clearly where it was best to draw generic lines. While it must be admitted that there are many minor differences in the generic concepts exhibited in the scholarly and monumental works to which I here refer, yet they establish a good usage, which on the whole has a considerable measure of uniformity, and goes far to establish the rank of such categories as genus, species, and variety.

Let me urge that, while we remit no effort to secure further light on this subject, there should be a general agreement to treat the accepted and traditional interpretation of large and important genera as sacred and binding until we can furnish definite and convincing evidence that change is needful, and that for the welfare and dignity of our science all should unite in opposing changes of the artificial sort, which consist merely in the shifting of ranks and modification of standards.

Investigations and Commercial Tests in Connection with the Work of an Engineering College.1

In any school it is necessary, in securing the best efficiency in instruction, that the professors shall be able to speak with authority on the subjects which they teach. In technical schools those who teach the practical engineering subjects cannot speak with authority unless they have had practical experience. Investigations and commercial tests may serve to give them this practical experience, and the question naturally arises, Is it a good policy for professors to conduct such work in connection with their regular college duties?

Let us consider the various ways in which a professor in an engineering school may acquire the practical experience which is necessary in his work.

First, he may be called to a professorship from the practical field.

Second, after teaching for a time and finding how necessary a practical experience is in his work, he may turn to the practical field, and then return to teaching.

Third, he may undertake practical work in connection with his college duties, and gain his experience in this

way.

Each method possesses its own advantages and disadvantages. Starting with the first, it must be admitted that many of our best instructors have entered the teaching line after they have had experience in the practical field. Such a man has an advantage in being able to make use of this experience immediately, when he starts at his teaching work. There is a disadvantage, however, in the fact that should he have secured a mature experience in the practical field, he will necessarily be no longer a young man, and it may be hard for him to teach and properly to adapt himself to the theoretical part of his course.

to

The advantages of the second system of securing a practical experience, where the professor leaves the teaching field, takes up outside work, and then returns teaching, are that during his practical career he will be very much alive to the points he should look into, and, furthermore, if he returns to teaching he will possess the advantage of having experience both as a teacher and as a practical engineer.

We will now take up the third method, where a professor obtains his practical experience by conducting outside work in connection with his college duties. The outside work undertaken by a professor should be that of a scientific or strictly engineering type.

The advantage to a college in having its professors do research and outside work is that what reflects to the credit of the professor will reflect to the credit of the college. Furthermore, the college will be looked to as a source from which an unbiased opinion can be obtained, and in maintaining this standard it will be fulfilling a high and useful mission. The results of the investigations may be made the subjects of scientific papers to be read before the various societies, and any reputation that a professor gains in this way will benefit his college.

The day is past when there can be a strict line drawn between the work of the consulting engineer and that of the professor who teaches in the same field. The ideal

1 From the address of Prof. D. S. Jacobus, president of Section D, Mechanical Science and Engineering.

professor in a given line should be able to take up the work of the consulting engineer in that line, and the ideal consulting engineer should possess enough technical knowledge to fit him for being a professor. There should be no jealousy, but rather a bond of friendship in that the fundamentals which each should master are the same.

UNIVERSITY AND EDUCATIONAL
INTELLIGENCE.

OXFORD.-The results of a census undertaken each year by the Magazine show that there are 2722 undergraduates actually in residence this term, as compared with 2621 in Hilary term, 1905. The increase is probably due to the Rhodes scholars and to the fact that a larger proportion of undergraduates now complete three years of residence than was the case a few years back. The three largest colleges are Christ Church, New College, and Balliol, with 211, 210, and 181 members in residence.

A long vacation course in geography will be held in Oxford between August 7 and 25, provided that sufficient names are sent to the Reader in Geography, Old Ashmolean Building, Oxford, by June 1. The course will include lectures and demonstrations in the School of Geography, and surveying and map-drawing in the field.

66

At a meeting of the Junior Scientific Club, held on Wednesday, February 14, at the museum, papers were read by Prof. Miers on Spontaneous Crystallisation, ' and by Mr. C. G. E. Farmer on The Use of Finely Divided Metals in Organic Chemistry."

CAMBRIDGE. The regulations for the diploma in mining engineering were passed by the Senate last Thursday. Among the chief of these regulations is that the candidate may take such parts of the natural sciences tripos and of the special examination in mechanism and applied sciences bear upon the subject of mining engineering, or candidate may take honours in the mechanical sciences tripos. Details of the examination and the schedules in the art of mining and in metallurgy will be found in the Cambridge University Reporter for December 5, 1905.

as

a

The Smith's prizes for 1904 have been adjudged as follows, the names being in alphabetical order :-C. F. Russell, Pembroke, for his essay on The Geometrical Interpretation of Apolar Binary Forms"; F. J. M. Stratton, Gonville and Caius College, for his essay on "A Problem in Tidal Evolution Suggested by the Motion of Saturn's Ninth Satellite."

Mr. J. W. Nicholson, of Trinity College, has been elected to the Isaac Newton studentship in astronomy and physical optics, of the value of 250l. for one year, for study and research in astronomy.

Mr. R. H. Rastall, late scholar of Christ's College, Harkness scholar in 1903, has been elected to a junior fellowship at Christ's College. Mr. Rastall has worked chiefly in the Geological Museum at Cambridge, and has written on the Blea Wyke beds of Yorkshire and on The Buttermere and Ennerdale Granophyre" of Lakeland.

66

DR. C. H. LEES, lecturer in physics and assistant director of the physical laboratories of the University of Manchester, has been appointed professor of physics at the East London College.

THE King's Speech, read by His Majesty at the opening of Parliament on Monday, promised that, at the earliest possible moment, a Bill would be introduced "for amending the existing law with regard to education in England and Wales."

THE Lancashire County Education Committee has recommended the council to make a grant of 1ool, a year to the fund for the establishment of a department in economic botany in the University of Liverpool. The cost of the proposed department has in consequence now been completely guaranteed.

THE Senate of the University of St. Andrews has resolved to confer the following honorary degrees, among others, at the graduation ceremonial on April 3-LL.D., Dr. A. C. L. G. Gunther, F.R.S., in appreciation of his lifelong and distinguished labours in zoology, Prof. J. C. Wilson, Oxford, and Prof. A. H. Young, Manchester.

AN open competitive examination for not fewer than twelve situations as assistant examiner in the Patent Office will be held by the Civil Service Commissioners in April next. The examination will commence on April 23, and forms of application for admission to it are now ready for issue, and may be obtained on request addressed by letter to the secretary, Civil Service Commission, Burlington Gardens, London, W.

AT the annual conference of the Labour Representation Committee held on February 16 considerable discussion took place on the following resolution :-"That this conference condemns the educational policy of the Government as laid down in the Act of 1902, and demands the formulation of an educational programme based upon the principle of equal opportunities for all, such programme to aim at securing-(1) that immediate provision be made for giving at least one free meal per day to all school children; (2) that all grades of education shall be free and State maintained; (3) that all education shall be free, and that secondary and technological education be placed within the reach of every child by the granting of bursaries or maintenance scholarships to all children whose usefulness would be enhanced by such extended education; further, that adequate provision be made for children to continue at school until the age of sixteen years, or until such age as the university course begins; (4) that provision be made to continue the education of capable students through the university courses; (5) that the standard of capacity shall be judged by work previously accomplished, and not by competitive examination; (6) that the education in all State-supported schools shall be secular; (7) that all State-supported schools shall be under the control of and their affairs administered by the directly elected representatives of the people; (8) that each educational district shall be required to train the number of pupil teachers demanded by local needs, and for this purpose to establish training colleges, preferably in connection with universities or university colleges; (9) that the cost of the abovementioned reforms shall be borne by the National Exchequer out of revenue obtained through broadening the basis of taxation, and by the restoration and democratic administration of valuable misappropriated educational charities and endowments." "This conference, therefore, instructs the committee (or such body as may be appointed for the purpose) to draft a Bill embodying the principle of the said resolution, with a view to the Labour group introducing it early into Parliament." A division having been taken, the result was declared as follows:-817,000 votes for the resolution and 76,000 for its rejection. The resolution was therefore carried. In view of the growing importance of the labour interest, it is satisfactory and gratifying to find a large and representative body of labour delegates appreciating the fact that the future welfare of the country is closely bound up with the provision of a rational system of national education.

THE publication on February 19 of a correspondence between Mr. A. H. D. Acland, formerly Minister of Education, and Mr. Birrell, President of the Board of Education, is gratifying evidence that at last something is to be done in the direction of providing adequate Exchequer grants for English secondary education. Mr. Birrell, in reply to a series of suggestions made by Mr. Acland, announces that provision is to be made in the Estimates for this year for a considerable increase of the Exchequer grants (1) in aid of secondary schools; (2) to alleviate the burden now placed upon local authorities in respect of the education of teachers; and (3) to assist further the building of training colleges for teachers by the local authorities. How much higher education in this country has suffered from the inadequate education of boys in our secondary schools, which, through want of funds, are too often under- and inefficiently staffed and equipped, has been pointed out in these columns with patient persistence. It is earnestly to be hoped that the findings of the Royal Commission on Secondary Education of ten years ago will now be considered seriously, and a statesmanlike attempt made to secure for the pupils in whose hands our future success as a manufacturing nation lies, a rational and complete secondary education which will enable them to take proper advantage of

higher technical instruction. The promise that local education authorities are to be helped-in a degree commensurate with modern needs-in the pressing work of supplying more training college accommodation is heartily to be welcomed. The proportion of fully trained teachers in our elementary schools is at present scandalously low; and this is due primarily to the fact that until quite recently the only training colleges were those built-with the aid of special State grants like that of 1835-by the National and the British and Foreign School Societies, and supported largely by Government grants on each teacher in training. Though in recent years the work of day training departments in connection with university colleges has improved the facilities for the training of teachers, much yet remains to be accomplished if English elementary education is to take advantage of modern educational enlightenment. Local education authorities, with their knowledge of local needs, will be in a position, when helped by the promised Treasury grants, to start the much needed provision of more colleges where teachers may become acquainted with the principles upon which successful teaching must be based. In carrying out this important work, the need of training for secondary school teachers must be forgotten. Most masters in secondary schools begin their work knowing only what to teach, and nothing of how to teach.

not

SOCIETIES AND ACADEMIES.

LONDON.

Royal Society, January 25.—"On the Effect of High Temperatures on Radium Emanation." By W. Makower. Communicated by Prof. Arthur Schuster, F.R.S.

(1) The activity of radium emanation in radio-active equilibrium with its products A, B, and C, is changed by heating above 1000° C.

(2) The effect increases with the temperature up to 1200° C., and possibly beyond this temperature.

(3) The effect increases with the time of heating for about the first hour, but subsequent heating is without effect.

PARIS.

In

Academy of Sciences, February 12.-M. H. Poincaré in the chair. Some properties of the a rays emitted by radium and by bodies rendered active by radium: Henri Becquerel. Some experiments of Prof. Rutherford recently published have led the author to repeat some of his earlier work on the deviation of the radium rays. the present paper full details are given of the strength of the magnetic field, and the dimensions and arrangement of the apparatus. As a result, M. Becquerel definitely rejects the interpretation deduced by him from his earlier experiments and the hypothesis of an increase in the radius of curvature along the trajectory, and accepts the explanation of Prof. Rutherford, all the measurements confirming the existence of a reduced velocity for the a rays when traversing a leaf of aluminium. There is no difference in the behaviour of a rays arising from radium salts or from bodies rendered active by the emanation.-The internal pressure of fluids and the equation of Clausius : E. II. Amagat.-Some lemmas relating to quasi-waves of shock: P. Duhem.-Observation of the eclipse of the moon of February 9, 1906, made at the Observatory of Paris: P. Salet. Note on the time of contact, with especial reference to the difference observed between the photographic and visual observations in different eclipses. --Observations of the Brooks comet (1906a) made at the Observatory of Algiers with the 31.8 cm. equatorial: MM. Rambaud and Sy. Observations on the apparent positions of the comet and the positions of the comparison stars were made on January 31 and February 2. January 31 the comet had the appearance of a round nebulosity with an eccentric nucleus, with a lustre comparable with that of a star of the eleventh magnitude.-Observations of the sun made at the Observatory of Lyons with the 16 cm. Brunner equatorial during the third quarter of 1905: J. Guillaume. Fifty-six days were available for observations during the quarter, the results of which are summarised in three tables showing the number of spots, their distribution in latitude, and the distribution of the faculæ in latitude.-Integral functions: Ed. Maillet.-A

On

hyperelliptic Hessian: Louis Remy.-The extinction of a solitary wave propagated along a horizontal elastic tube : A. Boulanger. A comparison of the time of discharge in an X-ray tube and of a spark in series with the tube producing the rays: Bernard Brunhes. Remarks on a recent paper by M. André Broca, and directing attention to a paper published by the author in 1900 on the same subject. The recombination of the ions in saline vapours: G. Moreau. The ions of salt vapours, both by their mobilities and by the values of the coefficient a, for temperatures between 170° C. and o° C., are intermediate between the ions of ordinary gases and the large ions due to the oxidation of phosphorus. Their mass diminishes as the temperature rises, and in a flame, for the negative ion, they become comparable with kathodic particles, and, for the positive ion, with the atom of hydrogen.-Remarks on the combinations of the rare metals of the cerium group and on their sulphates: Camille Matignon. A reply to a claim for priority made by M. Otto Brill.-Calcium iodomercurates: A. Duboin. These salts are prepared by alternately adding calcium iodide and mercuric iodide to water, finishing with a slight excess of the calcium salt. The solution had a density of 2.89 at 16° C., and three crystalline compounds were isolated from the solution. -The existence of sulphides of phosphorus: H. Giran. Various mixtures of phosphorus and sulphur were heated in sealed tubes to 200° C., and the melting points taken after solidification. The results are given graphically. The four maximum points correspond exactly to the proportions of sulphur indicated by the sulphides P,S,, P2S,, P2S and PS The preparation and properties of strontium : MM. Guntz and Ræderer. Strontium amalgam is heated in a current of hydrogen until the whole of the mercury is expelled, strontium hydride remaining. This hydride, heated in a vacuum at 1000° C., is dissociated, the vapours of strontium being condensed a cool tube. The metal thus produced contained 99-43 per cent. of strontium, and was utilised in redetermining some thermochemical data.-The action of some esters of some dibasic acids on the halogenmagnesium derivatives of the primary aromatic amines : F. Bodroux. The constitution of the sulphates chromium: Albert Colson.-The existence of bicarbonates in mineral waters, and on the supposed anomalies of their osmotic pressure: L. C. Maillard and Lucien Graux. For one specimen of mineral water it is shown that the cryoscopic results are not opposed to the idea of the existence of bicarbonates in mineral water.-A new mode of extraction of oil of anise: Ph. Eberhardt. The oil can be extracted from the leaves as well as the fruit.-The anti-coagulating power of the blood serum of the lower animals J. Sellier. Serum extracted from some fishes and invertebrates has the power of preventing the coagulation of milk by rennet. The annelids of the Red Sea : Ch. Gravier. The salivary glands of the snail (Helix pomata): M. Pacaut and P. Vigier. The mechanism of the pathological modality special to each organ in the course of a general disease: A. Charrin.

on

DIARY OF SOCIETIES.

THURSDAY, FEBRUARY 22.

of

ROYAL SOCIETY, at 4.30.-On the Coefficient of Viscous Traction and its Relation to that of Viscosity: Prof. F. T. Trouton, F.R.S.Contributions to our Knowledge of the Poison Plants of Western Australia. Part I. Cygnine: E. A Mann and Dr. W. H. Ince. INSTITUTION OF ELECTRICAL ENGINEERS, at 8-Crane Motors and Controllers: C. W. Hill.

FRIDAY, FEBRUARY 23. ROYAL INSTITUTION, at 9.-The Internal Architecture of Metals: Prof. John O. Arnold. PHYSICAL SOCIETY, at 5.-A Note on Talbot's Bands: J. Walker.Secondary Röntgen Radiation: C. G. Barkla.-Records of the Difference of Potential between Railway Lines, and a Suggested Method for the Observation of Earth-Currents and Magnetic Variations: C. W. S. Crawley and F. B. O. Hawes.

INSTITUTION OF CIVIL ENGINEERS, at 8.-The Graphical Determination of the Deflection of Beams: C. H. Sumner.

ROYAL GEOGRAPHICAL SOCIETY, at 8.30.-Travels on the Boundaries of
Bolivia and Peru: Baron Erland Nordenskjöld.
INSTITUTE OF ACTUARIES, at 5.-On a Form of Spurious Selection
which may arise when Mortality Tables are Amalgamated: W. Palin
Elderton.
TUESDAY, FEBRUARY 27.

ROYAL INSTITUTION, at 5.-Food and Nutrition: Prof. W. Stirling.
ANTHROPOLOGICAL INSTITUTE, at 8.15.-Anthropological Notes from
Lake Tanganyika: W. A. Cunnington.

INSTITUTION OF CIVIL ENGINEERS, at 8.-Adjourned Discussion: A Plea for Better Country Roads: G. R. Jebb.-Country Roads for Modern Traffic: J. E. Blackwall.

WEDNESDAY, FEBRUARY 28.
SOCIETY OF ARTS, at 8.-London Traffic: Captain G. S. C. Swinton
THURSDAY, MARCH 1.
ROYAL SOCIETY, at 4.30.-Probable Papers: Experimental Inquiry into
the Factors which Determine the Growth and Activity of the Mammary
Glands: Miss J. E. Lane-Claypon and Prof. E. H. Starling, F.R.S.-
The Specificity of the Opsonic Substances in the Blood Serum: Dr. W.
Bulloch and G. T. Western.-The Internal Anatomy of Stomoxys:
Lieut. F. Tulloch.
Part IV.

CHEMICAL SOCIETY, at 8.30.-Studies of Dynamic Isomerism.
Stereoisomeric Halogen Derivatives of Camphor: T. M. Lowry.
ROYAL INSTITUTION, at 5.-The Physiology of Plants: F. Darwin,
F.R.S.

LINNEAN SOCIETY, at 8.-On a New Type of Stem from the Coal-
measures: Dr. D. H. Scott, F. R.S.--Notes on Some Species of Nereis
in the District of the Thames Estuary: Dr. H. C. Sorby, F.R.S.
CIVIL AND MECHANICAL ENGINEERS' SOCIETY, ar 8.-Coast Lines
Protected by Chain Cable Groynes: R. G. Allanson-Winn.

The New Orleans Meeting of the American Associa tion

Practical Science for Schools. By Prof. J. Perry, F.R.S.

401

402

402

406

407 408

The Nile Quest. (Illustrated.) By J. W. G.

The Time of France. By H. P. H.
The Colombian Earthquake
Notes. (Illustrated.)

Our Astronomical Column:

Comet 1906a.

University and Educational Intelligence Societies and Academies

Diary of Societies.

THURSDAY, MARCH 1, 1906.

MATHEMATICAL ASTRONOMY. The Collected Mathematical Works of George William Hill. Vol. i. Pp. xviii+363. (Washington: The Carnegie Institution, 1905.)

IT

advance which their solution implies. The latter paper was the first which threw any real light upon the general problem of three bodies, and it is well worth notice how large a part arithmetic plays in its success. The analysis is pregnant in the extreme, but it is the actual calculation of a whole sequence of periodic orbits which a moon might occupy that gives it shape and name.

If this memoir may be said to be the first significant word on the problem of three bodies, the former one, on the motion of the lunar perigee, seems to be almost the last word on a question that had outrun calculation from Newton's day to Delaunay's. It is doubtful whether the more determined effort to calculate this quantity was made by Newton or

under a certain Dr. Strong. Dr. Strong "était un homme de tradition, un laudator temporis acti; pour lui Euler était le Dieu des Mathématiques, et après

lui la décadence avait commencé; il est vrai que c'est là un dieu que l'on peut adorer avec profit," and if it led Hill to the study of originals, we may overlook the depreciation of the moderns. From New Jersey he went to Cambridge to continue his studies at Harvard; very soon here, by a paper contributed for a prize to a mathematical miscellany, he attracted the notice of Runkle, the editor, who was Newcomb's predecessor at the office of the American Ephemeris. Hill became attached to the Ephemeris as computer, and remained in discharge of these duties for thirtytwo years. At first he worked at his own home, as was then the custom; but under Newcomb's management, and in order to complete his theory and tables of Jupiter and Saturn he lived for some years at Washington, incessantly absorbed in his task. “The only defect of his make-up of which I have reason to complain," Newcomb has written, "is the lack of the teaching faculty." In 1892 he withdrew to the little farm where his boyhood was passed, and where he still lives, asking nothing but the liberty to continue his labours.

The present volume carries us up to 1881, and includes most, but by no means all, of his best known papers. The essay which attracted Runkle's notice is No. 3, "On the Conformation of the Earth," and was written at the age of twenty-three. It is perhaps not of any permanent importance, yet it is marked by the clearness and the firm hand of his later writings and the same salutary determination that theory should give an account of itself arithmetically. It is natural to compare it with Stokes's memoir "On the Variation of Gravity," written some twelve years before, when he also was a young man, and the comparison shows strikingly how Stokes is the physicist and Hill the analyst.

The two great memoirs by which Hill is best known are No. 29, "On the Part of the Motion of the Lunar Perigee which is a Function of the Mean Motions of the Sun and Moon," and No. 32, "Researches in the Lunar Theory." These writings have been greatly praised, but it seems impossible to praise them too highly, whether for their difficulties or the way these are overcome, or the greatness of the

approximation they attained were very different, they had this in common, that they proved the inadequacy

of the methods employed. Hill first, with the smoothness of a conjurer, gives form to the intractable equations, and then shows how the solution is contained in a certain transcendental equation, an infinite determinant. It affords striking evidence of Hill's

power to contrast his treatment of this determinant with that of Adams, who followed a similar route, sed

longo intervallo, as he said himself. The complexity arising from an infinite sequence of equations might

seem to preclude any general conclusions from being drawn, but Hill uses this very feature in the most beautiful manner to derive the eliminant in a transcendental form in the shape cos c= a known quantity, and from this equation determines c, the required ratio. The secret of the success is now apparent. c is nearly equal to unity; hence it is very much easier to approximate to cos c, where we are in the neighbourhood of a stationary value, than to c directly; and though the difficulty recurs when we seek to find the arc c from a cosine in the neighbourhood of its minimum, it is then an insignificant one, for we are past the true complexities of the problem.1

The remaining papers are naturally not of equal moment with these, but we may be grateful to the Carnegie Institution for making them accessible in the present collection. Several of them arose in con

a

nection with Hill's duties as computer to the Ephemeris, but even on such hackneyed subjects as eclipse computing and reduction of star places he has something good to say. He is a true artist; nullum quod tetigit non ornavit. Of considerable general interest are No. 18, "Remarks on the Stability of Planetary Systems," and No. 14, "A Method of Com puting Absolute Perturbations," which contains rescension of Hansen. Even the smaller papers, like No. 22, "On the Solution of the Cubic and Biquadratic Equations," are usually marked by some analytical felicity that makes one wish that Hill had been able to bring his great powers to bear upon a material not so invariably intractable and overloaded with tradition, and limited in its problems, as practical astronomy. But if we feel that his hand

1 The point of th's approximation is put somewhat incorrectly by M. Poincaré in his introduction.

66

the other hand, sedimentation rapidly takes place under the influence of gravity, and its rate is little influenced by the withdrawal of water or by the addition of electrolytes. The suspension may be reformed by purely mechanical means.

(3) Alterations in the total energy of the system are frequently associated with the process of coagulation. These have been measured in several cases by means of the calorimeter.

(4) Colloids in solutions are capable of undergoing reactions with one another, which closely simulate purely chemical reactions.

The next section of the book deals with the classification of colloid solutions or hydrosols. The classifications of the hydrosols have been based on two principles, namely, the size of the particles and the reversibility or irreversibility of the hydrosol (Hardy). On plate i. the author gives a graphic representation of a classification of colloids founded on these principles. The reversible colloids differ from the irreversible in not being readily coagulated by the addition of electrolytes. It is noteworthy that irreversible colloids may be partially protected from the coagulating action of electrolytes by the addition of a reversible colloid to their solutions. Great quantitative differences are found to exist in the extent of protection given by different reversible organic colloids to irreversible gold hydrosols.

tion would lead to a further limitation of the term "solution." In order to include the colloids Zsigmondy defines solutions as uniform distributions of solids in fluids, which are transparent to ordinary light, and not separable into their constituents by the action of gravity or by filtration.

In order to gain a clearer idea of the nature of colloid solutions, it is necessary to find criteria for distinguishing such solutions from those of crystalloids on the one hand and from suspensions on the other. Zsigmondy only refers very briefly to the distinguishing characteristics of the former, as this subject has been previously treated by Bredig in his monograph on Inorganic Ferments." He deals more fully with the properties of colloid solutions which distinguish them from suspensions. In this connection he mentions the following as the chief features distinguishing colloid solutions from suspensions :

(1) The particles in colloid solutions are much smaller than in suspensions. In colloid solutions the average diameter of the particles varies from 5 to 20 μ.μ. This difference is, however, one only of degree.

(2) Many colloids are capable of undergoing irreversible changes. Separation of a metal from its colloid solution may be readily brought about by the withdrawal of water or the addition of electrolytes. In this process the metal has undergone an irreversible alteration or coagulation. For the reformation of the colloid solution, chemical or electrical means must be employed. In the case of suspensions, on

A historical account of the preparation and pro

next section of the book.

The author next gives an interesting account of the development of the method of ultramicroscopy by Siedentopf and himself. A full description is also given of the necessary apparatus and of the method of using it.

The succeeding sections give details of the results of his own researches on gold hydrosols. By means of the ultramicroscope he was enabled to determine approximately the average size of the gold particles, their colour, and the rapidity of their movements both translatory and oscillatory. The limit of size determinable by the ultramicroscope appears to be about 6 μ in the case of gold hydrosols. Still smaller particles (amicrones) are also present in gold hydrosols. Their presence may be proved by the coagu lation of the hydrosols on the addition of electrolytes.

An excellent summary is also given of the results obtained by other observers through examination of various colloid solutions by means of the ultramicroscope.

Brief reference only is made to some points of great theoretical interest, namely, the causes of the stability of colloid solutions, and the mechanism of their formation.

The book concludes with a short summary of what is known with regard to the products of coagulation of colloid solutions.

The work as a whole is to be regarded as a valuable monograph indispensable for those interested in the ultramicroscope and its applications.

J. A. MILROY.