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 PREFACE 
 
 7? OUR booklets have been issued similar in 
 * form to this one, and describing respectively 
 the Abbe Refractometer , the Hilger W^avelength 
 Spectrometer and Nutting Photometer, the 
 Hilger Wavelength Spectrometer and High 
 Resolving Power Accessories, and the Polarimeter, 
 as made by this firm. These instruments are 
 widely and increasingly used by chemists for the 
 identification and estimation of substances, in the 
 control of industrial processes, and so forth. 
 
 But measurements of refractive index, wave- 
 length, absorption, and rotatory power have a 
 deeper significance than appears in such practical 
 applications, and it is the purpose of the present 
 essay to remind the members of our staff and 
 our clients how greatly the study of these pro- 
 perties has already contributed towards our 
 present knowledge of the structure of matter, 
 and what a wide and promising field it offers 
 for further investigations. 
 
 The present, second edition of the essay, is a 
 slightly amplified reproduction of its first edition 
 published in May, 1919. 
 
 ADAM HILGER LTD 
 
 .\ ;:,.;;': ....... ... JUNE, 1920
 
 REFRACTIVE INDEX 
 
 ABSORPTION WAVELENGTH AND 
 ROTATORY POWER in RELATION TO 
 
 MOLECULAR STRUCTURE 
 
 TTNTRODUCTION. Everybody is familiar 
 with the fact of the simple behaviour of light in 
 empty space, in such regions of space, that is, from 
 J. which palpable, ponderable matter has been either artifici- 
 ally removed or highly attenuated, or better still, in those 
 immense interplanetary or interstellar regions where owing 
 to some natural conditions there is, comparatively speaking, 
 but very little left of it. In such empty regions or practical 
 vacua luminous waves are propagated with constant, and for 
 all directions the same, velocity, amounting very nearly to 
 300,000 kilometers, or 3-io 10 cm. per second, independent of 
 intensity or amplitude, of wave-length or colour, and of the 
 state of polarisation. The rays of light, which in this case are 
 generated by the normals to a wave-surface in its successive 
 positions, are, and remain throughout, straight lines. The 
 energy stored in light, although spreading over greater and 
 greater regions, remains invariable in its total amount. Nor 
 does light in empty space show any tendency to change its 
 .nature in any respect whatever, apart from the weakening of 
 its intensity while it inundates ever-growing regions of space. 
 These simple properties are all more or less profoundly 
 modified when light encounters and has to fight its way 
 through material bodies, those marvellously complicated 
 assemblages of atoms and molecules, themselves highly com- 
 plex systems of structural elements still more minute, elec- 
 trons and what not, about which modern science has only just 
 begun to give us some reliable information. This change of 
 propagational and other properties of light due to the 
 presence of matter manifests itself not only in the case of 
 those very dense aggregates known as solid and liquid bodies, 
 but also, to a well-observable degree, in gases under ordinary 
 
 5 
 
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 75A Camdeu Road, London. N.'W.l
 
 conditions. Moreover, and this is a fact not only beautiful in 
 itself, but of the greatest practical importance, the amount 
 and the nature of the modification of light properties is pro- 
 foundly characteristic for each kind of matter, placed under 
 definite, controllable conditions (such as temperature and 
 pressure or the intensity of an impressed magnetic field), in 
 distinction from others. 
 
 K1FRACTIVE INDEX. The most 
 prominent, and longest known, among these changes 
 is what is most familiar as refraction, at any oblique 
 incidence', but what in its deeper aspect is best 
 described as the change the velocity of propagation of light 
 experiences in various kinds of matter, as compared with 
 its velocity in empty space. In fact, what is called the 
 refractive index of a substance is the ratio of the sines of 
 the angles of incidence (from a vacuum or practically from 
 air) and of refraction ; this ratio is, apart from some cases 
 in which absorption plays a very strong part, constant 
 i.e., independent of the incidence angle and its numerical 
 value is equal to the ratio of the vacuum-velocity to the 
 velocity of propagation within the substance (medium) in 
 question. In a piece of glass, for example, of index n= 1*5, 
 the light velocity is but two-thirds of its undisturbed 
 value, i.e., 200,000 km. per second, and so on. And 
 this is true either on the old-fashioned elastic theory or the 
 now firmly established electromagnetic theory, according to 
 which light differs from other electromagnetic waves (such 
 as are used in wireless telegraphy) only by its considerably 
 smaller wave-length. 
 
 Thus we can say in the literal sense of the word that a 
 Refractometer enables us to measure the velocity of propaga- 
 tion of light in a substance, this velocity, being inversely 
 proportional to what is called the index of refraction. Now, 
 the propagation velocity being undoubtedly the result of a 
 very delicate and intimate reaction upon light waves of the 
 molecules and of their configuration within the lump of 
 matter, its value, and therefore that of the refractive index, 
 6 
 
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 75x Camdeu Road, London. N. W. 1
 
 will stand for some very characteristic attributes of the 
 substance in question. In fact, the refractive index, /u, 
 especially as it can now easily be measured to four or five 
 decimal figures,* offers one of the most reliable means of 
 identifying a substance. 
 
 It goes without saying that the value of this index depends 
 not only upon the chemical nature of the substance, but to 
 a considerable extent also on its temperature and pressure ; 
 these, however, can easily be controlled, and it can be said 
 without exaggeration that the value of the refractive index, 
 even if accurate only to four decimals, together with 
 the temperature and pressure under which it has been 
 obtained, are much more characteristic for a substance 
 than, say, its specific gravity, even when measured with 
 very refined means. 
 
 DISPERSION. We have hitherto spoken 
 of the propagation velocity and of the refractive 
 index of a substance only for the sake of shortness 
 and in order to keep the various aspects of the 
 action of matter upon light a little apart from one another. 
 We should have spoken of a velocity and of an index M X 
 corresponding to light of a given colour or of a definite 
 wave-length. And this brings us to the second of the subtle 
 changes wrought in light propagation by the presence of matter. 
 In fact, the velocity of light within material media is not only 
 in general different from (and as a rule, to which sodium, silver, 
 gold and copper are an exception, smaller than) that in empty 
 space, but its value, and therefore also that of the index /i, 
 differs very considerably for different colours or wave-lengths. 
 This beautiful phenomenon distinguishing material media 
 from empty space is generally known by the name of Dis- 
 persion. What is called the dispersion curve of a substance 
 (under given physical conditions) is the graphic representa- 
 tion of this property ; the abscissae of the curve giving, in 
 
 * By some recent methods based upon the principles of interferometry 
 variations of the refractive index can be detected which amount only to one- 
 tenmillionth and less. 
 
 ADAM HILGER LIMITED 
 75x Camden Road, London, N.W. 1
 
 some conventional scale, the wave-lengths A, and the ordi- 
 nates, the corresponding values p. of the refractive index, 
 or of some particularly interesting function of p, such as /u 2 - 1, 
 or perhaps ^ I //t 2 + 2 , and so on. Now if the refractive index 
 for some chosen light quality, say the D of the sodium light, 
 is already characteristic of a substance, its dispersion curve 
 is still more so. Moreover, while n itself varies considerably 
 with the physical conditions of the substance, the dispersion 
 curve, although not independent of such agents, retains more 
 permanently or more tenaciously its features, and especially 
 those singularities associated with what are known as free or 
 natural oscillation frequencies (manifesting themselves as 
 absorption lines or bands), and there is but little doubt 
 that it does not merely represent properties of the whole 
 structure, the piece of solid or the drop of liquid, but reveals 
 to us some very intimate secrets of the individual bricks, the 
 molecules or atoms. As such the dispersion should attract 
 the greatest attention of the chemist as well as of the mole- 
 cular physicist. 
 
 ^yrSbCJlvr 1 1UJN. Intimately connected with 
 I ^ the dispersion is the Absorption of a substance 
 / % which, when carefully investigated, always turns 
 1 JL-Out to be, in a more or less pronounced way, selec- 
 tive i.f. t again dependent on the wave-length or the oscilla- 
 tion frequency of the impingent light. Many substances, 
 shortly called transparent, have their absorption regions, 
 absorption lines or bands, in the ultra-violet or the infra- 
 red. Other substances, visibly coloured, show absorption 
 bands (mostly very broad and without sharp limits) in 
 the more accessible, that is, in the visible part of the 
 spectrum. At any rate there is scarcely a substance devoid 
 altogether of absorption ; in more recent times even 
 all such " transparent " gases as oxygen have been 
 incontestably proved to possess well-defined absorption 
 bands ; nor do such transparent crystals as is the diamond 
 show an exception to the rule. But what is of prime 
 importance is that the absorption, which in its dependence 
 
 8 
 
 ADAM HILGER LIMITED 
 75x Camden Road, London. N.W. 1
 
 upon the wave-length can again be represented by a curve 
 or (in pronouncedly selective cases) by a number of isolated 
 peaks, is eminently characteristic for a substance or for a 
 chemical group, and thus offers a most welcome supplement 
 to the dispersion curve. Moreover, the absorption bands, 
 and especially the finer and sharply defined lines, correspond 
 directly to the free frequencies of a substance (attributable 
 to a great extent to the molecules themselves, and only in 
 part modified by the interactions within the aggregate) 
 which mould, so to speak, the character of the whole dis- 
 persion curve. Those constants in every modern dispersion 
 formula (of the Ketteler-Helmholtz type) which are known by 
 the name oifree wave-lengths^ and which, in conjunction with 
 the remaining constants, serve for the actual numerical 
 calculation of the refraction index for any desired wave-length, 
 represent directly the free oscillation frequencies for which 
 the absorption is strongest. 
 
 A careful study of both the absorption and the dispersion 
 curve of a substance will thus be seen to be a most important 
 step towards the exploration of the intimate structure of the 
 aggregate and, under favourable conditions, also of its mole- 
 cules, and therefore of the physico-chemical aspect of a 
 substance. 
 
 It is true that some generalisations, made chiefly by organic 
 chemists on the great road opened by Gladstone and Dale, 
 and by Bruehl and Landolt, associated with the names and 
 concepts of molecular and atomic refraction, were too rash 
 and notably too naive. But there is undoubtedly a profound 
 and intrinsic link or a correspondence between the dispersive 
 and the absorptive properties on the one hand, and the 
 structure of a molecule out of its atoms, and of these out 
 of subatomic entities, on the other hand. The simple 
 " additivity " of atomic refractions, those two- or three- 
 figure numbers given in the practical chemist's tables, is 
 certainly not the rule, but rather the exception ; although it 
 cannot be denied that it has in many cases aided and will still 
 be helpful to the organic chemist in selecting a constitution 
 formula among two or more otherwise equally possible ones. 
 
 ADAM HILGER LIMITED 
 75x Camden Road. London, N.W. 1
 
 But let us look more deeply and in a less sanguine manner 
 upon the whole question. Then we shall certainly have to 
 confess that the supposed simple additivity of optical pro- 
 perties is but a rough, and even very rough, first approxima- 
 tion to the truth, that it is actually swept away by innumerable 
 instances of evidence of a much more delicate and immensely 
 more complicated constitutivity of optical properties, the 
 manifestation of an intricate interaction of atoms. But this 
 is exactly the reason why we are justified in asserting that in 
 this direction an inexhaustible field of inquiry lies open for 
 many generations of chemists and of physicists, theorists as 
 well as experimentalists. And the practical, technical or 
 industrial advantages of this as of any other kind of investi- 
 gations, even if originally undertaken with the purest scien- 
 tific aim in view, will not fail to follow of themselves. The 
 more complicated a class of natural phenomena appears to be, 
 the more reason there is to investigate it with increasingly 
 powerful and accurate instruments, aided by careful methods 
 and cautious criticism. 
 
 W'AVE-LENGTHS OF EMIS- 
 SION AND ABSORPTION 
 ^* H<v^ 1 ix/\. In many cases the rough 
 knowledge of the general aspect of the absorption spectrum of 
 a substance is by itself of great service, enabling us to identify 
 the substance in question. This means of investigation, how- 
 ever, as well as that based upon the emission spectra, becomes 
 much more powerful when the wave-length of the spectrum 
 lines is measured. This has in recent times become an ex- 
 tremely accurate process, enabling us to measure the wave- 
 length of a well-defined, sharp line to one-hundredth or in 
 the case of comparisons or shifts (St. John, Evershed) even 
 to a few thousandths of an Angstrom unit, i.e., of a few thou- 
 sandths and perhaps even down to one- thousandth of io~ 8 cm. 
 Such accurate measurements require, of course, a particularly 
 refined and costly apparatus ; but even those easily acces- 
 sible wave-length spectrometers which enable one to discern 
 
 10 
 
 ADAM HILGER LIMITED 
 75A Camden Road. London, N.W. 1
 
 and to measure 2 or 3 A.U. (about J distance of the familiar 
 two D-lines of sodium), offer many new possibilities in the 
 field of optical investigation of substances. The wave- 
 length which, if originally measured in air, can easily be 
 reduced to its vacuum-value A (for this purpose good tables 
 are available), gives us directly the natural or free period, T, 
 of oscillations of the emitting or the absorbing atoms or 
 molecules ; for, if c be the standard or empty-space velocity 
 of light, we have simply \=cT. While the first-mentioned 
 optical properties, such as the refractive index for a given 
 wave-length, are to a great extent the outcome of an entangled 
 co-operation of a great number of molecules composing a 
 lump of matter, the spectral lines of emission or of absorp- 
 tion, especially of gases, are the work of the individual mole- 
 cules, or even atoms,* but very slightly, if at all, modified by 
 the interaction of neighbours. A spectrum line with its 
 accurately measured wave-length is thus the most direct 
 manifestation of the intrinsic properties of a molecule, and 
 even of its component atoms ; for the lines (and also many 
 features of their distribution) of a chemical element are still 
 recognisable in the spectra of its various compounds. 
 
 Apart from slight shifts due to strong pressure variation, 
 the spectrum, consisting in the majority of cases of an amazing 
 number of separate lines, visible and ultra-violet or infra-red, 
 is also one of the most -permanent properties or manifesta- 
 tions of a substance or, better, of its component particles. 
 Under such circumstances it is manifestly impossible to 
 overrate the physico-chemical importance of the quantita- 
 tive investigation of the emission- and the absorption- 
 spectra of substances. 
 
 While in the case of refractivity and of dispersion the 
 physico-chemists, notably those of the Landolt-Bruehl school, 
 
 * The intensity or the amount of emitted or absorbed light is, of course, due 
 to the minute contributions of many individual oscillators, but the period T, 
 and therefore also the measured wave-length, belongs to each of them taken 
 separately. And, to judge from the extreme fineness of the lines, especially if 
 (in the case of complex ones) ultimately split by a Lummer plate, the individual 
 differences of period, if at all existing, are extremely small. 
 
 11 
 
 ADAM HILGER LIMITED 
 75A Camden Road. London. N.W.I
 
 aimed from the beginning at a quantitative representation of 
 the properties of compounds by means of the additivity law, 
 of a handful of more or less provisional atomic refractions or 
 " refractive equivalents," and of various extra-terms, such 
 as those due to double or treble bonds, of " exaltations," and 
 so on, their line of research concerning emission and 
 absorption spectra has assumed a more qualitative character. 
 The results are here, on the whole, stated with more criticism 
 and caution, and it would seem that, owing perhaps to the 
 latter circumstance, the results hitherto gathered, although 
 not professing to be a mathematical synthesis and prediction 
 of numerical coefficients, are, comparatively speaking, very 
 valuable. The tangible result of a long series of investiga- 
 tions aiming at a discovery of the relations between absorp- 
 tion spectra and the chemical constitution of mainly organic 
 compounds, dating from 1879 (Hartley), consists in a wealth 
 of absorption curves (relative layer thickness plotted against 
 oscillation frequency) of many substances. One of the most 
 characteristic features of the conclusions which it was possible 
 to draw from these experimental data is that, in the ultra- 
 violet and the visible regions of the spectrum, the absorption 
 is eminently constitutive, the only ascertainable trace of 
 " additivity " consisting in the effect of what is called 
 " weighting the molecule," when the absorption bands are 
 shifted towards the less refrangible end of the spectrum. 
 According to Drude's (electronic) theory the longer, infra-red, 
 natural oscillation periods belong to the whole atoms, not to 
 the sub-atomic electrons, as vibrators. In conformity with 
 this theoretical conclusion the infra-red absorption should 
 exhibit some additive features ; and, in fact, Abney and others 
 were able to detect certain permanent or almost permanent 
 bands " due to " hydrogen, carbon, and hydroxylic oxygen. 
 Again, a valuable feature, already enunciated as a general 
 rule by Hartley, and more recently confirmed in many new 
 and subtle cases, is that chemical compounds of closely 
 allied structures have similarly shaped absorption curves. 
 The pushing of the bands towards the red by the addition 
 of certain groups to some (not too complicated) compounds 
 
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 7Sx Camden Road. London, N.W. 1
 
 seems also to be a well-established effect. A number of 
 other more special regularities have, no doubt, been cor- 
 rectly observed and tabulated. But it is undeniable that 
 infinitely more remains yet to be done in this vast and 
 promising domain of physico-chemistry. New fields of vital 
 interest and of a peculiar charm have also been opened for 
 high-precision spectroscopy (especially of emitted spectra) by 
 the several triumphs of Bohr's new theory of spectra (1913), 
 based on Planck's concept of quanta, and enriched by Sommer- 
 feld's theory of the fine structure of spectrum " lines " or, 
 rather, groups with all their marvellous intricacies a domain 
 bristling with new, difficult, yet most attractive problems for 
 the experimental as well as the theoretical spectroscopist.* 
 
 K)TATORY POWER. The intimate 
 relation of the natural (as distinguished from the 
 magnetic) Rotatory power of a substance with the 
 space-configuration of the atoms within its mole- 
 cules was already suspected and even fully grasped in its 
 deeper aspect by the great Pasteur (1848), who has deserved 
 well of humanity in another more familiar field. 
 
 As was mentioned at the outset, light propagated in vacuo 
 does not experience any change of its state of polarisation. 
 Thus, for instance, rectilinearly polarised light remains so 
 and the plane of polarisation containing the magnetic and 
 perpendicular to the electric force is preserved throughout 
 the course of the waves ; more generally, if the light be 
 elliptically polarised, the axes of the ellipse retain their 
 orientation i.e. 9 remain at their successive positions parallel 
 to themselves. This is still the case in common media 
 such as air, or water, glass, and so on, which differ optically 
 from a vacuum only by their refractive, dispersive and 
 absorptive power. But certain crystals (having no symmetry 
 plane), and notably quartz, possess the remarkable property 
 of rotating the plane of polarisation of light fari -passu with 
 
 * A first acquaintance with this beautiful domain of modern spectroscopic 
 researches may be obtained from my Report on the Quantum Theory of Spectra , 
 just published (May, 1920), by Adam ffilger, Ltd. 
 
 13 
 
 ADAM HILGER LIMITED 
 75x Camden Road. London. N.W.I
 
 its propagation ; a quartz plate of I mm. thickness, cut 
 perpendicularly to the crystal's optical axis, turns the plane 
 of polarisation of D-light through as much as 21 43' (and of 
 violet light, in the neighbourhood of the hydrogen line, H, 
 even through somewhat more than 51). Of course, in such 
 crystalline, active bodies the amount of rotation depends, 
 cceteris paribus, on the orientation of the wave-normal, and 
 the phenomenon, although conditioned, no doubt, by some 
 properties of the molecules, is essentially influenced by the 
 manner in which they are arranged in the crystal. 
 
 There is, however, a vast number of non-crystalline, 
 isotropic substances, such as turpentine and the most familiar 
 sugar solutions, which, although in a much weaker degree 
 than quartz, are also endowed with " natural activity " or 
 rotatory powei; easily detectable and measureable, some 
 dextrogyrous, others levogyrous i.e., right- and left-handed. 
 It is this, macroscopically speaking, isotropic class of naturally 
 active substances, in which therefore no direction of propaga- 
 tion is privileged, that is most particularly interesting from the 
 physico-chemical point of view, for owing to the complete 
 absence of macroscopic structural regularity of the whole 
 aggregate (a liquid or a liquid solution), the rotatory effect in 
 such substances is manifestly attributable to its building 
 stones, pointing, that is, to some sort of intrinsic chirality of 
 its separate molecules. 
 
 A first hint as to the possible nature of this peculiarity of 
 the molecules of active substances was given by Pasteur's 
 discovery of an instance ot the phenomena now known under 
 the head of optical isomerism. In 1848 Pasteur found that 
 besides the already known dextrogyrous tartaric acid there 
 exists a levogyrous one, a substance, that is, isomeric with the 
 former acid, and coinciding with it completely in all chemical 
 points, the only difference being that solutions of the two 
 isomeric acids turned the polarisation plane in opposite senses, 
 and what is equally remarkable c&teris paribus, through 
 the same angle. Moreover, the salts of these two acids 
 had, correspondingly, a right- and a left-hand hemihedral 
 crystal face. This led Pasteur to assume an asymmetric 
 
 14 
 
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 arrangement of atoms within each of the two isomeric 
 molecules, the features of the arrangement being irrecon- 
 cilably different for the two kinds of molecules. But there 
 was no such possibility in the bidimensional, planar formula 
 universally adopted by the chemists of those days. Pasteur 
 made therefore the bold leap into the third dimension, 
 pointing out for the first time the possibility of explaining 
 the phenomena of optical isomerism by a three-dimensional 
 arrangement of atoms, and consequently by the use of 
 three-dimensional constitution formulae. This was a step 
 on the path of progress whose importance it is impossible 
 to overrate. Pasteur even stated the true relation of the 
 isomeric molecules, by describing it (1860) as " an asymmetric 
 arrangement of atoms of such a kind as cannot be superposed 
 upon its mirror-image," one of such configurations belonging, 
 say, to the molecule of the dextrogyrous acid and its exact 
 image to that of the levogyrous acid. 
 
 Other instances of optical isomerism, exhibiting exactly 
 the same features, were discovered (1873) by Wislicenus, and 
 the next year saw the definitive inauguration, in almost simul- 
 taneous papers by van't Hoff and Le Bel, of Stereo-chemistry. 
 Stereo-chemical investigations centered at first almost 
 exclusively round the carbon compounds, the prominent 
 heuristic concept being the asymmetric carbon atom. But 
 in more recent times such investigations have been extended 
 to other elements, to nitrogen, and also to sulphur, selenium 
 and tin. It would carry us much beyond the scope of the 
 present little essay to describe the many triumphs of stereo- 
 chemistry which, moreover, can be looked up in good and 
 easily accessible monographs on the subject. We shall, 
 therefore, limit ourselves to pointing out that these successes 
 have, hitherto, at least, consisted not so much in calculating 
 beforehand (as in the case of refractivities) the numerical 
 value of the specific rotatory power of a compound, as in the 
 prediction of the possibility of new stereo-isomeres and in 
 their actual discovery or synthesis guided by such theoretical 
 prediction. Thus, to quote only one of the most striking 
 examples, all the ten stereo-isomeric forms of a certain com- 
 
 15 
 
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 75x Camden Road, London. N.W.I
 
 plicated kind predicted theoretically by van't Hoff were 
 experimentally found embodied in as many saccharic acids 
 (Emil Fischer), to wit, eight active forms and two inactive 
 ones, in perfect agreement with theoretical expectation. 
 
 The vast field of even these qualitative, stereo-chemical 
 researches aided by optical observation is, undoubtedly, yet 
 far from being exhausted, and should as such engage the 
 keenest interest of the chemist and the physicist. 
 
 Concerning the amount of rotation or the value of the 
 specific rotatory power practically nothing has hitherto been 
 achieved, the formula proposed (about 1890) by Guye and 
 by Crum Brown representing the rotatory power as propor- 
 tional to the product of all the six differences of the molecular 
 weights of the four groups united to the carbon atom lacking 
 not only any plausible theoretical support, but also failing to 
 cover even coarsely the facts of quantitative experience. Quite 
 recently an interesting and deeply reaching theory of natural 
 rotation, on electro-magnetic lines, has been proposed by 
 Oseen. This attempt seems very promising, but requires 
 for its completion still much work on both the theoretical 
 and the experimental sides. The older roughly sketched 
 electron theory of the rotatory power given by Drude accounts 
 for the essential features of the phenomenon without entering 
 into the structural details of the molecular systems. This 
 theory gives for the dispersion of rotatory power formulae 
 of the Helmholtz type in which the natural periods are 
 those belonging to what are called the " active " elec- 
 trons. To a first approximation these formulae reduce 
 to the well-known empirical formula of Biot (rotatory 
 power inversely as A 2 ) which for many cases is sufficient. 
 For larger portions of the spectrum the full formulae of Drude 
 represent the observations much more accurately than Biot's, 
 and even excellently as, for instance, in the case of quartz 
 from 0*219 U P to 2>I 4 microns.* They cover also the cases 
 of anomalous rotatory dispersion observed in absorbing 
 organic liquids. A further development of even this rough 
 electrical theory and the comparison of its results with obser- 
 
 * A micron is one-thousandth of a millimetre or 10,000 A.U. 
 16 
 
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 75A Camden Road. London, N.W.I
 
 vations would not be without value for the exploration of 
 the interior of molecules and of atoms, especially as it sheds 
 some interesting light upon the " active " or " rotatory " 
 electrons, which in general appear as distinct from those 
 responsible for the ordinary dispersion and absorption 
 phenomena. An important quantitatively additive property 
 relating to compounds containing two or more asymmetric 
 carbon atoms which was already announced by van't Hoff 
 (1875) has more recently been verified experimentally in a 
 very satisfactory manner. It asserts that the rotatory power 
 of such a compound is equal to the algebraic sum of the 
 powers due to the separate " asymmetric carbon atoms " 
 together, of course, with their satellites. This so-called 
 law of optical superposition seems to be the only notable 
 consolation of the " additivity " hunter in this beautiful 
 domain of optical phenomena. It is, no doubt, based upon 
 a rather loose association of the two or more component 
 parts of the whole compound. The intrinsic physics of each 
 of them as a system is still awaiting its thorough investigation. 
 If an ultimate reduction to atoms is aimed at, the phenomenon 
 of natural rotation of the plane of polarisation is manifestly 
 a constitutive one par excellence. But the more it is so, the 
 more active attention of the physico-chemist does it deserve. 
 Last, not least, the rotatory power of organic compounds 
 seems also to open a very promising field for the biologist 
 owing to the well-established selective attitude of certain 
 living organisms or of their enzymes towards stereo-isomeres, 
 a class of phenomena already discovered and utilised by 
 Pasteur himself, and more recently studied by Emil Fischer 
 and others. According to the latter's simile, in order that 
 an enzyme may successfully attack one of two enantiomorph 
 molecules, there must be between the aggressor and its 
 victim some similarity of intramolecular configuration, some- 
 what as between " lock and key." But the very appeal to 
 this, no doubt, most happy comparison will serve to remind 
 the modern bio-chemist how much remains still to be done 
 in the rich field of this class of phenomena. 
 
 May, 1920. L - SILBERSTEIN. 
 
 17 
 
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