arteries are discussed in the next two chapters, and the authors point out that in physiological occlusion, Nature does not think it necessary to rupture the two inner coats of the artery, and that she does not divide the artery to reduce the longitudinal tension. In pathological obliterations, likewise, the rupture of the coats is by no means essential to occlusion, and, the coats not being ruptured, hæmorrhage does not occur.

In 1889, Messrs. Ballance and Sherrington published in the Journal of Physiology a valuable paper on the formation of scar-tissue, which has been practically re-copied into this book. The authors have made use for their experiment of Ziegler's method of placing glasschambers under the skin of animals, and examining their contents at various intervals after their introduction. Messrs. Ballance and Sherrington have been unable to trace the development of the so-called plasma-cells from the ordinary cell-forms of blood and lymph, and incline towards the opinion that plasma-cells are derived from the connective-tissue elements, and ultimately develop into fibrous tissue.

I cannot help thinking that Ziegler's method is by no means satisfactory when the object in view is to study the formation of cicatricial tissue; for in such investigations the most important point is that all the tissues to be examined should be removed intact, and examined after fixation. It is impossible to do this with glass-plates, but satisfactory results may be obtained by introducing soft material, such as filter-paper. The surrounding tissues and the paper can then be removed, and serial sections made through the whole. The examination of preparations made in this way make it doubtful whether Messrs. Ballance and Sherrington's views are correct, and would rather lead me to believe that the plasma-cells are originally derived from leucocytes.

Messrs. Ballance and Edmunds proceed to investigate the conduct and fate of the coats and of the ligature, and it is clear that they have taken immense trouble in ascertaining, by experiments on animals, how quickly a ligature becomes absorbed after being applied. Numerous and beautifully-executed plates, show the microscopical and macroscopical appearances of ligatures made of tendon, silk, floss silk, silkworm gut, &c., at varying intervals after their application to blood-vessels in man and animals; whilst special chapters are devoted to the ligature, of the knot, of the force used in the tying, &c. It may be noticed that the authors describe a new form of knot, which they recommend, and to which they give the name of stay-knot, whilst the old-fashioned reef-knot is entirely discarded. Moreover, the authors condemn in no uncertain terms the practice of rupturing the coats of arteries during, and the division of vessels after, ligature -points of the greatest practical importance.

The other chapters on the operation and the fate of the patient are of clinical interest chiefly; but special mention should be made of excellent chapters on suppuration occurring after ligation, and on the pathology of hæmorrhage, as well as of the full account of the experimental investigations made by Messrs. Ballance and Edmunds. It is only right to mention that most of the experiments were made at the Brown Institution.

The book is beautifully printed, and profusely illustrated with 10 plates and 232 figures. It will be widely read

by all surgeons, histologists and pathologists, and forms a most valuable addition to surgical and pathological science. M. ARMAND Ruffer.

OUR BOOK SHELF.

Precious Stones and Gems: their History, Sources, and Characteristics. By Edwin W. Streeter, F.R.G.S., M.A.I. Fifth Edition. (London: George Bell and Sons, 1892.)

BOOKS dealing with the fascinating subject of precious stones naturally fall into three classes-mineralogical treatises, archæological essays, and works adapted for experts and commercial men. Among the last class, the well-known work above cited, which has now reached a fifth edition, takes a prominent place. The enterprise and energy of the author in seeking out and developing new sources of ornamental stones is well known, and many of the facts contained in the present volume have been collected or verified by Mr. Streeter himself, by his sons, or by their agents. The chapters, which in earlier volumes were devoted to the description of famous diamonds, and to pearls and pearl-fishing, are now omitted, these subjects having been dealt with in separate books from the author's pen, the space thus obtained being devoted to an account of the Ruby Mines of Burma, the sources of sapphire in Siam and Montana, and those of the emerald in Egypt. In all these cases Mr. Streeter's agents have taken an active part in the work of exploring not hitherto available to the public. the districts, and he is able to furnish much information While the commercial aspects of these gem-stone localities naturally receive the greatest amount of attention, it is only fair to the author to point out that much care has evidently been exercised in order to prevent the creeping in of those of this class. The author acknowledges in his preface errors on scientific points which too often disfigure works the assistance which he has received from Mr. Rudler, the Curator of the Jermyn Street Museum, in dealing with scientific questions. The new edition, like its predecessors, is admirably got up and well illustrated. Air and Water. By Vivian B. Lewes, F.I.C., F.C.S., Professor of Chemistry, Royal Naval College, Greenwich, &c. (London: Methuen and Co., 1892.) THIS little book is one issued in connection with a series of University Extension manuals. The author may be congratulated upon the selection of his subject, which is one of those capable of being adequately treated in a course of a dozen lectures; and he has been no less happy in his treatment of it, for by following the historical method he has succeeded in maintaining the interest of his readers, while he fairly covers the whole ground with which an elementary treatise on this topic may be expected to deal. The story of the researches of Galileo, Torricelli, and by an admirable résumé of the latest achievements of Pascal, of Priestley, Cavendish, and Lavoisier, is followed chemical science, and this in turn by a clear statement of the problems involved in the maintenance of proper supplies of fresh air and pure water. The warmest votaries of other branches of science will not quarrel with our be the "most beautiful of the sciences"; possibly, howauthor when, in his enthusiasm, he declares chemistry to ever, some may demur to the statements in the following passage: Although the amount of oxygen present in the air amounts to 1,233,010 billions of tons, still it is only one two-millionth of the total oxygen, and had not this small fraction been left over in the creation of the world, neither animal nor vegetable life could have existed." The author must hold very decided views as to how far down extends that condition of oxidation which is so constantly found at the earth's surface.

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.]

Ornithology of the Sandwich Isles.

IT is easy to make assertions which, however improbable, it is not easy to disprove. I would therefore invite Mr. Albert F. Calvert to furnish documentary evidence of those he has advanced (suprà, p. 485). They do not indeed materially affect what I had said; yet, for the sake of accuracy, it might be as well to know on what foundation they rest. Those who are interested in the growth of ideas will be pleased to find that Sir Joseph Banks was so far in advance of his time as, on his return from his voyage with Captain Cook, which ended in 1771, to have "several cases of birds carefully mounted and arranged according to the localities in which they were collected"; and, among them, a "group of land birds from Owhyhee -an island which Cook did not discover until 1778-or seven years later. As these assertions alone concern the subject on which I wrote, I refrain at present from offering any remarks on the others; but your correspondent seems to have been the victim of a delusion or something worse. ALFRED NEWTON.

I OBSERVE in NATURE (p. 485) a letter from Mr. Albert F. Calvert, in which he states that certain cases of birds, which were collected by Sir Jos ph Banks during his voyage in the Endeavour with Captain Cook, "were in the custody of the Linnean Society of London until 1863, when they formed part of their natural history sale."

This is not the fact. The birds belonging to Sir Joseph Banks were never in the possession of this Society. It is true that the Society at one time possessed the insects and shells which formed part of the Banksian collections, and these in 1863 were presented, not sold, to the Trustees of the British Museum. Had there been any birds, they would doubtless also have been presented at the same time.

Where, then, did the cases, which Mr. Albert F. Calvert says are still carefully preserved in the museum of his ancestor Mr. John Calvert," come from? Certainly not from the Linnean Society. In view of Prof. Newton's valuable communication to NATURE (March 17, pp. 465-69), it is of importance that this inquiry should be answered, and a list furnished of the species contained in these cases, the reported existence of which will agreeably surprise ornithologists.

As Mr. A. F. Calvert has disclosed a source of information likely to be useful, perhaps he may be able to answer another question.

In 1860 the late Mr. J. D. Salmon, a well-known oologist, bequeathed to the Linnean Society a valuable collection of birds' eggs. This collection was known to contain the eggs of many British birds which were then becoming scarce, and have since become still rarer, if not quite extinct as breeding species; such, for example, as the golden eagle, osprey, kite, buzzard, honey buzzard, raven, chough, dotterel; and amongst some of the rarer species not found breeding in Great Britain, the spotted eagle, gerfalcon, black kite, rough-legged buzzard, Lapp owl, &c., and, above all, an egg of the great auk (Alca impennis), the value of which alone would almost equal that of all the other eggs in the collection.

I am informed that on the death of Mr. Salmon this collection was intrusted by his executors, for the purpose of being catalogued, to someone known to Mr. A. F. Calvert's ancestor; and by some accident, when it came to be handed over to the Linnean Society, the egg of the great auk was (some time afterwards) found to be missing, as also the eggs of certain species above mentioned, with several others that might be named.

Possibly they may have been removed at the time for safe custody, and were forgotten to be returned when the collection was deposited in its present resting-place.

Perhaps Mr. A. F. Calvert will say whether by chance these eggs (like the Banksian birds) have been "carefully preserved in the museum of his ancestor." If so, I presume that, on proof of the bequest of the collection to the Linnean Society, and proper identification of the eggs by their numbers, initials, or other

marks, Mr. A. F. Calvert would be willing (on behalf of his ancestor) to restore them to the cabinet from which they have been so long missing. J. E. HARTING. Linnean Society, Burlington House, March 28.

Poincaré's Thermodynamics.

IT is clear that I was justified in attributing the gist of M. Poincaré's first letter to his not having sufficiently read my notice of his book. He has not even yet fully apprehended the bearings of that notice, as a few words will show. Far from being unable to uphold any one of my critical remarks, as M. Poincaré is pleased to hint may be the case, I reassert every one of them, and could easily add to their number.

Let us begin with the particular item of my criticism which M. Poincaré persists in regarding as the most important. My words were:-"in his assumed capacity [of pure analyst] he quite naturally looks with indifference, it not with absolute contempt, on the work of the lowly experimenter." As an illustration of this I instanced M. Poincaré's ignoration of the thermo-electric researches of Sir W. Thomson, Magnus, &c. Then I quoted (in full) two of his remarks on the "Thomson effect." In the first of these he used the very peculiar phrase "Sir W. Thomson admet qu'il existe une force &c.":and in the second he said

"si l'effet Thomson a pu être mis en évidence par l'expérience, on n'a pu jusqu'ici constater l'existence des forces electromotrices qui lui donnent naissance."

To these he has, in his recent letters, added other like statements. Now, as I understand the matter, Lord Kelvin proved (which, as I take it, means a good deal more than might be implied by "constater") the existence of the electromotive force which depends on the so-called "Thomson effect" (giving also thereby the means of measuring its amount) by showing that the Peltier electromotive force does not in general fully account for the observed current in a thermo-electric circuit, and may even be directly opposed to it; while no other source of electromotive force can exist save the gradation of temperature in one or both of the metals. He then proceeded, by experiment, to measure the amount of the "Thomson effect for unit current in various metals, unequally heated. the passages above quoted from M. Poincaré's work are compared with the facts just stated, my comments on them will be seen to be fully justified.

When

As to the three chief objections I made to the work of M. Poincaré, every one (author, critic, or onlooker) is entitled to form and express his opinion. I need not restate mine, though I continue to adhere to every word of it :-but I may make the following additional remarks on these objections severally.

1. What sort of title to completeness can be claimed for a Treatise, on Thermodynamics, in which no mention is made of the grand principle of Dissipation of Energy; nor of Thermodynamic Motivity, "that possession the waste of which is called Dissipation"?

2. With regard to the measurement of Absolute Temperature, what I did say was that the experiments of Joule and Thomson, which justified them in basing it on Carnot's Function, were not mentioned by M. Poincaré in this connection. The omission by M. Poincaré of the italicized words makes an absolutely vital change in the meaning of my statement; and enables him to make what, (at first sight only), appears to be an

answer to it.

3. As regards the foundation of the Second Law, it is unfortunately clear that M. Poincaré and I must continue to differ-so that further discussion of this point would be unprofitable. For I presume that M. Poincaré has not formed his opinion without careful study of all that Clerk-Maxwell said on the point :-so that even a perusal of Lord Kelvin's latest paper (Fortnightly Review, March 1892) is not likely to induce him to change it. P. G. T. 26/3/92.

M. Poincaré and Maxwell.

In his recent treatise on "Électricité et Optique," M. Poincaré professes to give a description of Maxwell's theories of electro

magnetic actions. M. Poincaré appears to think that Mossotti's theory is consistent with and differs but little from Maxwell's. On this Maxwell says (§62):-"The theory of direct action at a distance is mathematically identical with that of action by means of a medium... provided suitable hypotheses be introduced when any difficulty occurs. Thus Mossotti has deduced the mathematical theory of dielectrics from the ordinary theory of attraction." Maxwell anyway repudiated Mossotti's theory. M. Poincaré introduces a "fluide inducteur" as the name of a thing displaced in the dielectric, when what Maxwell calls electric displacement occurs. This is all very well. It is anyway not inconsistent with Maxwell, even though Maxwell says distinctly that he does not know what the change of structure is like which he calls electric displacement. It might be a bending or twisting or lots of things, but M. Poincare is partially justified in fixing the idea thus. He calls this "fluide inducteur" elastic, though at the same time he calls it incompressible. It is not quite clear what "fluide" means here. M. Poincaré certainly observes that the elasticity of the "fluide inducteur" is quite different from that of material bodies, and in fact acknowledges that it is such as can hardly be fairly attributed to an incompressible fluid. Indeed, how can an incompressible fluid be elastic at all? There must be something besides the fluid; there must be some structure fixed in space which offers an elastic reaction to the fluid when driven past it, or else there must be the two liquids he objects to that are driven past one another. It is hardly a fair representation to talk of an elastic incompressible fluid, and then to invent difficulties, when the phenomena could not confessedly be represented by any such thing, but only by a fluid with some other mechanism superadded.

M. Poincaré's statement, "La méthode précédente n'est pas la seule que l'on puisse employer pour déduire de la théorie de Maxwell les lois de la distribution électrique," coupled with his further statement of "une autre méthode . sans supposer l'existence de ce fluide," seems at variance with his implication that this elastic incompressible fluid is part of or involved in Maxwell's theory.

This leads to the question of how far Mossotti's theory can fairly be considered as a substitution for or as a development of Maxwell's. It does not in any real sense get over action at a distance. There are the horrid old electrical charges acting upon one another across a space full of some non-conducting medium. This is practically no advance as far as a theory of electrical action is concerned. It is an advance no doubt as far as the behaviour of the medium is concerned, inasmuch as it enables a time propagation through space to occur; but as a theory of electric action it is a distinctly retrograde step on Maxwell's scientific position that he did not know what was the structure of the ether.

He

M. Poincaré proceeds to criticize Faraday's theory of the stresses in the dielectric, which he attributes to Maxwell. He begins by suggesting that the forces should have been explicable by the elasticity of the inductive fluid, in the same way as mechanical forces are due to the elasticity of matter. has in this quite forgotten that what he calls the elasticity of this fluid, is not a bit like the elasticity of any matter, and would require either a second fluid, which he rejects, or some structure other than the fluid, to explain its properties. Granting such an additional structure, then the elastic energy of the medium, fluid and structure combined, does exactly explain the motions of conductors. Nobody has explained exactly how conductors differ from non-conducting space in structure, and can or do move, and this is not a bit clearer on Mossotti's hypothesis than on any other, not even when the Maxwell non-conducting diaphragms are made infinitely thin.

nection with the "fluide inducteur," and not at all due to another fluid with peculiar properties. If the stresses are due to the connections of the "fluide inducteur" there is no great difficulty in supposing them proportional to the squares of the displacements of the "fluide inducteur," just as the increased tension of a stretched horizontal string due to a small weight at its centre is proportional to the square of this weight. In fact, a suggested model working upon this sort of principle has been published as illustrating this very point, and Dr. Lodge's model ethers, in the first part of his "Modern Views," are all of this kind.

M. Poincaré proceeds to find "une difficulté plus grave." He creates this by assuming that the energy of the medium is all due to the work done by these mechanical stresses deforming it. This is a most gratuitous assumption. Take the case of the stretched string with the weight on it. The increased energy of the system is not due only to the work done by the increased tension. At last he confesses, however, that if the energy in the dielectric be kinetic and not potential these difficulties would disappear. "Mais on ne peut encore adopter cette interprétation de la pensée de Maxwell sans se heurter à de grandes difficultés." And why? Merely because Maxwell afterwards calls the electric energy potential while he calls the magnetic energy kinetic. Has M. Poincaré forgotten that potential energy may in any case be the kinetic energy of an associated system? or can he not imagine two modes of motion of the same medium? Anyway, if the potential energy may be the kinetic energy of an associated system, and if M. Poincaré's difficulties are inapplicable to a kinetic explanation of the phenomena, it se ms impossible but that they are really inapplicable to a potential system if this system be judiciously devised. It is just here that M. Poincaré fails. He revels in elastic fluids, and yet he continually harps upon the same difficulty-namely, "How can an incompressible liquid be elastic at all?"-and instead of once for all solving this by acknowledging that there must be some structure, he reverts to it as if it were a new difficulty whenever he comes across its consequences.

As a mere mathematical work the book is admirable and clear, if a little prolix. GEO. FRAS. FITZGERALD. Trinity College, Dublin, March 24.

Prof. Burnside's Paper on the Partition of Energy, R.S. E., July 1887.

IN his criticism on a paper of mine on the partition of energy in a set of non-homogeneous spheres (NATURE, March 31, p. 512), Mr. Watson says that the conclusions are vitiated owing to my having omitted to introduce the frequency factor of collisions before proceeding to take the averages. This is not exactly accurate, since a frequency factor is introduced, viz. the relative speed of the centres of inertia of the impinging spheres parallel to the line of impact.

In the spring of 1888 Prof. Boltzmann published a criticism of the same paper in the Sitzungsberichte of the Vienna Academy, in which he contended that the correct frequency factor should be the relative speed of the points of impact of the spheres parallel to the line of impact; and in which he showed that the result of averaging with this frequency factor is to make the mean rotational energy equal to the mean energy of translation. Had I been entirely satisfied at the time of the cogency of Prof. Boltzmann's reasoning, I should, of course, have published a short note calling attention to the correction he proposed to make; and I regret now that this was not done, as it would have prevented the waste of a certain amount of valuable time and trouble. W. BURNSIDE.

Royal Naval College, Greenwich, April 1.

DR. WATSON has shown in his letter to NATURE of March 31 (p. 512) how the general methods of Maxwell and Boltzmann may be applied to the particular problem discussed by Prof. Burnside. He has also pointed out an error in Burnside's reasoning-namely, the non-introduction of the factor u whereby Burnside's conclusions at variance with the MaxwellBoltzmann law of partition of energy are vitiated.

U + ca,

You may, perhaps, allow me space to point out a little more Burnprecisely in what, as appears to me, the error consists. side has to find the average value of the expressionU + cw) {2w - c(K + k) (u U) Now, we may take averages in two ways:

(u

It

ON a blood orange being cut open by my little daughter yes. terday, a small orange was found inside, which, although no larger than a hazel-nut, was yet perfect in form and colour. showed no point of difference, other than that of size, as compared with the parent orange, and there was nothing in the appearance of the uncut fruit suggestive of the miniature of itself carried within. My sole right to write upon this subject is one you have always recognized in your journal, viz. that to record an interesting fact. GERALD B. FRANCIS.

Katrine, Surbiton.

METALS AT HIGH TEMPERATURES.1 PROPOSE this evening to consider, first, the methods of measuring high temperatures, and, second, to describe certain effects they produce on metals.

I

Geber, writing in the eighth century, gives directions for obtaining high temperatures, but points to the difficulties that arise in practice, " because fire is not a thing which can be measured, sed quoniam non est res ignis, quæ mensurari possit."2 It is not sufficient to attain

I A Lecture delivered at the Royal Institution by Prof. W. C. RobertsAusten, C.B., F.R.S., on Friday evening, February 5, 1892.

From the edition of his "Summa Perfectionis Magisterii," p. 28, published in Venice, 1542.

temperatures that are known to be high; it is necessary, for the purpose of modern investigation, to measure them with accuracy; and few of the early chemists in this country did more in affording a basis for the study of metals at high temperatures than Robert Boyle, the application of whose well-known law to solutions of metals in each other has been made evident by recent work. The 30th of December last was the third centenary of his death; it is well, therefore, that this lecture should begin with a tribute to his memory. He suggested improvements in the ordinary mercurial thermometer,' constructed what would appear to be the first air thermometer with an index; and although he did not do much for thermometry at high temperatures, he appears to have been struck by what must have been a quaint device for regulating high temperatures, for he points out that "the great mechanic, Cornelius Drebel 2 made an automatous musical instrument and a furnace which he could regulate to any degree of heat by means of the same instrument." He indicates various degrees of intensity of heat by reference to the colour of a glowing mass of fuel, and says that, "tho' we vulgarly say in English, 'a thing is red hot,' to express a superlative degree of heat, yet, at the forges and furnaces of artificers, by a white heat they understand a further degree of ignition than by a red one." It is not a little strange that for three centuries after his death the same vague expressions have constantly been used in describing high temperatures.

A great step in advance was made in 1701 by Sir Isaac Newton, who applied the law of cooling to the measurement of temperatures beyond the range of the mercurial thermometer, and in the notes which accompany his

66

Scala graduum caloris " he showed that he knew that the freezing-point of lead differs slightly from its meltingpoint.

Eighty years later, Josiah Wedgewood (1782), aided by one of my predecessors, Mr. Alchorne, Assay Master of the Mint, determined a few melting-points of metals, and, in communicating a description of his "thermometer for measuring the higher degrees of heat" to the Royal Society, we find him, one thousand years after Geber had said that "fire cannot be measured," still lamenting the want of suitable instruments, saying: "How much it is to be wished that the authors [to whom he refers] had been able to convey to us a measure of the heat made use of in their valuable processes; . . . a red heat, a bright red, and a white heat are," Wedgewood adds, "indeterminate expressions, and even though the three stages are sufficiently distinct from each other, they are of too great latitude, and pass into each other by numerous gradations which can neither be expressed in words nor discriminated by the eye." Another ninety years brings us to the last time that the measurements of high temperatures formed the subject of a Friday evening discourse in this Institution. On March 1, 1872, the late Sir William Siemens addressed you on the measurement of "heat by electricity"; and, speaking of the mercurial thermometer, said: "When we ascend the scale of intensity we soon approach a point at which mercury boils, and from that point upwards we are left without a reliable guide, and the result is that we find, in scientific books on chemical processes, statements to the effect that such and such a reaction takes place at a 'dull red, such another at a ‘bright red,' or a cherry red,' or a 'white heat-expressions which remind one," he adds, "of the days of alchemy rather than of chemical science at the present day."

It is not a little singular that the same lament should have been uttered, with so long an interval between, by two prominent technical men, and it suggests that but little experimental work had been done in the meantime with a view to the measurement of high temperatures. This is, however, far from being the case. A vast amount of work was done by physicists and metallurgists whose chief masters were "indefatigable labour, the closest inspection, and hands that were not afraid of the blackness of charcoal"; and their more noteworthy efforts were based on the employment of the air thermometer, in which the expansion of air replaces the expansion of the mercury in the ordinary thermometer, the bulb being of some fire-resisting material. For this purpose, Princep (1827) used a bulb of gold, Pouillet (1836) one of platinum, and finally, Deville and Troost, in a truly splendid series of investigations, adopted bulbs of porcelain, with iodine vapour as the elastic fluid. They ultimately reverted to the use of air.

66

You will remember that old mercurial thermometers had much information, supposed to be useful, engraven on their scales, and such statements as water freezes," "water boils," "blood heat," "fever heat," summer heat," were considered indispensable. It is by exposure to known temperatures that a thermoscope can be converted into a pyrometer for measuring intense heat; and the air or gas thermometer has, in the hands of Deville and Troost, rendered excellent service by enabling such gradations to be effected. The gas thermometer is not, in itself, a handy appliance, for it requires much subsidiary apparatus, and elaborate corrections of various kinds have to be introduced into the numerical data it affords; but it has given many fixed temperatures-such as melting-points and boiling-points of elements, and of compounds which may safely be made use of in graduating pyrometers. For very high temperatures, 900° C. and over, we rely on the excellent work of M. Violle 2 on the specific heats of platinum, silver, gold, palladium, and iridium, which have enabled the melting-points of the respective metals to be calculated.

I

The determinations of temperatures between 300 and 1000°, which are now generally accepted, also rest upon data accumulated by the aid of the air thermometer, which has thus enabled the graduation to be effected of instruments widely differing from it, that can be trusted to give rapid and accurate indications in daily use. can only bring before you two of the many kinds which have been devised; they are, however, by far the best that are available, and for the determination of temperatures up to the melting-point of platinum, leave little to be desired-

(1) A pyrometer which depends on the increase in the resistance of a heated conductor through which a divided electrical current is passing; and

(2) One in which the strength of an electric current, generated by the heating of a thermo-junction, is used as a measure of the heat applied to the thermo-junction.

The principle of the electrical resistance pyrometer was indicated by Sir William Siemens ("Collected Papers," vol. ii. "Electricity," p. 84, 1889) in a letter addressed to Dr. Tyndall, dated December 1860, and the nature of the instrument may be made clear by the accompanying diagram, Fig. 1. A divided current passes from the battery B, to a platinum wire, C, coiled round a clay cylinder, and to a resistance coil, R. At the ordinary temperature the resistance of the platinum coil is balanced by the standard resistance R. If, however, the platinum coil be heated, its resistance will be increased, and, this increase of resistance, which can be measured in various ways, indicates the temperature of the coil c. The coil itself may be adequately protected and exposed to tem

See the excellent bibliography given by C. Barus, Bull. Geological Survey, U.S.A., No. 54, 1889. 2 Comptes rendus, vol. lxxxix. p. 702, 1879; vol. xcii. p. 866, 1881.

The Report of a British Association Committee showed in 1874 that the instrument is liable to changes of zero, but Mr. H. L. Callendar has recently (1887) restored confidence in the method which had been shaken by the Committee. He has proved that if sufficiently pure platinum wire be used, and if the wire be carefully annealed and protected from strain and contamination,1 resistance pyrometers may be made practically free from changes of zero even when used at temperatures as high as 1000 C. He attributes the changes of zero to which the Siemens pyrometers are liable to the action on the wire of the clay cylinder on which it is wound, and of the iron tube in which it is inclosed. As the result of his experiments he has introduced certain modifications, which render the instrument not only trustworthy but very sensitive. He winds the platinum wire on a thin plate of mica, and incloses it in a doubly glazed tube of hard porcelain. He uses the zero method of measuring the resistance; but for these and other details of manipulation his own very interesting papers must be consulted. I will only add that I have had the pleasure of working with him in the Mint Laboratory, and I am satisfied that at temperatures about 1000 the comparative results afforded by his method are accurate to the tenth of a degree, a result which would certainly have been deemed impossible a year or two ago.2

The necessity for working with small volumes of fused metals, into which the tube of Callendar's pyrometer could not be plunged, has led me to prefer to adopt a method that would be classified under the second heading I have given. A very small thermo-junction may, in fact, be employed in such cases. The use of thermo-junctions for measuring high temperatures appears to have been sug

Phil. Trans. Roy. Soc. vol.178 (1887). A, pp. 161-230, and vol. 182 (1891), A, pp. 119-157; Phil. Mag. vol. xxxii. July 1891, p. 104, and vol. xxxiii. Feb. 1892, p. 220.

2 As this statement has been received with some surprise, it may be as well to state briefly how this degree of accuracy and sensitiveness is attained. The resistance-box is compensated for changes of temperature, and changes of resistance in the wires leading to the pyrometer are automatically eliminated. The resistance itself is measured by a modification of the well-known Carey-Foster method. The balancing resistance of the Wheatst ne bridge employed, is composed partly of resistance coils and partly of a bridge wire along which a contact key slides. The resistance of a centimetre of this wire is made to correspond to the increase of resistance of the pyrometer produced by a rise of 1 C. The galvanometer can easily be made sensitive to one-hundreth of a centimetre of this bridge-wire, so that one-tenth of a centimetre, which corresponds to one-tenth of a degree, can, of course, be measured with certainty.