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as the foundation of that side of medical and surgical the author before his work was in the printer's hands : 5 practice which is based on a sound knowledge of regional do not say that a perusal of it would have called for as, anatomy. The incomplete recognition of the physio- further notice than a reference. Symbolic methods come logical aspect of anatomy is, we think, the weak part of in for their due meed of recognition and employment. The
author satisfies himself with referring the student to the the book, and it is especially shown in the scanty notice table of contents for the topics included and the order which is taken of the action of the muscles and their pursued in treating them. The work consists of twelve association with the movements of the joints.
chapters divided up into twenty-four sections: i. (1) des To enable both these lines of anatomical study to be cusses the nature and meaning of a differential equatia pursued, the student is accustomed to employ at least between two variables ; ii. (2, 3, 4,) equations of the firs
order and degree ; iii. equations of the first order, but no two text-books ; the one in connection with his syste- of the first degree, (5) singular solutions (discriminarz
, matic work, the other as a guide to the dissection of the cusp-, tac-, and node-loci), (6) Clairaut's equation, body. Prof. Macalister apparently expects, as, indeed, geometrical applications, orthogonal trajectories; iv
. A he states in his preface, that his text-book should stand equations of the second order ; V. (9, 10) linear equations in the place of the two customarily employed. We doubt, with constant coefficients, in (10) symbolic methods art however, whether this expectation will be fulfilled. For his employed ; vi. (11-13) linear equations with variable text-book, in addition to what is essential in topographical hypergeometric series ; ix. (17) special forms of differentia
efficients; vii. (14, 15) solutions in series ; viii. (161 the description, by containing an account of the microscopic equations, as Riccati's equation due reference is made to structure of tissues and organs, a section on embryology, Dr. Glaisher's classical paper in the Phil. Trans. for 1881. and a detailed description of the bones, is necessarily a Bessel's equation, and Legendre's equation (reference a work of considerable size and weight, and too cumber- made to text-books and memoirs) ; X. (18-20) equatias some to be conveniently carried to and fro by the student, ential equations of the first
order ; xii
. (23, 24) partial din
involving more than two variables; xi. (21, 22) partial dis as is required with a dissecting-room manual. On the ential equations of higher order. Examples for practice se whole, therefore, we prefer the old and well-accustomed added at the end of each section. Though Prof. Johess lines on which text-books have for so long been written, cannot lay claim to have made here any additions to o to Prof. Macalister's modified plan.
knowledge of the subject, he has produced an excele But whilst expressing our inability to regard the method introductory hand-book for students, and this, we expect, which has been followed in the descriptive anatomy of We have omitted to state that all use of the compita
was the object he proposed to himself in its compilatine the soft parts as an improvement on the customary variable is eschewed. arrangement of systematic text-books, we recognize with The Land of an African Sultan : Travels in Mara. pleasure the clearness of the descriptions and the many
1887, 1888, and 1889. By Walter B, Harris, F.R.G." suggestive hints, both morphological and practical, which (London : Sampson Low and Co., 1889.) the book contains. The volume is profusely illustrated A GOOD deal has been written about Morocco lare, with upwards of eight hundred wood-cuts, about one half and Mr. Harris's volume is an interesting, althougb net 3 of which are original figures.
very important, contribution to the literature of the subject He describes first a journey through northern Morocco,
then a journey with H.B.M. Special Mission to the court OUR BOOK SHELF.
of the Sultan at Morocco city, next a visit to Waran and A Treatise on Ordinary and Partial Differential Equa- a ride to Sheshuan; and in a final chapter he sums up
tions. By W. W. Johnson. (London: Macmillan, the impressions produced upon him by the Moors and the 1889.)
country. In the chapter on his ride to Sheshuan, he de We have read Prof. Woolsey Johnson's work with some upon by Christian eyes." Mr. Harris does not pretend
scribes a place which had been "only once before looked interest : his style is clear, and the worked-out examples to have produced an exhaustive work on Morocco, bu well adapted to elucidate the points the writer wishes to he presents clearly what he himself has had opportunities bring out. He appears to recognize Boole, but, so far as the text is concerned, does not acknowledge the existence
of observing. of Mr. Forsyth's fine work. We do not say that he was Wayside Sketches. By F. Edward Hulme, F.L.S.,F.S.L under any obligation to do so, but nowadays we are so (London : Society for Promoting Christian Knowledge accustomed to see a list of authors upon whom a writer
1889.) has drawn that we missed it here. “An amount of space This is a pleasantly conversational book on all sorts of somewhat greater than usual has been devoted to the subjects more or less connected with natural history of geometrical illustrations which arise when the variables country life: birds, caterpillars, flowers, snow-crystals are regarded as the rectangular co-ordinates of a point. and the forms of clouds, all come in for a share of attenThis has been done in the belief that the conceptions tion. Without having any scientific pretensions of 5 peculiar to the subject are more readily grasped when own, the book may well serve to rouse a first interest in embodied in their geometric representations. In this many branches of science. The numerous illustrations connection the subject of singular solutions of ordinary are very good indeed. differential equations, and the conception of the characteristic in partial differential equations may be particularly mentioned.” This is certainly the most prominent feature
LETTERS TO THE EDITOR. of the early chapters, and it is, to our mind, clearly put (The Editor does not hold himself responsible for opinions et before the student. Reference is duly made to Prof. pressed by his correspondents. Neither can he undertake Cayley's work in the Messenger of Mathematics (vol. ii.), to return, or to correspond with the writers of, rest which initiated the present mode of treatment of the sub
manuscripts intended for this or any other part of NATULI. ject, but not to Dr. Glaisher's “ Illustrative Examples”
No notice is taken of anonymous communications.] (vol. xii.), nor to Prof. M. J. M. Hill's paper (London
Influenza. Math. Soc. Proc., vol. xix.), in which the theorems stated The following paragraph, taken from Sir David Brewster": by Prof. Cayley are proved. This paper, though read “Life of Sir Isaac Newton," is not uninteresting at the prestas before the Society, June 14, 1888, may not have reached time :
"Some light has been recently thrown on the illness of
Osteolepidæ. Newton by Dr. Dowson, of Whitby, who, at a meeting of the
R. L. is mistaken in condemning so absolutely Philosophical Society there on the 3rd of January, 1856, read a
the above form. The word “Osteolepus " would be a legitimate paper on the Sapposed Insanity of Sir Isaac Newton,' in
adjective expressing the same idea as the substantive Osteolepis ; which he has shown that the malady with which he was afflicted
and the patronymic of the “Osteolepi” would be simply la September 1693 was probably influenza or epidemic catarrhal
"Osteolepidæ," and not "Osteolepididæ." ferer, which prevailed in England, Ireland, France, Holland,
It may be useful for R. L. and some others to apprehend this and Flanders in the four last months of 1693. This distemper,
principle in word-building-viz, that compound Greek adjectives which lasted from eight or ten days to a month, was so general,
do not take the lengthened genitive as root; thus the correct that few or none escaped from it'; and it is therefore probable, as
Latin equivalent for the corresponding Greek adjective is not Ds. Dowson believes that Newton's mental disorder was merely
“echinodermatus” but “echinodermus," not i distomatus" the delirium which frequently accompanies a severe attack of but " distomus.” Hence, the correct form for the neuter plural influenza. See Dr. Theophilus Thomson's Annals of Influenza of the former is “Echinoderma ;” and for the neuter singular of of Epidemic Catarth in Great Britain,' published in 1852 by the latter is Distomum. And it would be wrong to write "Disthe Sydenham Society. See also the Philosophical Transactions tomatidæ" as the family name, and correct to write “Distofor 1094, vol. xviii. pp. 105-115." W. GREATHEED.
midæ." Hence Osteolepidæ and the like are admissible, since they may be considered as formed from adjectives, and not from
the substantive (of questionable form itself) in -is. Arout forty-five years ago I paid a visit with a friend to the
R. L. + E. laboratory of the celebrated chemist Prof. Schonbein, the dis(verer of ozone in the atmosphere and the cause of influenza. Just prior to our visit the Professor had obtained some ozone,
Exact Thermometry. and had inhaled it for the purpose, as he said, of giving himself Since the publication of my letter in NATURE (December 19, influenza, in order to ascertain how it would affect him. We 1889, p. 152) on the cause of the rise of the zero-point of a thertoth distinctly observed most of the ordinary symptoms of the mometer when exposed for a considerable time to a high malady.
AUGUSTUS HARVEY. temperature, two letters on the same subject have appeared, one 12 Landridge Road, Fulham, January 17.
from Mr. Herbert Tomlinson (January 2, p. 198), the other from Prof. E. J. Mills (January 9, p. 227), who replies to my
objections to the plastic theory. Rainbow due to Sunlight reflected from the Sea. Mr. Tomlinson considers that my experiments seem to leave I HAVE never heard of a rainbow, due to the image of the sun
no doubt that compression, due to the plasticity of the glass, is not
the main cause of the rise of the zero-point, but he considers that in water, having been seen ; and I think the following letter, it is not merely the prolonged heating, but also the change of from an old studient of mine of sixteen years ago, may interest temperature (heating or cooling), that is effective in bringing your readers,
about the change. I have not yet had time to make any special The University, Glasgow, January 7.
experiments to test this point, but I may perhaps mention that On September 18, 1889, I saw a rainbow, caused, not by the such data as I possess seem rather to point to the conclusion that direct rays of the sun, but by their reflection from the sea.
long-continued steady heating is more effective than alternate We were at the height of goo feet; the sky was all clouded heating or cooling. As the following experiment, made about a except along the western horizon ; the sun, an hour before set year ago, seems to bear on the point, I give the results :ting, was hidden; but its rays were reflected from the sea. A Approximate
6 drizzle was falling, and my companion was remarking how strong
3 6 6
31 6 61
rise of the light from the sea was, when it occurred to me that it might Rise of zero- } 1060°15 09085 0°50°'* 1° 2 0° 0° give a bow. And there it was behind us--not the usual recum
404 bent bow, less than a semicircle, but an overhanging one, greater Two other thermometers, heated each day for about six hours, than a semicircle. The clouds were drifting from the west, so showed after nine days rises of zero-point of 30:8 and 4°'I rethat the sun came into view; and the usual rainbow became spectively, but in these cases the change was apparently not visible with its secondary bow; so that three rainbows were seen quite complete. The temperature was in each case 280°, and 9 Nct. The sea-bow and the usual bow were identical at the all these thermometers belonged to the same batch as those horizon. The angle between them was greater than the sun's employed in my experiments already described in NATURE.
Prof. Mills does not regard the experiments as conclusive, but criticizes my results in the following words: “The zero movement, however, only ranged from 1 to 1°:2-small readings which might very possibly have been obtained, or not, on either of the thermometers at other times.” This criticism, in striking contrast to the rest of the letter, appears to be rather unkind either to me or to my thermometers, I hardly know which. I sincerely hope that none of my thermometers are capable of such erratic behaviour as to show changes of zero-point of 1° (or even twice this amount if the plastic theory is correct) without extraordinary treatment, or that my readings of temperature are
reliable only to within ro or so. But to make the matter more HORIZON
certain, I will continue the heating of the two thermometers, A and C, under the same conditions as before, and will also heat
two more thermometers under similar conditions to about 360°. angular heighit, but not double. It seemed as if the complemen Prof. Mills mentions the very curious behaviour of lead-glass tuo segment of the rim had been folded up from beneath into thermometers at different temperatures, but his objection on that view, but that the colours were not reversed. The sea-bow was score to the temperature 280° does not seem to apply, as my just as bright as the secondary bow, which it intersected. thermometers are all made of soft German soda-glass. It may,
From the fact that the three were seen together, for over 3 however, be useful to heat two more thermometers to a temperaminutes, at least in part, I would argue that it is no unusual ture of about 220° in order to compare the total rise with that at sigiat
, and that in Scotland, where bows are so frequent, and 280° and 360°. plenty of comparatively smooth water available, this sea-bow With regard to the statement that the final state of a thermomay be looked for and seen.
meter kept at the ordinary temperature for an infinite time i may mention, also, that I saw a fourth bow that evening. I would differ from that of the same thermometer after being subAfter the sun had set, á bow of one colour, an orange-pink, took jected to prolonged heating at a high temperature, I am not the place of the usual bow. The source of light, I thought, was prepared to give a decided opinion either one way or the other, 2 cloud just over the place where the sun had set.
but it does appear to me to be rather a daring procedure to
WILLIAM SCOULLER. make observations of the minute changes of zero-point over a $6 Calle de la Independencia, Valparaiso, November 9, 1889. ) few years, and to extrapolate from a decade or so to eternity.
I am also quite willing to admit that there may be other involuntary host of Dr. Lewis's Filaria, and his leg the sea! v causes tending to raise the zero-point besides the equalization of Elephantiasis lymphangiectodes, accompanied by hypertrophy tension, such, for instance, as the chemical changes alluded to of many integumentary structures of the limb. Oak-spangle hy Prof. Mills ; but I should like to ask, as I am ignorant on the on the other hand, are to my mind comparable to the circler point, whether there is any experimental evidence of their nests of ringworm, or to the sprouting epithelium of a Ver. nature or existence.
SYDNEY YOUNG. necrogenetica. Such comparisons may be of little scientis University College, Bristol, January 11.
value, yet I take it they are as tuseful in their place as attempte to gauge the amount of "disinterestedness" shown by a calitage
when it becomes the unwilling host of the gall-producing Foreign Substances attached to Crabs.
W. AINSLIE HOLLIS. In your issue of December 26, and also in exhibiting his
Brighton, December 30, 1889. collection of crabs before the Linnean Society, Mr. Pascoe cast some doubt on the function of the two pairs of modified legs of Dromia vulgaris, which are usually supposed to be adapted to
The Evolution of Sex, the retention of the sponge with which it covers its carapace. The interesting note of Mr. M. S. Pembrey in your issue of
That these legs were really used for this purpose I was enabled January 2 (p. 199), induces me to draw the attention of your to observe, during my stay at the zoological station in Naples correspondent to a short paper of mine just published 1973 last winter. I had in my tank several specimens, in some of course of publication) in the Ibis, where I communicated the which the sponge had also extended on to the ventral surface, experiences of a friend, who had hatched a series of price over the edge of the carapace, thus securing a firm hold apart
eggs, belonging to the genus Eclectus, in which the young from the action of the legs. In all specimens, however, there males are green, the young fernales ret. It is remarkable tha are seen, when the sponge is removed, which requires con- by far the larger number of the birds hatched were tales
. I siderable force, two oblique depressions into which the legs fit, each case only two eggs were laid, and the breeder himself
, wat giving them thus a distinct hold on the sponge. If the latter be, however , removed from the animal but left in out being able to tell why, is of opinion that nearly all the
hatche consisted of male birds. As there are still many embryos the tank, the crab soon sets to work to regain possession of its of those Eclectus in my hands, the sex of which is not yet if covering, and can be seen to use its modified hinder pairs of termined, I hope to be able to make known the result of my 3legs most effectually for this purpose. It would seem therefore vestigation later, whether the pairs are always males, or aluar beyond. doubt that these modified legs serve not only for holding females, or consist of a male and a female bird, at least sometimo on the sponge, but also for getting hold of a new sponge, should Meanwhile, I should be glad to hear if anything more is known the old one get injured or die, as must happen not unfrequently. about the sexes of birds which lay only two eggs at a time. F. ERNEST WEISS.
A. B. MEYER. The Zoological Laboratory, University College, January 6.
Royal Zoological Museum, Dresden, January 5.
“Manures and their Uses. ** Prof. Romanes and Dr. St. George Mivart in suggesting that
Allow me to thank the well-known writer "W." for bithey wished to assail the theory of natural selection in their review of the above-mentioned book. "W." does not bold recent communications to Nature on this subject. They must, with the view that "farmyard manure is erroneously support however, pardon me for saying that I still think the extract to contain all the necessary plant-foods required for the growth to which I alluded in my note admits this interpretation. As of plants." I believe, with M. Ville and others, that the my views of the relations of gall-formation to the theory of farmer who uses nothing but farmyard manure exhausts be natural selection are clearly at variance with those of your corre land.” “W." speaks of this as an "obvious fallacy." If the spondents, perhaps you will allow me space to give briefly the statement is wrong, would " W." kindly answer the quotatia grounds upon which I base my conclusions.
given on p. 76 of the book in question. The quotation "rues There are in England about ninety well-known varieties of as follows :galls, and of this number fully a third are found in the oak.
“M. Grandeau (the French agricultural authority) recently About half the oak-galls are formed on growing leaves. In estimated that one year's crop in France represents 298.220 nearly one-third of the total number the grub is hatched, and tons of phosphoric acid, of which only 151,200 tons were * the gall is fashioned in a developing bud. We can readily covered from the stable dung, thus leaving a deficit of 147.00 imagine, in the case of a tree with deciduous leaves, that the tons, equal to over one million tons of superphospłate, to be presence of a few galls upon its foliage would not greatly affect made good by other means. its chances of survival, if its fitness was in other respects com
“M. Grandeau also estimated that the entire pumber of fare plete. It is otherwise when a gall occupies the position of a animals in France in 1882, representing a live weights developing bud, especially when the bad is a terminal one. In 6,240,430 tons, had accumulated from their food 193,453 tons of this case there occurs coincidently with, and as a result probably mineral matter containing 76,820 tons of phosphoric acid. of, the adventitious formation, an arrest of normal development These figures give some idea of the enormous quantities of phos and growth. Indeed, I believe “the gnarled and twisted oak” phoric acid required to restore to the soil what is continuary owes many of its gnarls and most of the twists to the common being carried away by the crops sold off the farm." oak-apple and other bud-galls. If a tree endowed with less It must be borne in mind that in the above estimate, M. developmental vigour and with fewer supplementary buds than Grandeau includes the purchase of oil-cakes and other feeding the oak had been exposed to the repeated attacks of the insects stuffs
. Therefore, if farmyard manure only contaias about hali for many generations in a struggle for existence, it would doubt the amount of phosphoric acid (to say nothing of nitrogen, less have long ago succumbed, and it would have done so by a potash, &c.) required to retain the land in a fertile condita process of natural selection operating in the ordinary manner,
how can I have attached “too much prominence to chemical and not " at the end of a long lever of the wrong kind," what: manures, and too little importance to stock-feeding as a manuna ever that may mean. This selective process in the case of gall-agency"?
A, B, GRIFFITES bearing trees has left possible traces of its action to-day, for I [DR. GRIFFITHS assumes that because, as asserted by M. am unaware that any other English tree than the oak is attacked Grandeau, the balance of fertilizing matter in France is again by terminal bud-galls. The terminal leaf-galls of certain Salices the land,'" the farmer who uses nothing but farmyard manus and Conifers can scarcely affect their growth and development exhausts his land." This is arguing from general principles to to the same extent as the bud-galls.
special cases, and there is no sequence in his reasoning, When we compare pathological tumours in the higher animals nation may be rushing to ruin, bat that does not prevent and with these vegetable excrescences, we must make due allowances dividual from growing rich. Phosphates and nitrates may for the different conditions under which each lives., I cannot diminishing, but that does not prevent them from accumulating then see that the "morphological specialization" of galls, on any particular farm. We traverse Dr. Griffiths's statemea which, for the most part, are composed of hypertrophied repro- without qualification, that the farmer who uses nothing elsels ductions of the simpler vegetable tissues, is greater than that farmyard manure exhausts his land. We believe he improves exhibited by man himself, when, for instance, he becomes the his land.—THE REVIEWER.)
ent order of phenomena. As the temperature rises, the deflection on the galvanometer diminishes very slowly
till a high temperature is attained ; then the rate of WHEN one considers that the magnetic property is decrease is accelerated until, as the temperature at destroyed by the admixture of some foreign body, as is reached, the rate of diminution becomes very manganese-one would naturally expect that its existence rapid indeed, until, finally, the magnetism of the would depend also on the temperature of the body. This iron disappears at the same time as for small forces. is found to be the case. It has long been known that iron Instead of following the magnetization with constant remains magnetic to a red heat, and that then it somewhat forces for varying temperatures, we may trace the curve suddenly ceases to be magnetic, and remains at a higher of magnetization for varying forces with any temperature temperature non-magnetic. It has long been known that we please. Such curves are given in Diagrams 9 and 10. the same thing happens with cobalt, the temperature of In the one diagram, for the purpose of bringing out change, however, being higher; and with nickel, the tem- different points in the curve, the scale of abscissæ is 20 perature being lower. The magnetic characteristics of times as great as in the other. You will observe that the iron at a high temperature are interesting. Let us return effect of rise of temperature is to diminish the maximum to our ring, and let us suppose that the coils are insulated i magnetization of which the body is capable, slowly at with a refractory material, such as asbestos paper, and that the ring is
WROUGHT IRON made of the best soft iron. We are
11000 now in a position to heat the ring to a
MAGNETISING FORCE 0-3. high temperature, and to experiment upon it at high temperatures in exactly
10000 the same way as before. The temperature can be approximately determined by the resistance of one of the copper coils. Suppose, first, that the current in the primary circuit which we use for magnetizing the ring is small; that from time to time, as the ring is heated and the temperature rises, an experiment is made by reversing the current in the primary circuit, and observing the deflection of the galvanometer needle. At the ordinary temperature of the air the deflection is comparatively small; as the temperature increases the deflection also increases, but slowly at first ; when the temperature, however, reaches something like 600° C., the galvanometer deflection begins very rapidly to increase, until, with a temperature
3000 of 770 C., it attains a value of no less than 11,000 times as great as the deflection would be if the ring had been
6 2000 made of glass or copper, and the same exciting current had been used. Of course, a direct comparison of 11,000 10 i cannot be made : to make it, we must introduce resistance into the secondary circuit when the iron is
700 785 800°C used; and we must, in fact, make use of larger currents when copper is
Fig. 8. used. However, the ratio of the inducton in the ca-e of iron to that in the case of copper, at first, and rapidly at the end. It is also very greatly to 770°C., for small forces is no less than 11,000 to 1. Now diminish the coercive force, and to increase the facility mark what happens. The temperature rises another with which the body is magnetized. To give an idea of 15C: the detection of the needle suddenly drops to a the magnetizing forces in question, the force for Fig. 8 value which we must regard as infinitesimal in comparison was o'3; and as you see from Figs. 9 and 10, the force to that which it had at a temperature of 770° C. ; in fact, ranges as high as 60. Now the earth's force in these at the higher temperature of 785° C. the deflection of the latitudes is 0-43, and the horizontal component of the galvanometer with iron is to that with copper in a ratio earth's force is Oʻ18. In the field of a dynamo machine not exceeding that of 1'14 to 1. Here, then, we have a the force is often more than 7000. In addition to the most remarkable fact : at a temperature of 770° C. the general characteristics of the curve of magnetization, a magnetization of iron 11,000 times as great as that of a very interesting, and, as I take it, a very important, fact non-magnetic substance; at a temperature of 785° C. comes out. I have already stated that if the ring be sub!ron practically non-magnetic. These changes are shown mitted to a great current in one direction, which current in Fig. & Suppose now that the current in the primary is afterwards gradually reduced to zero, the ring is not in circut which serves to magnetize the iron had been great its non-magnetic condition, but that it is, in fact, strongly instead of very small. In this case we find a very differ- magnetized. Suppose now we heat the ring, whilst under
the influence of a strong magnetizing current, beyond the " lezugural Address delivered before the Institution of Electrical En- critical temperature at which it ceases to have any magpiper, og l'hursday, January 9. by J. Hopkinson, M.A., 'D.Sc., F.R.S., Prendert. Conusued from p: 254
netic properties, and that then we reduce the current to
zero, we may in this state try any experiment we please. cent. of nickel is non-magnetic as it is sure to come from Reversing the current on the ring, we shall find that it is the manufacturer; that is to say, a substance compounded in all cases non-magnetic. Suppose next that we allow of two magnetic bodies is non-magnetic. Cool it, howthe ring to cool without any current in the primary, when ever, a little below freezing, and its properties change : cold we find that the ring is magnetized ; in fact, it has a it becomes very decidedly magnetic. This is perhaps distinct recollection of what had been done to it before it not so very remarkable: the nickel steel has a low critical was heated to the temperature at which it ceased to be temperature-lower than we have observed in any other magnetic. When steel is tried in the same way with magnetizable body. But if now the cooled material be varying temperatures, a similar sequence of phenomena | allowed to return to the ordinary temperature it is mag
netic; if it be heated it is still mag.
netic, and remains magnetic till a tem16000
perature of 580° C. is attained, when it very rapidly becomes non-magnetic, exactly as other magnetic bodies do
when they pass their critical tempera12000-6
ture. Now cool the alloy: it is nonWROUGHT IRON
magnetic, and remains non-magnetic till the temperature has fallen to
below freezing. The history of the 8000 TEMPR. 775 TO 767°C
material is shown in Fig. 11, from which it will be seen that from -20°C. to 580° C. this alloy may exist in either of two states, both quite stable-a magnetic and a non-magnetic—and
that the state is determined by whether MAGNETIZING FORCE
the alloy has been last cooled to - 20° 70 C. or heated to 580° C.
Sudden changes occur in other properties of iron at this very critical temperature at which its magnetism disappears. For example, take its elec
trical resistance. On the curve, Fig. 10000 WROUGHT IRON
12, is shown the electrical resistance
of iron at various temperatures, and 8000
also, in blue, the electrical resistance of copper or other pure metal. Observe the difference. If the iron is heated, its resistance increases with an accelerating velocity, until, when
near the critical temperature, the rate MAGNETIZING FORCE
of increase is five times as much as the copper ; at the critical temperature the rate suddenly changes, and it assumes a value which, as far as experiments have gone, cannot be said to
differ very materially from a pure metal. The resistance of manganese steel shows no such change; its temperature coefficient constantly has the value of 0'0012, which it has at the ordinary tenperature of the air. The electrical resistance of nickel varies with temperature in an exactly similar manner. Again, Prof. Tait has shown that the thermo-electric
properties of iron are very anoMAGNETISING FORCE 6.7
malous—that there is a sudden change at or about the temperature at which the metal becomes
non-magnetic, and that before this TEMPERATURE O
600C temperature is reached the varia
tions of thermo-electric property are quite different from a non
magnetic metal. is observed ; but for small forces the permeability rises to Prof. Tomlinson has investigated how many other proa lower maximum, and its rise is less rapid. The critical perties of iron depend upon the temperature. But the temperature at which magnetism disappears changes most significant phenomenon is that indicated by the rapidly with the composition of the steel." For very soft property of recalescence. Prof. Barrett, of Dublin, obcharcoal iron wire the critical temperature is as high as served that if a wire of hard steel is heated to a very 880° C. ; for hard Whitworth steel it is 690° C.
bright redness, and is then allowed to cool, the wire will The properties of an alloy of manganese and iron are cool down till it hardly emits any light at all, and that curious. More curious are those of an alloy of nickel then it suddenly glows out quite bright again, and afterand iron. The alloy of nickel and iron containing 25 per wards finally cools. This phenomenon is observed with