must everywhere press the attempt at mechanical explanation as far as possible" (p. 638). Further, he quotes from Karl Ernst von Baer : "The naturalist must always commence with details, and may then afterwards ask whether the totality of details leads him to a general and final basis of intentional design" (p. 639). Again, he says: "We now believe that organic nature must be conceived as mechanical. But does it thereby follow that we must totally deny a final universal cause? Certainly not; it would be a great delusion if anyone were to believe that he had arrived at a comprehension of the universe by tracing the phenomena of Nature to mechanical principles" (p. 710). In truth, this revolt of teleology against Darwinism is a little ungrateful. For, if Darwinism has done anything, it has carried on and indefinitely extended its work. In the last century, teleology was, it seems to me, a valuable motive-power in biological research. Such a book as Derham's "Physico-Theology" (1711) may be read with interest even now. I well remember that my first ideas of adaptive structures were obtained from the pages of Paley. Thirty years ago I do not know, except from them and the notes to Darwin's "Botanic Garden," where such information was to be otained. The basis of research was, however, too narrow to continue; it did not look beyond the welfare of the individual. The more subtle and recondite springs of adaptation opened up by the researches of Darwin, which look to the welfare of the race, were not within its purview. Conse quently it dried up, and virtually expired with the Bridgewater Treatises. To return, however, to the Duke of Argyll. "Neither mechanical aggregation, nor mechanical segregation, can possibly account for the building up of organic tissues.’ Who has said they did? The Duke has entirely misunderstood the matter. Prof. Lankester never suggested that it was possible to put so much protoplasm into a vessel, and shake out a cockatoo or a guinea-pig at choice. His image of the kaleidoscope had nothing to do with the building up of organisms, only with the varied combination of the elements known to take part in the formation of the fertilized ova from which organisms originate. I am not sure that I perfectly comprehend what follows. Perhaps some further emendation than that already published is needed in one of the sentences. But it seems evident that the Duke is re-stating his old doctrine of "prophetic germs." He has already defined what he means by these (NATURE, vol. xxxviii. p. 564). "All organs," he says, "do actually pass through rudimentary stages in which actual use is impossible." Here, again, as in the case of the transmission of acquired characters, what one wants is not a reiteration of the assertion, but some definite observed evidence. For the production of this, if only in a single instance, Prof. Lankester pressed the Duke more than a year ago (NATURE, .c. p. 588). None, however, has as yet been forthcoming; and it appears to me that it is not permissible to persist in statements for which he does not attempt to offer a shadow of proof. The Duke exults in a very amazing fashion over what he strangely calls Prof. Lankester's admission that "natural selection cannot account for the pre-existence of the structures which are prescribed for its choice." I am afraid I have already trespassed on your space too much with quotations; but I have done so in order to show, in some measure at any rate, what is the consensus of opinions amongst students of Darwinism; and must answer the Duke with one more from Prof. Huxley's a dmirable biography. It is true that the Royal Society publishes these things in the least attractive way possible; but this particular paper could hardly have escaped attention, as it won the notice and admiration of even a journal so little occupied with scientific discussion as Truth. "There is another sense, however, in which it is equally true that selection originates nothing. Unless profitable variations occur, natural selection can do nothing' ('Origin,' ed. I, p. 82). 'Nothing can be effected unless favourable variations occur' (.c., p. 108). What applies to one animal will apply throughout time to all animals-that is, if they vary-for otherwise natural selection can do nothing. So it will be with plants' (c. p. 113. Strictly speaking, therefore, the origin of species in general lies in variation; while the origin of any particular species lies, firstly, in the occurrence, and, secondly, in the selection and preservation of a particular variation. Cleainess on this head will relieve one from the necessity of attending to the fallacious assertion that natural selection is a deus ex machiná, or occult agency." And the Duke says he has been waiting for this for thirty years. One can only wonder what Darwinian literature has been the subject of his studies during that time. W. T. THISELTON DYER. Royal Gardens, Kew, January 6. The Microseismic Vibration of the Earth's Crust. IN Mr. White's article on British earthquakes (NATURE, Jan. 2, p. 202) he refers to me as having discovered the microseismic AS old as any part of electrical science is the knowledge that a needle or bar of steel which has been touched with a loadstone will point to the north. Long before the first experiments of Galvani and Volta the general properties of steel magnets had been observed-how like poles repelled each other, and unlike attracted each other; how the parts of a broken magnet were each complete magnets with a pair of poles. The general character of the earth's magnetism has long been known that the earth behaves with regard to magnets as though it had two magnetic poles respectively near the rotative poles, and that these poles have a slow secular motion. For many years the earth's magnetism has been the subject of careful study by the most powerful minds. Gauss organized a staff of voluntary observers, and applied his unsurpassed powers of mathematical analysis to obtaining from their results all that could be learned. The magnetism of iron ships is of so much importance in navigation that a good deal of the time of men of great power has been devoted to its study. It was the scientific study of Archibald Smith; and Airy and Thomson have added not a little to our practical knowledge of the disturbance of the compass by the iron of the ship. Sir W. Thomson, in addition to much valuable practical work on the compass, and experimental work on magnetism, has given the most complete and elegant mathematical theory of the subject. Of late years the development of the dynamo machine has directed attention to the magnetization of iron from a different point of view, and a very great deal has been done by many workers to ascertain the facts regarding the magnetic properties of iron. The upshot of these many years of study by practical men interested in the mariner's compass or in dynamo machines by theoretical men interested in looking into the nature of things, is, that although we know a great many facts about magnetism, and a great deal about the relation of these facts to each other, we are as ignorant as ever we were as to any reason why the earth is a magnet, as to why its magnetic poles are in slow motion in relation to its substance, or as to why iron, nickel, and cobalt are magnetic, and nothing else, so far as we know, is to any practical extent. In most branches of science the more facts we know the more fully we recognize a continuity in virtue of which we see the same property running through all the various forms of matter. It is not so in magnetism; here the more we know the more remarkably exceptional does the property appear, the less chance does there seem to be of resolving it into anything else. It seems to me that I cannot better occupy the present occasion than by recalling your attention to, and inviting discussion of, some Inaugural Address delivered before the Institution of Electrical Engineers, on Thursday, January 9, by J. Hopkinson, M.A., D.Sc., F.R.S., President. of those salient properties of magnetism as exhibited by iron, nickel, and cobalt-properties most of them very familiar, but properties which any theory of magnetism must reckon with and explain. We shall not touch on the great subject of the earth as a magnet-though much has been recently done, particularly by Rücker and Thorpe-but deal simply with magnetism as a property of these three bodies, and consider its natural history, and how it varies with the varying condition of the material. To fix our ideas, let us consider, then, a ring of uniform section of any convenient area and diameter. Let us suppose this ring to be wound with copper wire, the convolutions being insulated. Over the copper wire let us suppose that a second wire is wound, also insulated, the coils of each wire being arranged as are the coils of any ordinary modern transformer. Let us suppose that the ends of the inner coil, which we will call the secondary coil, are connected to a ballistic galvanometer; and that the ends of the outer coil, called the primary, are connected, through a key for reversing the current, with a battery. If the current in the primary coil is reversed, the galvanometer needle is observed to receive a sudden or impulsive deflection, indicating that for a short time an electromotive force has been acting on the secondary coil. If the resistance of the secondary circuit is varied, the sudden deflection of the galvanometer needle varies inversely as the resistance. With constant resistance of the secondary circuit the deflection varies as the number of convolutions in the secondary circuit. If the ring upon which the coils of copper wire are wound is made of wood or glass -or, indeed, of 99 out of every 100 substances which could be proposed-we should find that for a given current in the primary coil the deflection of the galvanometer in the secondary circuit is substantially the same. The ring may be of copper, of gold, of wood, or glassit may be solid or it may be hollow-it makes no difference that with the vast majority of substances the deflection of in the deflection of the galvanometer. We find, further, the galvanometer in the secondary circuit is proportional to the current in the primary circuit. If, however, the ring be of soft iron, we find that the conditions are enormously different. In the first place, the deflections of the galvanometer are very many times as great as if the ring were made of glass, or copper, or wood. In the second place, the deflections on the galvanometer in the secondary circuit are not proportional to the current in the primary circuit; but as the current in the primary circuit is step by step increased we find that the galvanometer deflections increase somewhat, as is illustrated in the accompanying curve (Fig. 1), in which the abscissæ are proportional to the primary current, and the ordinates are proportional to the galvanometer deflections. serve that as the primary current is increased the galvanometer deflection increases at first at a certain rate; as the primary current attains a certain value the rate at which the deflection increases therewith is rapidly increased, as shown in the upward turn of the curve. rate of increase is maintained for a time, but only for a time. When the primary current attains a certain value the curve bends downward, indicating that the deflections of the galvanometer are now increasing less rapidly as the primary current is increased; if the primary current be still continually increased, the galvanometer deflections increase less and less rapidly. You ob This Now what I want to particularly impress upon you is the enormous difference which exists between soft iron on the one hand, and ordinary substances on the other. On this diagram I have taken the galvanometer deflections to the same scale for iron, and for such substances as glass or wood. You see that the deflections in the case of glass or wood, to the same scale, are so small as to be absolutely inappreciable, whilst the deflection for iron at one point of the curve is something like 2000 times as |