NEW BOOKS The Physics and Chemjstry of Surfaces. By Neil Kensington Adam. 84 X 16 cm; pp. 2 S82. Oxford: The Clarendon Press, 1990. Price: $6.00. “In this book the theory of surfaces is discussed from the standpoint of molecular theory. The subject of Capillarity, formerly one of the less interesting departments of Physics, has recently been transformed
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into amost fascinating study by the introduction of those ideas about molecules which have formed the basis of the theory of Organic Chemistry during the last three-quarters of a century. If the molecules are treated as tangible objects, with the shapes and mechanical and chemical properties required by the theory of Organic Chemistry, a flood of light is thrown on many phenomena hitherto classed as physical; the discovery of this intimate connexion between Physics and Organic Chemistry is likely to be of great service in advancing both sciences. It seems that an attempt to present the theory of surfaces consistently from this point of view may be worth while,” p. v. The book is divided into nine general parts: liquid surfaces, capillarity; surface films of insoluble substances; the size, shape, and other properties of organic molecules, deduced from X-ray studies and other sources; surface films of soluble substances, adsorption; results of the measurement of surface tension; solid surfaces, general properties; spreading and lubrication; fine structure of solid surfaces, adsorption and catalysis; the measurement of surface tension. “The free energy in the surface is of fundamental importance; a vast number of problems relating to the equilibrium of surfaces can be solved without knowing more than the magnitude of this free energy. In the solution of such problems a mathematical device is almost invariably employed to simplify the calculations;.it is to substitute for the surface free energy a hypothetical tension, acting in all directions parallel to the surface, equal to the free surface energy. This is what is generally known as the surface tension. It is always possible, mathematically, to replace a free energy per unit area of surface by a tension acting parallel to the surface. Such a tension has, of course, the same dimensions as a surface energy (mass/time.)Z; and it must have the same numerical magnitude. The work done in extending a surface which is pulling with a tension y dynes per cm., by one square cm., will be y ergs per sq. cm.: hence the free surface energy of such a surface will be y ergs per sq. cm.,” p. 3. “The great convenience of the hypothesis of surface tension, as the equivalent of free surface energy, combined with the fact that it was in use nearly a century before the conception of energy became definite, has given the words ‘surface tension’ a predominance in the literature of surfaces, which does not rightly or logically belong to them. The term ‘surface tension’ has often been strained to imply that liquids have in their surfaces some mechanism like a stretched membrane pulling parallel to the surface. The surface is said to be in a ‘state of tension.’ This view must not be pushed too far. Any mechanism possessing free energy in the surface will undergo the spontaneous contraction which has led to the ideas of surface tension; hence we can gain practically no idea of the actual nature of mechanisms in the surface, from this fact of spontaneous contraction alone. The view that there is some skin in the surface, pulling parallel to it, leads to great difficulties when the structure of the supposed skin is considered in terms of molecules,” p. 4. “There is very strong evidence that the change of density from liquid to vapour is exceedingly abrupt, the transitional layers being generally only one or two molecules thick. Perhaps the clearest evidence is that derived from the nature of the light reflected from the surfaces. According to Fresnel’s law of reflection, if the transition between air and a medium of refractive index n is absolutely abrupt, the light is completely plane polarized, when the angle of incidence is the Brewsterian angle, that is tan-%. But if the transition is gradual, the light is elliptically polarized. This happens to be a test of such sensitiveness that it will
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detect layers of the order one molecule thick. Jamin and others had found that the light reflected from most solid and liquid surfaces usually deviates considerably from this law, but Rayleigh showed that when the accidental layer of grease was carefully cleaned away from a water surface practially no trace of ellipticity remains. I t is known now that the layer of grease on water is only one molecule thick; hence the practical absence of ellipticity indicates that a water surface is a definite transition, within the thickness of about one molecule. “Raman and Ramadas, in recent experiments, find that there is still some small amount of residual ellipticity in their cleanest surfaces of water, and also that these scatter light to some extent. There appears t o he a slight discrepancy between these results and Rayleigh’s, but both agree that the transitional layer is about one molecule thick; the slight residual ellipticity is ascribed to the thermal agitation of the water molecules at the surface,” p. 7 . In capillary tubes “the liquid is not pulled up the tube by a hypothetical surface tension pulling on the walls, as is suggested by the explanation found in so many elementary textbooks-it has never been clear to the author what is the hook on the wall to which this surface tension attaches itself, nor how the hook contrives to move up the tube in advance of the rising meniscus. Nor is it driven up by the squeezing action of the attractions exerted by the walls of the tube on the layers of molecules remote from the Falls, as was suggested by Leslie, a suggestion which has been revived recently. The energy relations determine what is the stable contact angle; the fluidity of the liquid permits the molecules to move about till they rest a t this stable contact angle; the contact angle and the curvature of the tube curve the liquid surface; the pressure difference follows from the free energy resident in the surface, and the liquid then flows up the tube under the hydrostatic pressure,’’ p. 16. “The vapour pressure over a convex surface is grearer than that over a plane; and over a concave surface it is less. The difference depends on the fact that condensation of vapour on a small convex drop of a liquid increases its surface area, so that the surface tension tends to oppose the condensation and to increase the vapour pressure. On a plane surface condensation does not alter the surface area, and on a concave s;rface the surface area is diminished by condensation of more vapour,” p. 31. If the formula holds, a drop of water with a radius of 1 0 - 6 mm will have nearly three times the normal vapour pressure, while one with a radius of 10-6 mm will only show an increase of about eleven percent, p. 22. “It happened that all the oils used in the earlier researches formed films in which the thermal motions could be neglected, as the molecules had such a large attraction for one another laterally in the films that they were united into compact masses much too large to show independent thermal motion. For such coherent films Pockel’s law that the surface tension does not drop appreciably till the critical point is reached, and then drops suddenly, is true,” p. 28. “Langmuir established the existence of one type of coherent film, with insoluble substances, and of the gaseous films with soluble substances. Adam’s work has made it clear that four types of film exist with insoluble substances. These four types vary in the amount of lateral adhesion existing between the molecules. “There are ( I ) condensed films, in which the molecules are closely packed, and very steeply oriented to the surface, as with the acids and alcohols investigated by Langmuir; (2) gaseous films, in which the molecules are separate, moving about independently, and exerting a surface pressure by a series of collisions on the barriers; (3) liquid expanded films, in which the molecules adhere strongly to each other laterally, though the packing and adhesion is less strong than in the condensed films; and (4) vapour expanded f i i s , in which there is a great deal of adhesion, but not enough, as in types ( I ) and ( 3 ) , to keep the molecules together in islands. Class (4) is rather ill defined, and shades into.the gaseous films; but its properties are often so different from those of the gaseous films that a separate name seems desirable,” p. 43. “The liquid expanded films are liquid and coherent, forming a separate surface phase from the gaseous film, but their area is much greater than that of the condensed films. Since expansion does not occur at a single temperature, but over a range of about 1 2 O , and
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since the transition pressures between the expanded and condensed films are not constant, the change from the condensed to the expanded films is not, from the strict thermodynamical point of view, a change from one surface phase to another. “The coherence of the film (low vapour pressure) points to the molecules being in contact; the areas to their being neither upright nor horizontal. It seems perhaps most likely that the molecules are in a state of constantly changing orientation, from vertical to horizontal; but the difficulty in this view is to account for the film being definite in area, and coherent. The molecules may be rotating about a vertical axis, while considerably tilted; having regard to the fact that the chains are jointed at each carbon atom, it seems likely that they a d whipping about as well as rotating. The difficulty in these suggestions is that there is no obvious reason why the tilting or whipping should not continue until all the molecules lie flat, and to account for the cohesion,” p. 74. “Proteins have been examined by Gorter and Grendel, who found that the extent of spreading varies with the acidity of the water. Oxyhaemoglobin was the most thoroughly studied. Assuming that the molecule was not associated, and had the molecular weight 16,000, the spreading on neutral solutions was a t first quite small, increasing to a final value of about 1,500 sq.A. at no compression. On dilute acid, the final maximum area was practically instantaneously reached and was larger, about 3,200 sq.A. On buffered solutions of less acidity, intermediate values were obtained; the attainment of the final area was accelerated by a rise of temperature. “It appears that the maximum spreading is found when practically all the COXH groups in the molecule are in contact with the water, so that the molecule is lying flat and uncoiled. An isolated protein molecule would tend to coil up into a compact form, because of the mutual attraction of the different parts of the large and complicated molecule. On neutral solutions, it appears to take time to uncoil the molecule so that all the COSH groups are in contact with the water, and the process may never reach completion; on acid solutions, the attraction between water and CONH groups must be larger, since acid tends to hydrolyse these groups, and so the fully spread state is more easily reached. It should be noted that the spreading of a huge coiled-up molecule such as a protein probably differs little from that of a liquid composed of smaller molecules, apart from the fact that the single molecule cannot be further disintegrated, once every water-attracting group touches the water. Compression to 20 dynes diminished the area, in the fully spread films, to about half; the incompletely spread films were also rather compressible. As there is, between the C O S H groups, always some length of hydrocarbon chain, the diminution of area on compression may be due to the COKH groups being squeezed closer together, bending the hydrocarbon chains out of the surface. The protein films appear to be coherent, but measurements a t low surface pressures have not yet been made,” p. 80. “We find evidence from the X-ray studies just as from the surface films, that structures are present whose shape and construction, so far as it has yet been discovered, are very much like that of the models which were constructed to explain the results of Organic Chemistry. They have the mass that is attributed to the chemical molecules, and they have also, as nearly as can be estimated a t present, the same size and shape that was deduced from experiments on surface films. The conclusion cannot be resisted, that we are throughout dealing with the same molecules, whether their properties are discovered by Organic Chemistry, by surface films, or by the X-ray survey of crystals. Consequently the three methods can now eo-operate, and form a most powerful combination of instruments wherewith to explore the properties of molecules. Of the three methods, that of Organic Chemistry is the most fully worked out; that of the surface films is somewhat limited in its application, while that of the X-rays is still in its infancy, and gives almost untold promise of discoveries to come. I t should eventually reveal the exact position of every atom in the crystal, and from that it should be a perfectly feasible step to discover the forces between the atoms,” p. 93. The author postulates, p. 98, that graphite can be oxidized to mellitic acid; but this is probably not true, which rather plays havoc with his reasoning. “If there are two constituents of a solution, and the molecules of one have a higher attractive field of force round them than the molecules of the other, then if those mole-
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cules are present at the surface, a liquid with a comparatively high surface tension or free energy will result. If the molecules with the smaller fields of force are at the surface, the surface tension or field of force exposed at the surface will be smaller. Therefore, since the molecules are free to move, the liquid will always tend to cover itself, a t the surface, with the molecules of the constituent possessing the smallest field of force. The composition of a solution therefore always alters, so as to concentrate a t the surface those constituents which have, intrinsically, the smallest free surface energy. Thia concentration of constituents a t surfaces is called ‘adsorption’; positive adsorption is an increase of concentration, and negative, a decrease of concentration at the surface,’’ p. III. “The work of adsorption increases by a constant amount for each CHs group added to the hydrocarbon chain of the fatty acids. This must mean that each CHngroup is situated in the same relation to the surface as every other such group in the chain, and this can only be the case if the chains lie parallel to the surface. Hence Langmuir concluded that the molecules lie flat in the surface, in the gaseous adsorbed films. It haa been shown in Chapter I1 that there are two other independent lines of evidence that the molecules of the insoluble films lie flat, in the gaseous state. There appears thus to be no distinction, except that of solubility and to some extent of lateral adhesion between the molecules, between the adsorbed films of soluble substances, and the insoluble films spread on the surface. “This continuity points strongly to the adsorbed films being one molecule in thickness. Future research alone can show why there is a discrepancy between this conclusion and that drawn by McBain (p. I I ~ ) ,that the adsorbed films are more than one molecule thick,” p. 129. “Schofield and Rideal have calculated the adsorption of alcohol at the surface of mixtures with water, using the exact form of Gibbs’s equation, and solutions ranging from pure The adsorption increases, up to a maximum a t about 0.25 mol fraction; but a t higher concentrations of alcohol it apparently decreases, finally becoming nearly constant at little more than one-third of this value when the mol fcaction is about 0.7. The maximum adsorption corresponds to an area per molecule of 24 sq.A, which is a nearly close-packed monomolecular film. This apparent drop in the adsorption is probably to be explained, not by any decrease in the alcohol concentration in the outer layer, but by the formation, below the outer layer of alcohol, of a second layer of water,” p. 130. On p. 152 is the statement that “in the case of ordinary liquids, the author knows of no clear evidence of a change in surface tension with time (except of course when the time is sufficiently long for contamination from outside to occur).” It might be well to study the benealdoximes or any of the cases in which there are two forms in the melt which do not change rapidly one into the another. . The discussion of Eotvos’ law, p. 155,is not very satisfactory. While it certainly does not hold in its original form, a good deal better case can be made out for it than would be guessed from these pages. The author believes in the contact angle, which he has a perfect right to do; but he does not mention that many people do not believe in the existence of a contact angle, and that is rather serious. He also does not explain how it is that no person who makes accurate measurements ever fmds a contact angle. The author is also rather shaky on the subject of emulsions. He would still like to believe in the wedge theory of emulsifying agents, apparently because he can form a clear mental picture of that. “One thing seems certain that some degree of flexibility in the molecules attached to the faces in the films is necessary to reduce friction. Water and glycerine have good adhesion to most solid faces, but they are not boundary lubricants a t all. The molecules in the aliphatic series steadily improve as lubricants as the chains are lengthened, and the molecules acquire additional powers of bending to a deforming force. Some results in ( b ) , though perhaps less accurate than those just described, support this view in a general way. Ring compounds are usually rather poor lubricants, especially when, as with hydroquinone and salicyclic acid, there are two active groups in the molecule to adhere to the face (possibly the ring lies flat on the surface in these compounds, which only diminish t h e friction on
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bismuth faces by about 0 . 1 ) . Pentaerythritol also is a poor lubricant; this compound has four CH20H groups on one carbon atom, and if these are all adhering to the solid face, the molecule cannot have much flexibility. I n any extension of this work, it may be worth testing this hypothesis of flexibility more directly, but it already appears to the author practically certain that ‘oiliness’involves flexibility in the molecules of the film,” p. 227. “Catalysis a t solid surfaces is conditioned by two main factors: the adsorption of the molecules or atoms of the reacting substances at the surface, and their activation so that they react, when perhaps without this activation they would undergo no change. The action of promoters may he due to an effect either on the adsorption, or on the activation, of the adsorbed molecules. A simple case in which two solid substances in contact would be necessary for catalysis of a gaseous reaction to occur, simply because of the effect on the adsorption, would be the following. Suppose a reaction between two gaseous constituents, which could only occur when the two kinds of adsorbed molecules were in contact with one another on the surface, and suppose one gaseous constituent is adsorbed by one of the solid substances, and the second gas by the other, no adsorption of the gases occurring except on these particular substances. Then the only chance for adsorption of the two constitutents side by side would be a t the boundary between the soild substances on the surface, and the reaction would be catalyaed by neither solid substance separately, but would be catalysed by both together. “Another possible mode of action of the second solid would be to adsorb, and render harmless, a small amount of impurity in the reacting gases, which would poison a reaction really catalysed solely by the first solid, if this poison were not removed. “A third and very important way in which a promoter may act is by a peculiar state of the surface atoms, where two solid substances or phases meet, causing a peculiar reactivity in molecules or atoms which are adsorbed a t this surface. There is plenty of evidence that the atoms of solid subst,ances, a t the one-dimensional boundary or line where two solid phases meet on a two-dimensional surface, are often in an unusually reactive condition. This special state of the boundary surface atoms would very likely induce an unusual activation in molecules or atoms adsorbed a t the one-dimensional interface,” p. 236. Palmer and Constable found “that the activity of the [copper] cat,alyst [for the dehydrogenation of alcohols], produced by decomposition at a given temperature was alwayspractically the same, no matter whether one of the oxides, the formate, or a longer chain salt was used as the compound to he decomposed. The longest chain salt used was the valerate, which has five carbon atoms in the molecule; so that the spacing apart of the copper atoms in the original salt is clearly a matter of little or no importance to the activity of the catalyst resulting from its decomposition,” p. 275. “Recent work by Sheppard and the research staff of the Eastman Kodak Company has brought to light a very interesting case of the effect of a small local change in a solid surface. I n searching for the constituent of gelatine which is responsible for the increased sensitivity to light acquired by silver halides in the presence of gelatine, and is the basis of the efficiency of the photographic dry plate, it was found that the sensitivity was not due to any ordinary known constituents of gelatine, but to the presence of minute, accidental amounts of either organic isothiocyanates, or thioureas. Probably the sensitizing substance present in most samples of gelatine is allyl isothiocyanate, CaHsN:C:S. This substance and a few similar substances, which must, however, contain the C:S group, reacts with the surface of the silver halide grains, forming local patches of silver sulphide. In the neighbourhood of these patches, the stabilit,y of the silver halide is reduced, so that light produces decomposition more readily than elsewhere, thus starting a centre of reduced silver atoms which is developable by the usual developers. The proportion of the sulphur-containing compound in the gelatine is about one part in 300,000;much more than this produces fog, the bromide grains being rendered too unstable,” p. 282. “As to the mechanism of activation, Quastel makes the definite suggestion that it is due to an intense local electric field situated at some region of the active patch, which disturbs the arrangement of the electrons in the adsorbed molecules, so that they become reactive (a). I n this way he predicts that many molecules will become reactive a t the B carbon atom,
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so that the well-known biological oxidation a t that point of the chain will be induced to occur. The cause of these powerful electric fields is thought to be due to deformation of the molecules which form the building-stones of the cell surfaces, a particular building-stone being perhaps placed where it does not fit particularly well, and becoming distorted by the attractions of other parts of the patch. In this way groups which might otherwise combine chemically may be kept apart, and an intense local field of force be developed. These views are naturally somewhat Fpeculative and difficult to test at present, since the experimental technique for mapping the fine architecture of such complex surfaces is as yet in its very early infancy, and the reader must be referred to the original papers for details of the theory. Whatever the activating mechanism, however, its seems certain that the adsorption is fairly specific, and that, for activation and catalysis to occur, the specific adsorptLon must he followed by a specific activation, which can only occur if a particular structure is present,” p. 288. T i l d e r D. Bancroft Gmelins Handbuch der anorganischen Chemie. Edited by R. J . Meyer. System-number 69:Al and -42. Iron. Eighth editzon. 26 X 18 cm; p p . 312. B e r h Verlng Chemie, 1929. Price: 5’3 marks. There are to be two volumes on iron, the A volume dealing with the historical part, with the occurrence of the ores, and with the properties of metallic iron and its alloys. The B volume will deal with the compounds of iron. The historical part consists of a rather exhaustive bibliography, coming down to the beginnings of the Bessemer process. This is taken as the dividingline between yesterday and today. Under occurrence of iron, p. 61, the author adopts the general views of Washington and of Goldschmidt, that the center of the earth consists of an iron nucleus with a radius of about 2300 km. and a density over eight. Around this is an intermediate layer about 1700 km. in thickness, and having a density of about five or six. Outeide of this is a silicate shell, about 12 km. thick, and n i t h a density which drops from four in the innermost layers to about 2.8 in the outermost layer. The outermost layer is what is known as the earth’s crust. “In general an iron ore, nowadays, contains 45-557c of iron; but the locality counts. I n middle Europe one might be able to work a thirty percent deposit a t a profit, while an iron ore containing seventy percent of iron might be quite valueless if it were in some parts Of Africa, for instance. The purest iron ore in Europe comes from Sweden; but there is apparently an enormous amount of iron ore in Finland, which is low in manganese, p. 89. T h w ingite is green and contains 3 1 - 3 9 s FeO and 13.5% FeOj. Vindite is green and contains aboub 51 GFeO and 1 0 . 7 5 Fe20a,p. 159. Olivine is also green and has a high content of ferrous oxide relatively to ferric oxide, pp. 159, 166; but nobody seems to have tried to make any of them blue by reducing the magnetite further and increasing the percentage of ferrous oxide. Glaucophane is light blue to dark blue, and contains about five percent ferrous oxide and 1 - 9 s ferric oxide, p. 172. Pennin can apparently have any color from green through pink and violet to yellow and silver white, p. 170. The second part of the A volume contains the physical properties of pure iron and the electrochemical behavior, T x o isotopes of iron Fe(54) and Fe(56) with a ratio of I : 2 0 seem to be well-established. Harkins postulates the existence of Fe(j2), p. 225. The present value of the atomic weight is gj.84, p. 228. The eo-ordination number is 6, though values of 8 and 12 appear also to have been found, p. 229. From experiments with ammonia a t 500 mm, the affinities of the iodides come out: Xi > Co > Fe > &In> Cu > Cd >Zn > Mg, p. 229. The present electrochemical series is: Li, K, ?;a, Mg, Zn, Fe, Cd, Co, Ki, Pb, Sn, H, CU,Ag, Au, p. 302. If the metals are amalgamated, the electrochemical series is supposed to be: Zn, Cd, T1, Sn,Pb, Cu, &In, Fe, Bi, Co, Hg, Ni, Pt, p. 303. V’ilder D. Bancroft
The Aims of Mathematical Physics. By E. A. Milne. E$ X 16 cm; p p . 28. Oxford: The Clarendon Press, 1929. Price: 70 cents. This is an inaugural lecture delivered by the Rouse Ball Professor of Mathematics a t Oxford University. One might expect that a lecture on
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mathematical physics by a pure mathematician would not contain much of special interest to the chemist. There are three paragraphs, however, which have to be quoted practically in full. “Mathematical physics is a subject of its own, drawing its inspiration both from observational science and from pure mathematics, but it works in a medium of its own. It has its own thought-processes. Its greatest triumphs are special to itself, and not mere complicated deductions from experiment. I t influences the progress of science by suggestions which do not take their origin in experiment. Often, indeed, it indicates to the experimenter something to look for as a mere consequence of previous experiment. But more often it drops a suggestion out of its own blue. The fusion of ideas from physics with ideas from pure mathematics gives birth to new ideas far beyond deduction from experiment. Pure mathematics can tell us nothing about the outside world; mathematical physics suggests to us what to look out for in the outside world. The considerations brought forward by the mathematical physicist are the result of genuinely mathematical ideas, fermenting in an experimental medium. Mathematical physics is like yeast; it leavens the lump. “Mathematical physics has of course in the first instance its problems suggested by experiment and observation. The mathematical physicist must be a craftsman, competent t o work out problems. A higher form of activity is the formulation of problems; a mathematical situation produced by one problem may suggest another mathematical situation and so lead to another problem for the experimenter, another question to be put to nature. But it is often not the first problem which suggests the second; it is the mathematical situation which is responsible. The greatest role of the mathematical physicist is, however, the conjecturing of laws of nature, where he rises superior to the hypotheses with which he has been presented, and to satisfy some mathematical yearning suggests a new hypothesis as one worth testing. The new hypothesis may be directly inspired by mathematical symbolism. The mathematical physicist is then no longer the servant of his material-he has become its master,” p. 6. “The thermodynamics of Carnot, Clausius, and Kelvin proceeds by a series of syllogisms from one grand induction from experience. But it was left to Willard Gibbs to import a genuinely mathematical idea and so make thermodynamics applicable to chemistry. Willard Gibbs simply introduced partial differential coefficients into a situation in which the ordinary ‘physical’ notion of a partial differential coefficient is physically not realizable. He considered the increase in energy of a system which occurs when its volume and entropy remain fixed but the mass of a chemical constituent is altered. How on earth (or in physics, which is the same thing) one can experimentally import matter into a system without importing or exporting entropy, or indeed knowing what entropy has been imported or exported, I have never been able to see; and the difficulty is largely responsible for the obscurity which is felt when reading Gibbs. The fact is that the idea is a purely mathematical one. Gibbs’s success was due to his breaking away from pure physics and introducing what is in the first instance an ‘unobservable’ (namely, a partial potential) suggested by mathematics. The mathematical physicist is a t work in his own medium. Once this idea has been worked into the subject, the whole of classical analytical thermodynamics follows, P. 9. Wilder D. Bancrojt
The Use of t h e Microscope. By John Belling. 23 X 15 p p . n + $15. Y e w York and London: McGraw Hill Book Co., 1930. Price: $4.00. The title of this book might appropriately be qualified by the phrase “in Cytology and Related Studies”. The author has drawn on his many years of experience in this field, in formulating a guide to the manipulative technique required for obtaining optimum results from the microscope. The usual topics of microscopy are presented, with exceptionally full treatments of binocular microscopes, the water-immersion objective, the proper use of corrected condensers, the importance of cover glass thickness, and the restriction of illumination to the field visible in the microscope. There are rather surprisingly meager discussions of photomicrography, and of the interpretation of appearances, although chapters are devoted to Discoveries with the
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Microscope, The Past and Future of the Microscope. A Hundred Microscopical Ohjects of Biological Interest, and a list of study questions is appended. The growing use of polarized light in the study of biological objects is not discussed, probably because of its limited value in cytology. There is no mention of any of the various fields of technical microscopy or their wide variety of special apparatus, much of which is adaptable to biological investigations. The author evidently presupposes a moderate degree of experience in biological microscopy, and assumes that the theoretical background necessary for an intelligent appreciation of the reasons underlying his methods will be obtained from other works, a useful bibliography of which is given. The presentation is eminently “practical” in tone, with numerous “dont’s,” lists of causes for all possible defects in microscopic images, and rules for their improvement. The glossary of terms is not inclusive enough to be of much help to the student, and is hardly sufficiently precise to clarify the ideas of the advanced worker. A certain vagueness of phraseology throughout the book detracts from the emphasis which many of its valuable suggestions merit. If one makes allowances for this, and for the fact that many of the apparent generalizations are actually limited to cytological microscopy, there is a great deal of information that ought to be useful to all workers who are concerned with “critical” microscopy. The best chapter in the book-“Fifty Practical Exercises with the Microscope”, is a collection of experiments illustrating the chief points which the author stresses. If the reader works through these exercises and looks up the principles upon which they are founded, he will acquire, more clearly and usefully than from pages of discussion, and the technique and theory which are essential to all exacting microscopical studies (whether cytological or not) of fine detail a t high magnifications. C. W . Mason
Leipziger Vortrilge 1929: Dipolmoment und chemische Struktur. Edited by P. Debye. + 184. Ldpzig: S. Hirzel, 1929. Pnce: 9 marks. This small book is the outcome of a week’s course of lectures arranged by Professor Debye in the Summer of 1929: the lectures all being on subjects connected closely with the title of the course. R. Sanger (Zurich) described new measurements of the temperature variation of dielectric constants of gases and vapours, Estermann (Hamburg) lectured on the application of the molecular-ray method to the investigation of the polarity of molecules. J. Errera (Brussels) dealt with various subjects: (i) the alteration of the polarisation of polar bod& with concentration and temperature, (ii) dipolemoments and molecular constitution (in conjunction with Sherill (Brussels)), (iii) molecular association, (iv) atomic polarisation. L. Ebert (Wiirzburg) lectured on the internal molecular motions of large and especially of aliphatic molecules. F. Hund (Leipzig) gave some theoretical remarks on the question of electric movements and of the structure of the molecules. W. Hiickel (Freiburg i. Br.) dealt with dipolemoments and reaction velocity; and K. L. Wolf (Karlsruhe) delivered two lectures, (i) on the absorption spectrum of bisubstituted benzenes, (ii) on the Kerr effect and molecular structure. Dr. Debye is to be congratulated on this stimulatory series of lectures; his enterprise is well worthy of imitation. There is no account given of any discussion in connection with them; but, whether there was any formal discussion or not, one can very me11 imagine t h t informal interchange of ideas which took place in the intervals between the lectures. This small hook is to be highly recommended as a succinct account of a rapidly growing subject. A . W . Porter 28
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ERRATUM The price of Colloid Symposium Annual edited by H. B. Weiser (John Wiley and Sons) was erroneously given in the review (34, 887) as $3.75. The price of the book is $4.50.
