In India, in 1875, at my suggestion Colonel Waterhouse used bromide of silver collodion plates dyed with eosin. The success of his experiments demonstrates that eosin was the best optical sensitiser for collodion plates. Later Ducos de Hauron employed orthochromatic plates in his photochrome or three-colour printing process. Attout-Tailfer next introduced isochromatic gelatine dry plates dyed with eosin or its derivatives, in conjunction with an alkali. I used azaline-a mixture of quinoline blue and quinoline red-for the same purpose; whilst Eder, of Vienna, recommended erythrosin (tetra-iodo-fluorescein) as the best optical sensitiser for dry plates. In 1885 Obernetter and I showed that by the use of eoside of silver plates it was possible to dispense with a yellow screen for ordinary landscape photography. I find that the best results are obtained with a film containing 1,000 parts of collodion (containing seventeen of cotton) to three parts aurantia. The exposure required in this case is four times as long as when no yellow screen is employed. For landscapes only two parts of aurantia are necessary, and the exposure required is only two and a half times longer than without a screen. Another important factor in regard to orthochromatic plates is the action of the developer. My own results show that the impressions of the blue rays develop before those of the red and yellow, and, therefore, when using eosin dyed plates they should be developed until all the yellow parts of the picture are visible in the negative. It is usually stated that the relative value of colours in a landscape approximates more closely at sunrise and at sunset than at noon, and, therefore, that in the former case a yellow screen is unnecessary when using orthochromatic plates. This would be true were direct sunlight the only agent, but since diffused light also comes into play it is not the case, for Crova has shown that the proportion of blue rays in diffused light increases as the sun goes down, whilst the reverse holds for direct sunlight. With orthochromatic plates the results depend on (1) the colour sensitiveness of the plate; (2) the proportion of the different coloured rays in the diffused light: and this proportion, I find, varies from day to day. I gather from Captain Abney's paper in the Photographic Journal' that the sensitiveness of the plates he employed for yellow rays was only 4ths of that for blue. To this fact I attribute the unsatisfactory results he obtained. In Germany we use eosin dyed plates, the sensitiveness of which is five times greater for yellow rays than for blue rays. Such plates can be used for landscapes without a yellow screen. The prints exhibited to the Section show the comparative results obtained with ordinary plates and plates of this kind. The pictures were taken at 3 P.M. 2. On the Sensitising Action of Dyes on Gelatino-bromide Plates. By C. H. BOTHAMLEY. Although large numbers of dyes have been examined since Dr. H. W. Vogel's discovery in 1873, very few exert any marked effect in making gelatino-bromide plates sensitive to the less refrangible rays of the spectrum. Only cyanin and the dyes of the eosin group (including the rhodamines), with perhaps malachite green, chrysoidine, and alizarin blue, can be said to exert any useful effect. The main points established by previous observers may be summarised as follows: (1) The dyes that act as sensitisers are readily affected by light when they are in contact with fabrics, paper, &c.; (2) in order that a dye may act as a sensitiser it must. have the power of entering into intimate union with silver bromide, forming a kind of lake'; (3) and it must show a strong absorption band for the particular rays for which it is to sensitise. Although these statements hold good for all the dyes that are known to act as sensitisers, it is important to observe that the converse is not necessarily true. Several dyes having all these properties show no appreciable sensitising action. Experiments by Dr. E. Vogel on the rate of fading and the sensitising action of the eosin dyes, led him to the conclusion that the order of sensitising effect coincides with the order of fading when the dyes are exposed to light. The order in which Vogel places the dyes does not, however, correspond with the order of fading as observed in dyed fabrics, and the experimental method that he used is open to criticism. The author's observations on the fading of the various sensitisers, when exposed to light in contact with gelatin, lead him to the conclusion that, although all the sensitisers are readily affected by light, the order of sensitising effect does not necessarily correspond with the order of fading, whether the dyes belong to the same chemical group or not. There are two chief hypotheses as to the mode in which the dyes act, namely: (1) the view held by Abney that the dye itself is oxidised by the action of light, the oxidation product remaining in contact with the silver bromide; and when the plate is treated with the developer, the latter and the oxidation product acting simultaneously on the silver bromide bring about its reduction; and (2) the view first definitely formulated by Eder, and endorsed by Vogel, namely, that the energy absorbed by the dyed silver bromide is partially used up in bringing about the chemical decomposition of the silver bromide, instead of being almost entirely converted into heat, as when absorbed by the dye alone. The author has found that the less refrangible rays will produce a photographic image on the sensitised gelatino-bromide plates, when they are immersed in powerfully reducing solutions, such as a mixture of sodium sulphite and pyrogallol. This holds good for cyanin, the eosin dyes, the rhodamines, and quinoline red, whether the sensitiser has been added to the emulsion or has been applied to the prepared plate in the form of a bath. It is, therefore, impossible to attribute the sensitising effect to any intermediate oxidation of the dye. Experiments with various reagents such as potassium bromide, potassium dichromate, mercuric chloride, and dilute hydrogen peroxide seem to show that the chemical nature of the latent image produced by the less refrangible rays on the specially sensitised plates is precisely the same as that of the latent image produced by the more refrangible rays in the ordinary way. Further proof in the same direction is afforded by the fact that the effect of the sensitisers extends to the production of a visible effect by the prolonged action of light. The balance of evidence is therefore greatly in favour of the view that the dye absorbs the particular group of rays, and, in some way which is not at all clear, hands on the energy to the silver bromide, with which it is intimately associated, and which is thereby decomposed. For the present, for want of a better word, the phenomenon might be distin guished as photo-catalysis, and the sensitiser might be described as a photo-catalytic agent. As yet no connection can be traced between the chemical constitution and the general physical properties of a dye, and its sensitising action. 3. Report of the Committee on the Action of Light on Dyed Colours. See Reports, p. 263. 4. On some Stilbene Derivatives. By J. J. SUDBOROUGH, D.Sc., Ph.D., F.I.C. The author has prepared monochloro-, methyl-chloro-, and ethyl-chloro stilbene by the action of phosphorus pentachloride on deoxybenzoïn and on its methyl and ethyl derivatives. The monochloro-stilbene differs from that described by Zinin (Annalen,' 149. 375), as it is a solid, which crystallises from alcohol in large colourless plates. It melts at 53°-54°, and yields additive compounds with bromine, with chlorine and with nitrous acid.' These, together with the corre sponding compounds obtained from methyl- and from ethyl-chloro-stilbene, are described. An oily monochloro-stilbene, corresponding to that of Zinin, has also been prepared, and is being subjected to further examination in order to determine whether it is merely an impure form of the crystalline compound or a true stereoisomeride. 5. Note on the Constitution of Camphoric Acid. The author draws attention to the fact that, as regards its etherification, camphoric acid shows a marked resemblance to some of the polycarboxylic acids investigated by Victor Meyer and Sudborough (Ber.,' 27, 3146), and to hemipinic acid (Wegscheider, Monatsheft,' 16, 75). It is thought that this resemblance may throw some light on the constitution of camphoric acid, and that at any rate it can be used as an argument for or against any formula which is brought forward. The author then considers several of the more important formulæ already suggested, and regards those of Armstrong and of Bredt as best agreeing with the behaviour of camphoric acid. 6. Experimental Proof of van 't Hoff's Constant, Dalton's Law, &c., for very Dilute Solutions. By Dr. M. WILDERMANN. 7. The Formation and Properties of a New Organic Acid. When tartaric acid is oxidised under certain conditions in presence of a ferrous salt a substance is produced which acts as a powerful reducing agent, and which gives a beautiful violet colour with ferric salts in presence of alkali. This substance has after considerable difficulty been isolated, and proves to be a dibasic acid having the formula CIO, 2H,0. The constitution of this acid is now under investigation. Heated with hydrogen iodide it gives succinic acid, racemic acid being an intermediate product. Bromine in presence of water oxidises it quantitatively to dioxytartaric acid. Heated with water it is resolved into carbon dioxide and glycollic aldehyde. This aldehyde has been obtained as a viscid liquid, pure except for a trace of ether; and, on removing the latter by heating in a vacuum, the aldehyde undergoes polymerisation, a sweet-tasting solid gum being the result. Analysis and molecular weight determination show that this gummy substance has the formula C&H12O6: Further observations have recently been made as to the conditions under which this new acid may be obtained from tartaric acid. The presence of a ferrous salt is essential. Ferric, manganous, and various other salts have been tried with negative results. If moist ferrous tartrate be exposed to the air for a short time a certain quantity of the new acid is produced, and may be indicated by the characteristic violet colour given when caustic alkali is added. The effect is much more intense if the exposure be made out of doors, and the increased result was at first attributed to some constituent of the fresh air (e.g., hydrogen dioxide; ozone seems to be inoperative). But later experiments show conclusively that light is the cause. Air which has been purified by passing through potassium iodide and caustic potash solutions gives an effect about equal in intensity to that produced by fresh external air, if the exposure to light be the same in both cases. That oxygen (or some oxidising agent) is essential is shown by the fact that exposure in a vacuum, even to bright sunlight, gives a negative result. 8. On the Velocity of Reaction before Perfect Equilibrium takes place. By MEYER WILDERMANN, Ph.D. The solidification and the crystallisation of over-cooled liquids, as well as the melting of frozen and solidified liquids, belong to processes which occur most generally in nature; therefore it is of importance to know the velocity with which these processes take place, and to find the common equation for all of them. 1. On the Velocity of Solidification of Over-cooled Liquids, of Solutions, and of Liquid Mixtures. From the experiments of Moore, made in Ostwald's laboratory,' dx the author shows that the equation =c(ft) is to be applied for the velocity dz of solidification, where t is the actually existing temperature of the over-cooled liquid, to the temperature, where the solid and liquid solution are in equilibrium, since beginning with the greater differences (instead of as has been done by Moore, with the smaller) to-t, it is easy to show that to-,_(dax : dz)t, dx dz tot (da: az)t,, 2. On the Velocity of Crystallisation of Over-cooled Liquids "and Solutions.— The same equation = c(t−t) is found to be applied for the velocity of crystallisation. Now, since the separation of the solid solvent is accompanied by evolution of heat (latent heat of melting), and the increase of the temperature of the liquid is directly proportional to the quantity of separated ice, we can, instead of dt the above equation, put d=c'(to-t), where c' is directly proportional to the latent heat of melting, and inversely proportional to the specific heat of the liquid. Very careful measurements have been carried out. The liquid was at first over-cooled to below its freezing-point; the distance from the freezing-point was then measured on the 01° thermometer, and the time noted by my assistant to second. 3. On the Velocity of Melting of Solid Solvents in the Warmer Liquids and dt Solutions. For the process of melting, Newton's equation = c(t − t) for conducdะ tion is to be used; the convergence temperature is here that at which ice and liquid are in equilibrium, i.e., the freezing-point; the ice plays here the part of the cooling medium, abstracting heat from the liquid. Since now the velocity of reaction takes place through the ice-surface, the velocity of reaction at a given time ≈ will be also directly proportional to the surface of the ice present in the liquid at the time z. Our equation can therefore get the form=c'(t-t)0, where O is in proportion to the surface of the ice. The liquid or solution to be investigated is at first over-cooled 1° or 1°.2 below its freezing-temperature; the ice is then crystallised. After the separation of the ice we allow the ice to rise in the beaker to the upper part of the liquid, warm the liquid to about 0°3 or 0°4 above the freezing-point; the liquid is then stirred, the temperature rises at first, and after reaching its maximum falls. The time is measured to second. dt dz We have therefore investigated two classes of reactions before perfect equili brium takes place. The first is where the temperature of both parts of the heterogeneous system is below or above the temperature of equilibrium (solidification, dx crystallisation). For this class we have to apply the equation = c(t-t) or dz dt dz - =c' (t。 − t), which in its form, but not in its purport, is identical with Newton's equation for conduction. The second class is where one of the parts of the heterogeneous system is at the temperature of equilibrium and the other is above or below that of equilibrium (melting process in liquids). The velocity of these processes is regulated by Newton's law for conduction. As we know, we have two kinds of equilibrium, perfect and imperfect equilibrium. While in the case of perfect equilibrium (for example, ice and water) at a constant pressure, the smallest change of temperature is sufficient to cause one of the parts of the heterogeneous system to disappear, in the case of imperfect equilibrium (for example, acid + alcohol, ether + water) a small change of temperature produces only a small change in the state of equilibrium, while the relation Zeitschr. phys. Chem., vol. xii. p. 545. of the quantities of the acting parts changes in one or the other direction. The velocity of reaction, before imperfect equilibrium takes place, formed the subject of investigation of many scientists; Vernon-Harcourt and Esson, van 't Hoff, Guldberg, and Waage, should be specially mentioned. The author finds in the case of dx dt solidification and crystallisation that the equation = c(t − t), or = c'(to-t) is dx de dz to be applied, but would express the common equation as =f(to-t), since the velocity of the reaction can often be complicated by other phenomena. Let us now bring into connection the equations for the two kinds of velocity of reactions with the two kinds of equilibrium. In the case of imperfect equilibrium we have, before the state of equilibrium is arrived at, two reactions: In the case of and equilibrium is arrived at when c(A-x') (B-x') = c'a', i.e., both reactions take place simultaneously and the equilibrium is a dynamic one. perfect equilibrium we have before the equilibrium is arrived at dx and equilibrium is arrived at when to-t-0 (ie., at the freezing temperature); therefore at the point of equilibrium equals 0, that is to say, no further reaction dz takes place, and the equilibrium is a static one. 9. Chemical History of Barley Plants. By C. F. CROSS and C. SMITH. Work has been carried out over a period of two years (1894 and 1895) upon crops grown on the experimental plots at Woburn. Maximum plot 6 and minimum plot 1 were investigated with regard to the furfural and permanent tissue which they contain. A table of results is appended to the paper. From the table we draw the following conclusions: 1. The conditions of soil nutrition have very little influence upon the composition of the plant. 2. The feeding value of straws grown in wet seasons is high, and conversely the paper-making value of such straws is low. 3. The furfuroids are continuously assimilated to permanent tissue in a normal season, but in a dry season, on the other hand, the permanent tissue is put under contribution for nutrient material, which is ordinarily drawn from the cell contents. |