CHEMICAL NEWS, Efficiency of Selenium as a Director of Light. lenses of the eye-glasses, that the objects are seen at their real angle, so that the vision through a stereoscope is, as it were, a vision seen through a magnifying glass. For all these reasons the vision through a stereoscope differs sensibly from the normal vision. M. André Cheron, in a notice sent to the Academy of Sciences by Prof. Lipmann, shows that he has managed to construct a stereoscope whose aim is in a certain measure, as far as possible, to correct these inconveniences. The two pictures are projected on to one plane by two objectives of the same focus. Moreover, the two images are concentred and superposed on their plane of projection, thanks to the presence of an achromatic lens covering the two objectives projecting the images. On the plane of the formation of the images is a condensor, which makes the rays coming from the right picture converge towards the right eye, and those proceeding from the left picture towards the left eye. In this way the spectroscopic relief is integrally preserved and the eyes converge and accommodate themselves as if they were looking at objects really situated in space. The pictures are enlarged and looked at at the distance of distinct vision, without anything being interposed between them and the eyes of the observer. THE CONTAMINATION OF MILK. 4. Carbon disulphide yields a blue polymeric nitrogen sulphide, and polymeric carbon monosulphide. Chloride of sulphur gives ordinary yellow nitrogen sulphide. Stannic chloride and titanium tetrachloride also yield solid products. In the latter case nitrogen was proved to be present. 5. All organic compounds tried, except carbon tetrachloride, yield hydrocyanic acid freely, but not cyanogen, as was proved by chemical tests. When chlorine is present, cyanogen chloride is formed. Benzene yields (almost certainly) cyanobenzene. 6. The intensity of the cyanogen spectrum with organic compounds is no index of the quantity of hydrocyanic acid being formed. Preponderance of the red cyanogen bands is associated with cyanogen chloride or bromide. On a general view of the evidence, there does not appear to be any definite connection between the development of spectra by active nitrogen and the chemical actions in progress. "Electrical Emissivity and Disintegration of Hot Metals." By Dr. J. A. HARKER, F.R.S., and Dr. G. W. C. KAYE. Preliminary experiments have been carried out on the volatilisation and electrical emissivity of a number of metals, mostly in nitrogen at reduced pressures. The metals were heated by alternating current and no applied potential was employed. We all know how easily milk is contaminated. MM. Trillat and Fouassier, of the Paris Pasteur Institute, in a work on the frequency of the contamination of fresh milk by water containing infinitesimal quantities of typhoid bacilli, have recognised that the contamination might also take place even with water which from a bacteriological-due, probably, to the sudden release of occluded gas. analysis might be declared healthy and sound. The typhoid bacillus is unperceived during the first hours of contamination. The development of the culture then takes place quickly, as it were, like an explosion. 1. The emission of positive electricity occurs at temperatures from about 1000° to 1400° C. For metals which melt within this range, a sudden and marked increase in the positive current often occurred at the liquefying point MM. Trillat and Fouassier establish the fact that in its turn milk may become a frequent cause of the contamination of water in many circumstances, and this observation brings out clearly the reciprocal influence of water and of milk for their mutual typhoid fertilisation. PROCEEDINGS OF SOCIETIES. ROYAL SOCIETY. Ordinary Meeting, June 19th, 1913. 2. Oxygen appears to augment the positive current. 3. At higher temperatures, negative electricity predominates and increases rapidly with the temperature. The negative current attained with iridium at the melting-point was 80 milliampères, with tantalum at 1670° C. 220 microampères, with iron at the melting-point 90 microampères. In the case of carbon in air at atmospheric pressure, an ionisation current of 3 ampères was obtained. 4. The negative current at moderate pressures appears to be largely increased if the conditions are such that considerable sputtering of the metal occurs. 5. The negative currents are probably a consequence of chemical reaction between the metal and the surrounding gas. 6. Carbon becomes plastic in the neighbourhood of 2500° C. At such temperatures it also readily sublimes. "Method of Measuring the Viscosity of the Vapours of Sir ARCHIBALD GEIKIE, K.C.B., President, in the Chair. Volatile Liquids, with an Application to Bromine." By PAPERS were read as follows:: "Atomic Specific Heats between the Boiling-boints of Liquid Nitrogen and Hydrogen. I. The Mean Atomic Specific Heats at 50° Absolute of the Elements a Periodic Function of the Atomic Weights." By Prof. Sir JAMES DEWAR, F.R.S. "Active Modification of Nitrogen Produced by the Electric Discharge." (V.). By Hon. R. J. STRUTT, F.R.S. 1. An improved practical method of preparing and storing nitrogen for the experiments is described. 2. It is shown, notwithstanding criticisms of certain other experimenters, that the presence of traces of oxygen in the nitrogen used is not essential, or even favourable, to the phenomena. The nitrogen used, purified by cold phosphorus, does not contain oxygen to the extent of I part in 100.000. Passing it over red-hot copper in addition makes no difference. The intentional addition of oxygen does harm; 2 per cent obliterates the effects altogether. Hydrogen and carbon dioxide as impurities are much less harmful, but traces even of water vapour have a very bad effect. 3. Nitrides are formed by the admixture of active nitrogen with vapour of mercury, cadmium, zinc, arsenic, sodium, and sulphur. These are decomposable by water or potash solution, yielding ammonia. A. O. RANKINE, D.Sc. In this method of determining viscosities the rate of transpiration of the vapour through a capillary tube is controlled by the vapour pressures of the liquid itself, a difference of pressure being established in the process of virtually distilling the liquid through the capillary. The pressures can be estimated without the use of mercury gauges a state of affairs especially desirable in the case of the halogens. The viscosities of unsaturated bromine vapour over the approximate range 10° C. to 250° C. have been measured, and, except at the lowest temperatures, are found to agree well with Sutherland's formula, notwithstanding the fact that all the temperatures are below the critical. critical temperature; A = atomic weight), as compared with The constancy of the proportion ne2/A (ne = viscosity at chlorine, has been tested, and the agreement is good. "Efficiency of Selenium as a Detector of Light." By E. E. FOURNIER D'ALBe. The efficiency of a selenium preparation used as a detector of light is defined as the amount of additional conductivity imparted to it by the unit of incident light. It is best stated in "micro-ohms" per lumen. Since many factors affect the efficiency of a given selenium bridge, standard conditions are chosen, chief among them being an illumination of one lux. The law of light action is studied, and the total effect is shown to be proportional to the square root of the incident energy, while the instantaneous effect is proportional to the energy. This is verified down to an illumination of ooooor metre-candle. It is shown that selenium is the most efficient light detector known, that it is capable of discriminating minute differences of luminous intensity far beyond the capacity of the eye, and that, with suitable means of detecting minute currents, it should offer a means of testing the quanta theory of light by direct experiment. "The Hall Effect in Liquid Electrolytes." By A. E. OXLEY. | selection of the materials for the construction of physical laboratories and magnetic observatories, or serious disturbances may take place during observations with delicate magnetic instruments. "Synthesis of the Anhydrides of a-Aminoacyl Glucosamines." By ARTHUR HOPWOOD and CHARLES WEIZMANN. "Flexure of Telescope Mirror Discs arising from their Weight, and its Influence upon Resolving Power." By H. S. JONES. "On Fourier Series and Functions of Bounded Variation." By Prof. W. H. YOUNG, F.R.S. "Condition that a Trigonometrical Series should have a certain Form." By Prof. W. H. YOUNG, F.R.S. "Trigonometrical Series whose Cesaro Partial Summations Oscillate Finitely." By Prof. W. H. YOUNG, F.R.S. CHEMICAL SOCIETY. Experiments have been made on aqueous solutions of copper sulphate, silver nitrate, cadmium sulphate, and on copper sulphate gel. Each substance was placed in a small cell of glass or mica, and was subjected to a uniform magnetic field. A Paschen galvanometer was used to measure the transverse potential difference. In a uniform magnetic field this transverse potential difference is due partly to a true Hall effect (depending on the difference of the ionic mobilities) and partly to a concentration Hall effect (depending on the sum of the ionic mobilities). The latter effect is primarily the one which has been measured Prof. W. H. PERKIN, LL.D., F.R.S., President, in the in this research, and the former, which is smaller, is included. Eight experiments have been made, and the transverse potential differences, which changed sign on reversal of the magnetic field, have been found to agree with the calculated values. The relation between the transverse potential difference and the intensity of the magnetic field, for an aqueous solution of copper sulphate, is linear. "Displacements of the Particles and their Paths in some Cases of Two-dimensional Motion of a Frictionless Liquid." By Prof. W. B. MORTON. "Diurnal Variations of the Earth's Magnetism produced by the Moon and Sun." By S. CHAPMAN. "Electric Effect of Rotating a Magnetic Insulator in a Magnetic Field." By Prof. H. A. WILSON, F.R.S., and MARJORIE WILSON. "Magnetic Materials in Clay wares." By ARTHUR HOPWOOD. The author has found that white, cream, grey, yellow, buff, red, or brown claywares are feebly or moderately magnetic owing to the presence of unfused grains of unchanged ferruginous minerals and fused globules of com. plex ferruginous silicates; while flashed, brindled, or blue claywares are always strongly magnetic owing to the presence of complex ferruginous silicate and finely disseminated magnetic oxide of iron. The origin of the complex ferruginous silicates in clay: wares is quite different from that of the magnetic oxide of iron. While the latter is produced either by the orientation of the magnetite, originally present in the clays, or by the reducing action of the kiln gases on the precipitated or colloid oxides, hydroxides, or carbonates of iron disseminated throughout the clays, the former are produced by the fusion of the granular or concretionary ferruginous minerals, i.e., iron pyrites, siderite, hæmatite, magnetite, biotite, &c., occurring in the clays with the surrounding matrix. When clays are heated in strongly oxidising kilns ranging in temperature from 600° to 1000° C. they become magnetic owing to the orientation of unchanged magnetic minerals, and when heated in strongly oxidising kilns ranging in temperature from 1000° to 1350° C. they become magnetic, partly owing to the orientation of unchanged magnetic minerals and partly to the conversion of granular or ferruginous minerals to complex ferruginous silicates; but when clays are heated in reducing kilns ranging in temperature from 600 to 1350° C. they become magnetic owing to the formation of complex ferruginous silicates and magnetic oxide of iron. The amounts of the magnetic materials present in the different sorts of claywares made in various parts of the country show that great care must be exercised in the Chair. (Concluded from vol. cvii., p. 311). 184. "3-Naphthol Sulphide and iso-8-Naphthol Sulphide; and the Constitution of B-Naphthol." BY THOMAS JOSEPH Nolan and SAMUEL SMILES. The results of previous experiments were used in discussing the constitution of the two sulphides. It was concluded that the normal sulphide is the true a-sulphide of B-naphthol (I.), whilst the isosulphide, which is formed by reducing naphthasulphonium-quinone, is represented as in II., and may be regarded as the hydrosulphoniumquinone : The constitution of B-naphthol was also discussed, and it was concluded that in this substance, and probably also in naphthalene, the hydrocarbon nucleus exists in a symmetrical condition. 185. "Nitrites of Thallium, Lithium, Casium, and Rubidium." BY WALTER CRAVEN BALL and HAROLD HELLING ABRAM. Thallous nitrite, TINO2, is a bright orange-red soluble crystalline salt. Hydrated lithium nitrite crystallises with 1 molecule of water, as stated by Rây (Proc., 1908, xxiv., 75). Chem. Fourn., 1907, xxxviii., 616) closely resembles the Cæsium nitrite, previously examined by Jamieson (Am. potassium salt, as also does rubidium nitrite. 186. "Note on the Fat of the Seeds of Oncoba echinata ; Occurrence of Chaulmoogric Acid. By ERNEST GOULDING and NOEL CHARLES AKERS. Samples of the seeds of the "Gorli" plant (Oncoba echinata, Oliver) have been received at the Imperial Institute from Sierra Leone, and have been examined with the following results. The seeds contained 5.8 per cent of moisture, and, on extraction with light petroleum, yielded about 47 per cent of a hard opaque white fat of a crystalline appearance and possessing a peculiar characteristic odour. The fat furnished the following constants: D100°/15°5° 0.898, [a]D/17 +48.8°; acid value, 4'5; saponification value, 192:4; iodine value, 99'7; Hehner value, 96.5; ReichertMeissl value, nil; unsaponifiable matter, 1'5 per cent. It CHEMICAL NEWS, New Method to Illustrate the Walden Inversion. had no definite melting-point, but gradually liquefied above 35°, and was completely melted at 45°. The fatty acids obtained by hydrolysing the fat had an iodine value of 1051, [a] D/18 +52.5°, and consisted of a mixture of a crystalline solid and a liquid. By pressing the mixture between folds of filter-paper, a large proportion of the solid substance could be separated, and on re-crystallisation from warm alcohol it was obtained in thin lustrous plates, melting at 69°. This acid gave an iodine value of 90.5, and []D/17 +600°; it was identified as chaulmoogric acid, C17H31 CO2H (Power and Gornall, Trans., 1904, lxxxv., 846), by the analysis of the silver salt and the preparation of the methyl ester, which melted at 22°, and had [a]D/21 +55.8°. The liquid portion of the fatty acids, although saturated with chaulmoogric acid, gave an iodine value of 122, showing that the liquid acids are highly unsaturated; it darkened rapidly on exposure to the air. The investigation showed that the fatty acids consisted approximately of chaulmoogric acid, 87.5 per cent, and liquid acids, 12.5 per cent. 187. "A New Model to Illustrate the Walden Inversion." By WILLIAM EDWARD GARNER. This model consists of a wooden ball, divided vertically into three equal sections, which are bolted together so as to leave a space between each. Additional stability is conferred on the structure by rings situated at the top and the bottom of the model. To each of the bolts, which are placed as near the centre of the model as possible, is attached a steel arm, capable of being vibrated with an upward and downward motion. The three arms are connected by thin cord to the middle of a central glass or metal rod DD', which passes through the two rings. The method of attachment is seen by reference to the diagram (Fig. 2). By the movement of the central rod upwards through the rings, the three arms are caused to move simultaneously, and if this movement is made sufficiently great, they pass downwards into the enantiomorphous position. The Werner model, constructed by the author (Proc., 1912, xxviii., 65), was utilised to illustrate the change of maleic acid into fumaric acid, without the destruction of the double bond, and its employment in this connection naturally suggested its use in the transformation of syninto anti-oximes. In the latter case the inter conversion of the isomeride may be demonstrated in either of two ways, namely, (1) by the inversion of the valencies of the carbon atom, and (2) by the inversion of those of the nitrogen atom. If both the carbon and nitrogen atoms undergo inversion, no change in the oxime is produced. The three cases are illustrated below: The first method was easily carried out by means of the previously described model (loc. cit.), but in order to illustrate the second case it was necessary to devise a tervalent nitrogen model, the valencies of which could readily be inverted. The new model was constructed for this purpose (a model was first constructed in April, 1912). The central rod is then made of glass, and the valency arms, A, B, and c, represent the nitrogen tetrahedron of Hantzsch and Werner. Two of the valencies of this model are connected by thin rubber tubing to two arms of a carbon model, and the transformation effected, as has been previously described in the case of maleic and fumaric acids (loc. cit.). The appearance of the nitrogen model, when completed, suggested to the author that it might be applied to repre 7 | sent some of the properties of the asymmetric carbon atom; the ends A, B, and c of the three arms, together with the end of the rod D, form the vertices of a tetrahedron, and consequently may represent the four groups attached to a carbon atom. The ease with which this tetrahedron is inverted renders the model suitable for the illustration of racemisation and Walden inversion FIG. 1. A A -D D FIG. 2. phenomena. Gadamer (Chem. Zeit., 1910, xxxiv., 1004) had previously put forward the idea which it embodies, and had developed it fully to furnish an explanation of racemisation, and later has extended his theory to account for the differences in the behaviour of silver oxide and other bases on optically active chloro-acids (Frankland, Trans., 1913, ciii., 722). If the model represent a chemical molecule the group D may be replaced by another group with a change of configuration. In order to accomplish this, according to the Werner theory (Ber., 1911, xliv., 881), the entering group must approach the molecule A, B, C, D from the side opposite to that occupied by the group D, and attach itself by means of its partial valencies at D'; if it is attracted to any of the other faces, no inversion would be obtained. Simultaneously with this addition, the group at Dis gradually removed, and a corresponding movement of the other valencies occurs in a downward direction. The molecule will pass through an intermediate position II., in which the three valencies A, B, and c lie in one plane, and the fourth valency is divided into two halves at D and D', and in this position the entering and extruded groups are attached to the carbon atom with equal force. In other intermediate positions the length of the central valency above and below the ball will indicate the relative strength of the attachment of the two groups. Finally, the groups In a recent paper (Proc., 1912, xxviii., 271) A. W. Macbeth criticised the theory of fluorescence which Dr. Krulla and the author advanced (Trans., 1912, ci., 1469). It seems that this criticism is based upon a misconception of the theory itself and of the processes which take place. According to the theory the condensed force fields surrounding the molecules of a substance may be opened up in stages by the influence of a solvent and of light. These stages may be called 1, 2, 3, 4, &c., and each one absorbs light of different wave-lengths, A2, A3, A4, &c. Whereas previously only two of these stages had been recognised, the existence of several definite stages in the opening up of the condensed field of force has now been proved, and it is hoped very shortly to communicate the results of certain observations, which clearly show how by the use of suitable solvents several different stages can be produced absorbing different wave-lengths of light. emission. In any case of fluorescence or phosphorescence there are two processes: the absorption of energy and the emission of energy. If the velocity of the second process is equal to or greater than that of the first, then the substance fluoresces, but if the velocity of the emission is slower than that of the first, then phosphorescence takes place, namely, the persistence of the emission for an appreciable time after the exciting cause has been removed. In certain cases the velocity of the emission of the energy is exceedingly small, and in these circumstances the energy absorbed in the first process remains stored up in the substance for a very long time unless the velocity of the emission is increased by some means, such as the application of heat, when the phenomenon is known as thermoluminescence. There seems, indeed, no reason to assume any difference between the two phenomena, and if a theory can explain one of them, it follows that it can explain the other. According to the theory, the stages 1, 2, 3,... n, inasmuch as they are stages in the opening up of the condensed system of a single molecule, are intimately connected with one another, and although the process by which 2 is absorbed in the solvent is the only one which light itself is capable of bringing about, yet there can hardly be any objection to the probability that the disturbance to the whole system produced in this way will bring the next possible vibration periods into play, namely, stage 3 followed in less degree by stage 4, and so on. It is known that, given the necessary external con synchronous with wave-lengths A3, A4, &c. ; if these vibrations are brought into play by means of some other vibration they will emit light of the same wave-length. Macbeth seems to have fallen into an error as regards the relationship between the wave-length of the exciting light and the emitted light. In producing phosphorescence of wave-length A3, A3 is not absorbed. Now it is obvious that in the absorption of these light waves considerable damping must be present. If this were not so it is evident that the whole substance would on pro-ditions, stages 3, 4, &c., are characterised by vibrations longed exposure become diactinic. This is, however, absurd; owing to the damping that is present the light actually does work against the chemical forces, and is therefore changed, probably into heat, so that as the result a continuous and constant absorption of the light takes place. It would seem that Macbeth has not taken this damping into consideration. He states that if the substance can exist in stages 1, 2, 3, &c., these forms must be in equilibrium with one another. This is not a correct assumption. If the stage 3 were present in equilibrium with 1 and 2, and if, according to hypothesis, 13 or 23 can be produced by absorption of light A3, it is absolutely certain that A3 would be absorbed out of a light source containing these waves. It is an experimental fact that A3 is not selectively absorbed as long as the solvent necessary is absent. In other words, if a fluorescent material which absorbs A2 and emits A3 is screened from A2 no trace of selective absorption of X3 can be detected. This rules out of court the assumption that stage 3 is normally present in simple equilibrium with 1 and 2. Under the influence of the light a photodynamic equilibrium is set up, which is a very different thing from the chemical equilibrium assumed by Macbeth. Macbeth's first criticism therefore seems absolutely to fail, first, because he assumes that stages 1, 2, 3 are in simple equilibrium; secondly, because he neglected the damping; and thirdly, because he assumes that the substance when screened from Aλ2 must absorb λ3. Macbeth states further that the theory, although it cannot explain fluorescence, is more capable of explaining phosphorescence. It is now generally agreed that the two phenomena are really the same, and only differ in the relative velocity of the two processes, absorption and There does not seem to be any difficulty in reconciling all observations as regards the relations between temperature and phosphorescence. In the first place, the conditions may occur when the process 2 is very slow indeed, with the result that the process I→ 2 will take place with absorption of λ2. After some time the whole system will have absorbed considerable energy. On heating the system, or in many cases simply by rubbing or shaking, conditions are produced that enable the process 2 I to take place, with the result that a considerable amount of free energy escapes as heat, and the whole system gets disturbed, and some of the vibrations of stage 3 are called into play with emission of λ3. In this process no λ2 is emitted. Care must be taken not to look upon processes I2, 23, or 13 as being directly reversible, absorbing or emitting the same amount of light energy, for if this were so, it would simply resolve itself into a case of resonance phenomena. Macbeth has fallen into error here when he says that the reverse process 2 I will be accompanied by the emission of light A2, as demanded by Kirchhoff's law. Kirchhoff's law has nothing whatever to do with the case, for it is not a case of black body radiation. The reverse process 2 → I is not accompanied by emission of A2, but probably by emission of heat, and the process is not reversible, CHEMICAL NEWS, July 4, 1913 Some Experiments on Tinfoil Contact with Dielectrics. The next point is: Why do some substances fluoresce only at low temperatures when the free energy is less than before? The lowering of temperature will tend to produce a more completely closed system of force lines round the molecule. There must naturally exist a particular condition of this condensing together of the force lines for the particular type of fluorescence observed to take place, and it follows directly from the theory that this condition may be produced at low temperatures when it does not exist at higher temperatures. It has been shown (Trans., 1913, ciii., 91) that when a substance is opened up by a solvent and by light, the amount of light absorbed increases with the dilution up to a maximum, after which further dilution tends to decrease the amount absorbed, which is then followed by the disappearance of the selective absorption. There is thus an optimum condition of concentration as far as absorption of light is concerned. This agrees with and explains Lenard and Klatt's and Urbain's observations on phosphorescence, for these authors have clearly shown that there is always a definite condition of concentration of phosphorescence in the diluent at which an optimum of phosphorescence is observed. Clearly at this concentration the phosphorogen has its closed force field just sufficiently opened up, and in that condition best adapted to respond to the exciting vibrations in such a way that the next stages, 3, 4, 5, &c., are called into play. This optimum condition only refers to one particular temperature. At a much lower temperature that condition will not necessarily give the optimum, and indeed, perhaps, may not give any phosphorescence at all. Some other concentration will be more suited for the new temperature conditions, and whilst this new concentration may not suit the old temperature, yet on cooling, the phosphorescence or fluorescence makes its appearance. A specific example may make this clearer, namely, the phosphorescence of strontium sulphide. Becquerel (Ann. Chim. Phys., 1859, [3], lv., 5) found that this material gives at 200° an orange phosphorescence, and as the temperature falls the colour passes through yellow, green, and blue until at -20° it is dark violet. This observation has been confirmed for a great number of substances by Lenard and Klatt (Ann. Physik., 1904, [4], xv., 225, 425, 633). The reason of this effect is that the phosphorescent spectrum consists of at least five separate maxima having the above colours, and as the temperature is changed the relative intensity of the various maxima alters, and in the case of the strontium sulphide the tendency, with decrease of temperature, is for the maximum of the phosphorescence to move towards the shorter wave-lengths. If the separate bands in the phosphorescent spectrum be called A, B, C, D, E, then at the higher temperatures A will be the most intense, and as the temperature falls, B, C, D, E, in turn, show the greatest intensity. Each of these corresponds with a definite stage in the opening-up process, and the optimum condition for each stage, provided that the quantity of diluent remains the same, depends on the temperature. By varying the conditions of diluent, similar variations in the relations between the intensities of the different phosphorescent maxima can also be obtained under constant temperature conditions. This fact has an important bearing on the general theory. Since the whole phenomenon of phosphorescence is a property of diluted matter, and since the fall of temperature allows more and more free energy to escape from the system, it follows that the lower the temperature the less is the phosphorogen opened up by the diluent. The observations on phosphorescence therefore run pari passu with the observations on absorption, for here the more a compound is opened up by a solvent the nearer the absorp tion maximum lies to the red. In other words, the more complex the solvent-solute system, or, speaking generally, the more complex the system of the force field dealt with, the nearer to the red will be its absorption and also its phosphorescent or fluorescent maximum. Macbeth in his criticism, based on the fact that some substances do not phosphoresce or fluoresce at ordinary temperatures and do 9 so at low temperatures, has really advanced observations which strengthen and confirm the theory. Again, Macbeth attempts to draw an analogy between a spring in various stages of compression and the stages 1, 2, 3, &c. This analogy fails absolutely from the start, because it requires the same energy in different amounts to obtain the spring in the different stages of compression, while in the real case the stages 2 and 3, &c., require for their actual production not only a different solvent, but light of different wave-length. He also contradicts himself here, because in order for the analogy to be complete from his point of view, the states of the spring when in different stages of compression must be in equilibrium with one another, an assumption he made for the stages I, 2, 3, &c. Two further criticisms of Macbeth still remain to be dealt with. First, Nichols and Merritt's observations that the position of maximum fluorescence is independent of the wave-length of the exciting light, and that the latter may be on the red side of the fluorescent maximum. Macbeth says the processes are now reversed, and 3 is being absorbed and A2 emitted. This is by no means the case. An inspection of the absorption and fluorescent curves shows that these extend considerably on each side of the maximum in each case. They frequently, indeed, overlap, and therefore the very fact of Nichols and Merritt's discovery strongly supports the theory. The shape of the fluorescent curve is characteristic of the substance under the conditions of solvent and concentration. This fluorescence will be produced by any wave-length included in the absorption band, even if it happens by chance to be longer than those emitted. The substance responds to and absorbs the longer the wave-length, and it is natural to expect that it would produce the same effect as any other wave-length in the same absorption band. Nichols and Merritt also observed in the same paper (Physical Review, 1904, xix., 18) that if the fluorescent substance has a second absorption band of longer wave-length than the fluorescent light, the absorption of light in this second region does not produce fluorescence, which fact, of course, is in agreement with this theory. Finally, Macbeth quotes the observation of R. W. Wood, who showed that while fluorescing with light of definite wave-length a substance exerts no increased absorption of that light. On a theory of optical resonance "fluorescence absorption " might be expected, and the fact that it has been proved absent argues strongly against any simple resonance as a basis of fluorescence and absorption. Nothing whatever in the theory makes it probable that fluorescence absorption should take place. A criticism based on the fact that it does not take place seems there. fore somewhat irrelevant. The general conception may be made clearer by considering it in the following way:-Light energy (2) is absorbed and converted partly into heat and partly into light energy (3). In ordinary circumstances this reaction is not reversible, because A3 is not absorbed, but it might be considered that under certain special labile conditions it does become reversible. The absorption of A3 during fluorescence would mean that the same reaction was taking place in opposite directions at the same time, and absorbing energy on both counts. Whether the process is reversible or non-reversible, there is no reason why A3 should be absorbed during fluorescence. PHYSICAL SOCIETY. Prof. C. H. LEES, F.R.S., Vice-President, in the Chair. This paper describes some experiments showing how the accuracy of the different kinds of electrical measurements |