Principles of General Physiology

Further, when the periodic alternating electric field of light acts on the electrons, resonance comes into play, their energy content rises, and would do so indefinitely if it were not changed into heat by some kind of " damping " (possibly due to impacts). In the work of Luther and Nikolopoulos, referred to above, it was found that the steeper and higher the absorption curve, the more sensitive to light. Such a curve, in fact, means a small degree of damping, and great amplitude of vibration of the electrons set in motion by light.

The extent of the loss by damping determines the efficiency of the photo-chemical process, which may be very high, as we shall see when discussing the chlorophyll system. The following illustration given by Luther (1908) may perhaps make clear the possibility of a high efficiency. The substance sensitive to light is compared to a reservoir into which air is pumped, the compressed air representing the radiant energy of light. The pressure of the air (i.e., the energy of resonance) inside the reservoir would rise indefinitely except that a hole, D, is provided, through which air can escape, this loss representing the change to heat by damping. Suppose, however, that there is another hole, C, of adjustable aperture, through which air can escape. This represents the change of part of the energy of resonance to chemical work. The pressure will then decrease according to the area of C, and with it, the amount of escape through D ( = degradation to heat). If C is made very wide, all the air pumped in escapes through it, and none through D.

Resonance energy thus tends to decrease, either by change of rate of vibration of the resonator, or by increase of damping. Hence, in light of a given frequency of vibration, systems insensitive to it arise from those sensitive to it, Although light energy cannot act unless absorbed, it does not follow that, when absorbed, chemical change always results ; for example, acetic anhydride has an absorption band between wave lengths 320 and 240 pp, which is associated with decomposition. But it also absorbs the extreme ultra-violet, apparently by means of its CH3 group, and resonance of this group does not lead to change.

A fact common to all photo-chemical reactions may be mentioned here, namely, that the action of light is similar to that of a high temperature. The dissociation of carbon dioxide and of hydrochloric acid, the conversion of oxygen to ozone, and the polymerisation of anthracene may be referred to. For certain theoretical conclusions, drawn by Warburg from this fact, Weigert's monograph (1911, p. 94) may be consulted. A theory" has been developed by Bodenstein (1913) according to which the first effect of light is to decompose a group into electron and electro-positive remainder. Each of these gives rise subsequently to chemical changes of a particular kind.

When we proceed to examine the various reactions which occur under the influence of light, we meet -with great variety and complexity. It is well, therefore, to clear the way somewhat by reference to a not infrequent misconception of the nature of the action of light, in which it is spoken of as being catalytic. The initial phase of all photo-chemical reactions is accompanied by the actual consumption of light energy to set in motion a reaction, although it may afterwards proceed with the evolution of energy. We have seen in Chapter X. that a catalyst adds no energy to a reacting system, but merely accelerates the rate at which such a reaction arrives at equilibrium. Further, in many reactions, such as the decomposition of carbon dioxide by the green leaf, the reaction is actually caused to proceed in the direction opposite to that in which it goes naturally at the temperature of the reaction. But, in many cases, a catalyst is formed by the action of light, and this catalyst then proceeds, independently of the light reaction proper, to perform its usual function of accelerating the natural course of the reaction. In this case, contrary to that of the chlorophyll system, the net result of the change is a diminution in the free energy of the system.

It will, perhaps, assist the understanding of the question if we use an illustration due to Ostwald (1902, II. 1, p. 1087). The necessity of the supply of energy by light is obvious enough in that class of reactions in which the result is an increase in the energy content, but is not so clear when the final result is a decrease. Quantitative measurements show, however, that, even in the latter case, there is more liberation of energy than if the reaction had merely proceeded without light, the extra energy being that obtained from light in the initial process. Ostwald compares the system to that of a wedge-shaped block standing with its narrow edge upwards. In this position, that of " metastable equilibrium," the system would remain indefinitely if undisturbed, although by falling on its side energy would be given out. To cause this to take place, there is necessity for a certain expenditure of energy upon the block in order to tip it over ; in this process, its centre of gravity is raised and the energy required to do this is given out again when the block falls over. Another illustration that might be given is that of a billiard ball lying in the concavity of a clock glass on the top of a tripod. Although energy would be given out by the fall of the ball, supposing that the glass were to melt away, no change takes place naturally unless the ball is first raised over the edge of the glass by the application of a small amount of energy. This energy is given out again, together with that due to its original height, when the ball falls to the table. We may speak of reactions of such a kind as being prevented from taking place spontaneously by the existence of chemical "resistance," which is removed by the energy imparted by light.

Our further considerations will be facilitated by taking the classification of Weigert (1911, p. 75), together with an illustration of each class. This classification is based on the net result of the reaction, which results from the action of light, not merely on the actual part played by the light energy used. We have seen already that these reactions can be divided into two main groups, those resulting in increase of free energy and those resulting in a decrease. Each of these can be further divided. The first group may be either simple or complex, in both cases being completely reversible and similar to an electrolytic decomposition in which the electrodes become polarised, so that the reaction ceases at a certain point.

1. Simple Reactions with Increase of Energy. — In these cases, the reversion on removal of light takes place by the same route as the photo-chemical change. The simplest one is the polymerisation of anthracene to di-anthracene by ultraviolet light, as investigated by Luther and Weigert (1905). The stable condition in the dark is, at ordinary temperatures, that of pure anthracene. If we start with pure di-anthracene in the dark it changes spontaneously to anthracene, at a definite rate. Light causes the formation of di-anthracene. But, since the reverse change is unaffected by light, this change of di-anthracene to anthracene proceeds at its natural rate, and this rate increases by mass action as more dianthracene is formed by light. Under a given intensity of illumination, therefore, as much anthracene is formed by the " dark " reaction as di-anthracene is formed by light in the same time, so that a " stationary condition," simulating- a chemical equilibrium, is arrived at. Contrary to the latter, which would be a permanent one if left to itself, this stationary condition is only maintained by the continuous inflow of light energy, which becomes transformed to heat. It should be noted that, before the stationary condition is reached, part of the light energy becomes chemical energy. Another example is that of the formation of ozone from oxygen by ultra-violet light.

A striking fact, which may appropriately be mentioned here, is that the temperature coefficient of a light reaction is usually much lower than that of a chemical reaction proper. This follows from the fact that the rate of the photo-chemical change depends on that of the absorption of light, which varies only very slightly with temperature. The position of the stationary equilibrium in the case of anthracene, in relation to temperature, is controlled almost entirely by the change of chemical equilibrium by temperature. The temperature coefficient of the dark reaction (chemical) is 2'8, that of the light reaction, 1 '1 or less.

2. Complex Reactions with Increase of Energy. — These result from the combination of various purely chemical reactions with photo-chemical effects. Their reversal is by a different route from that taken in their production. The most interesting and important of these is that of chlorophyll and carbon dioxide, which will be treated of in a special section later. They are the most difficult class to analyse, since the various component reactions proceed both simultaneously and successively.

Those reactions resulting in diminution of free energy are always complex, as we have seen, and may be divided into two main classes : coupled and catalytic. They are non-reversible, in the sense that they do not change back spontaneously in the dark. 3. Coupled Reactions with Loss of Energy. — In these the products of photochemical change are immediately used up in another reaction. As a general scheme, we may take that of Weigert (1911, p. 36) : —

B is produced from A in the light, and would quickly return to A if it were not at once used up in the second reaction to form C. It is probable that the oxidation and reduction of alcohols by aromatic substances, observed by Ciamician and Silber, belong to this group. The important properties of the chemical sensitisers must be included also. A plate coated with silver bromide alone and exposed to the light belongs to our first class, so that when a certain amount of free bromine has been produced by light, a stationary, balanced condition is reached, owing to the recombination of silver and bromine, as in the dark. Such a plate would be of little use in photography, being comparatively insensitive. If, however, a substance such as gelatine is present, which combines with the bromine as it is produced, a much greater decomposition of the silver bromide takes place. There is no storage of energy, since the final system consists of brominated gelatine and metallic silver (or sub-bromide). The bromination of gelatine is associated with the giving off of energy, and the product has no affinity for silver.

An electro-chemical analogy to this group is that of an electrolytic process in which the products are used up in a reaction going on in the solution, so that depolarisation occurs and the current continues to flow. 4. Catalytic Reactions tvith Loss of Energy. — The second group of photochemical reactions in which there is diminution of free energy is that in which catalysts are produced by the action of light. In these cases the action of light leads to the same products which appear in the dark under the same conditions of temperature and solvent. Light merely accelerates the process by causing the formation of a catalyst for the reaction, which then obeys the usual laws of catalysis.

It is found that the catalyst may be formed from the reagents, or one of them, by the action of light, and may then disappear on the removal of the light. Or, in other cases, the catalyst may continue to exist and exert its action for some time after the light has been taken off. 4A. Reactions with Loss of Energy, in which a Catalyst is Formed by Light, the Catalyst lasting only as long as the Illumination, and vanishing in the Reaction. — One of the best known of all catalytic light reactions, namely, the combination of hydrogen and chlorine under the action of ultra-violet light, belongs to this group. A great number of investigations have been made on this reaction since the first exact research by Bunsen and Roscoe (1855-1859). Details of these will be found in Weigert's monograph (1911, pp. 44-56). Under the usual conditions of experiment, the effect is found to have a latent period, the so-called " Induction Period" during which no combination takes place. Subsequent investigation showed that this was due to the presence of impurities, especially on the walls of the vessel used. These impurities use up the catalyst for a time. What is the catalyst? From what was said above, it is clear that light energy is used up to produce it, and it appears to be chlorine in an " activated " form of some kind.

The fact that the drier the gases are, the more slowly does the reaction proceed, suggested to Mellor (1902) that there is formation of an intermediate compound (a;CL2, yH20, zH,2), as in certain other cases of catalysis, such as that of molybdic acid on hydrogen peroxide and hydriodic acid, as described above (page 324). The investigations of Burgess and Chapman (1906) directed attention to the cloud formation, due to the production of condensation nuclei in the illuminated gases. Whether these nuclei are identical with the hypothetical compound of Mellor seems doubtful, and it is more probable that they do not differ essentially from other cloud nuclei formed by radiations.

Chlorine is made " active " by light for other reactions also, such as for combination with carbon monoxide, sulphur dioxide, hydrocarbons, etc. Whatever the nature of the catalyst may be, it must consist of chlorine plus light energy, and therefore act chemically as chlorine itself. Accordingly, it disappears in the reaction. In such reactions it appears that the primary action of light is to form nuclei, which start a reaction in a way analogous to that in which they cause condensation of water vapour to drops of liquid. Weigert calls them " reaction nuclei," and points out that their mode of action is like that of other heterogeneous catalysts. The reacting substances are condensed on their surfaces by adsorption, and the reaction proceeds there more rapidly as a consequence of mass action.

Compared with the reactions in which light energy is stored up, and often in considerable amount, these catalytic reactions require little energy to form the catalyst, and are, as a rule, very sensitive. The phenomena of optical sensitisation belong to the present category of reactions. Light cannot act unless absorbed, and the question naturally arises whether the addition of some substance, such as a dye, to a system which is not affected by light of a particular wave length, is able to make it sensitive if the dye absorbs rays of this wave length. In point of fact, such is the case, although at first sight the reason is not obvious. The light is absorbed by the sensitiser, and must therefore produce changes in this, not necessarily in other, parts of the system. The key is given by the formation of catalysts from the sensitiser, which appear to be heterogeneous in nature and, at all events in many cases, require the presence of oxygen, "activating" it so that potassium iodide is oxidised as by ozone.

A simple experiment given by Wager (1914) shows this fact. Strips of paper containing starch are soaked in a solution of methyl violet, methyl green, eosin, fuchsin, or fluorescein, exposed to light, and then moistened with potassium iodide solution. Iodine is liberated, and stains the starch blue. It is interesting that cyanin, although bleached by light, does not give rise to active oxygen. The mode of action of optical sensitisers seems to be of a somewhat general

nature, since certain reactions can be accelerated by practically any wave length, so long as a dyestuff is present which can absorb these particular rays. The most important practical application of optical sensitisers is in the production of photographic plates sensitive to the whole extent of the spectrum. It is to be remembered that the adsorption of the dye by silver bromide does not make this itself more sensitive. It may be, as Weigert suggests (1911, p. 70), that the light absorbed by the dye makes it a better chemical sensitiser than it is in the dark, so that it takes up bromine with great avidity.

4s. Catalytic Photo-chemical Reactions in which the CataJ.yst remains after the Action of Light. — If the catalyst formed is not immediately used up in the reaction, it is clear that its activity may continue. Such a case is that of iodofonn in chloroform ; the iodine set free by light remains active after the light has ceased to act, and continues so for several days. Moreover, if a solution which has been exposed to the light be added to an unexposed one, decomposition of the latter sets in.

The capacity of being developed at any time after exposure, possessed by photographic plates, is another case. We cannot here discuss the nature of the latent image. The reduction-potential of the developer is not sufficiently high to affect unexposed silver bromide at any considerable rate ; but, where the light has formed a catalyst, metallic silver is produced in development. It appears that the acceleration is due to adsorption of developer on the surface of the heterogeneous catalyst, by which the concentration of the former is raised and, with it, the reduction-potential (see the remarks of Weigert, 1911, p. 74).

It is clear that, in these cases of catalytic action, if we could add the catalyst in any other way than by the action of light, the result would be the same. This is not so in the three first cases of our list, where the same products of reaction as those produced by light cannot be obtained in the dark, at the same temperature, by other means. Electro-chemical analogies for the catalytic action of light may be found in the saponification of an ester in a solution of neutral salt. The catalyst, in this case, is the alkali formed at the cathode, and it disappears by combination with the acid formed from the ester. If we take cane-sugar instead of an ester, the catalyst is the hydrogen ion formed at the cathode and it remains active after cessation of the current, provided that means are taken to prevent diffusion.

Bunsen and Roscoe (1862) showed that in order to produce a definite degree of darkening on silver chloride paper, the time required was inversely proportional to the intensity of the light. That is : — where i is the intensity of the light, and t the time of action. This is known as the Bunsen-Roscoe Law. When the exposure to light is followed by development, the law does not hold. Schwarzschild (1899) showed that, for silver bromide gelatine plates, the law must be expressed thus : —

The value of the exponent/) varies between 0-8 and 1, according to the brand of plate used. It seems probable that the exponential form of the equation may depend on the intervention of adsorption in this case, where development is made use of. Inertia. — There is a certain minimal duration of exposure of a plate to light below which no effect is produced. This is known as the " inertia " of the plate, and appears to be related to the photo-chemical induction already referred to.

There remain to be mentioned some phenomena connected with the absorption of light which are not obviously photo-chemical in nature, that is, chemical changes are not immediately obvious. It is very common to find that substances which absorb light of a particular wave length radiate it again, either at an increased wave length, or of the same wave length as that absorbed. The cases in which ultra-violet light is absorbed and given out again as visible light are the most striking. Such are : solutions of quinine salts, solid anthracene, and so on. In the cases mentioned, part of the light, as we have seen, is used for chemical change. A solution of eosin, which has an absorption band in the blue-green, gives a green fluorescence.

When we are dealing with colloidal solutions it is sometimes more difficult to state whether the phenomena observed are properly to be called fluorescence. For example, a colloidal solution of the acid of Congo-red gives an orange-red "fluorescence." The light transmitted is blue, and it seems that the particles really reflect orange red light in the same way as the dry solid itself does, like other solids of the same colour. The light transmitted would naturally be the complementary to this colour.

In examining colloidal solutions by the Faraday-Tyndall beam, confusion may sometimes be caused by fluorescence. When this is present, the path of the beam will be illuminated, whether colloidal substances are there or not. The distinction can usually be made by interposing screens of various colours between the light and the solution, in order to absorb that part of the light, usually the ultra-violet, causing the fluorescence. The colloidal phenomenon is, of course, found with any wave length of light, provided it is powerful enough. An interesting solution to examine is fresh urine, filtered to remove any coarse particles. Examined in white light, the beam is not extinguished in any position of the Nicol prism used to observe it. But that this is due to the fluorescence of the pigment is shown by the interposition of a yellow screen to absorb the violet end of the spectrum. The Faraday- Tyndall beam is still present, and can be abolished by rotation of the Nicol prism, showing the presence of colloids.

It seems probable that both fluorescence and phosphorescence are really cases of photo-chemical reactions with storage of light energy. Phosphorescence is the giving off of light, not only during illumination, but for a longer or shorter time afterwards. Weigert (1911, p. 26) suggests that a substance A is changed to a substance B, with storage of light energy. The spontaneous return of B to A is associated with the giving out of this energy, again in the form of light. The view is supported by the fact that fluorescence can be changed into phosphorescence at the temperature of liquid air, owing to the reverse reaction being retarded. Phosphorescence itself may be abolished at this temperature, but appears on warming.

The use of fluorescence in the observation of living tissues under the microscope, has been described above (page 9). When bodies are heated gradually they may be seen to begin to give off light when a certain temperature is reached. This light consists, when it first appears, at a temperature of somewhere about 1,000°, only of the longer wave lengths. As the temperature rises, shorter and shorter wave lengths are progressively added. The temperature called " white-heat," as is well known, is very high. Thus light of a particular wave length is associated with a particular temperature in the case of this form of radiation. But light is given of by many chemical reactions at a temperature much below that corresponding to the wave length of the light emitted, supposing it to have been produced by rise of temperature only. This phenomenon is known as chemi-luminescence and is not uncommon. It may be seen by taking a mixture of 10 c.c. of 10 per cent, pyrogallol, 20 c.c. of potassium carbonate, and 10 c.c. of commercial formalin. Add, in the dark, 30 c.c. of 30 per cent, hydrogen peroxide. An orange-red glow, accompanied by considerable foaming, will be seen. A list of the reactions in which similar emission of light can be seen, will be found in the paper by Trautz (1905). The reactions in which it occurs are themselves sensitive to light, and the wave length of this light is the same as that which is emitted in the reaction. Thus the system is set into vibration of its own particular rate by the chemical reaction itself (see Nernst's book, 1913, p. 815).

We may also speak of such reactions as being cases of direct conversion of chemical energy into light energy, without passing through heat. In the same way, certain photo-chemical reactions are direct conversion of light energy into chemical energy. The bearing of these phenomena on the emission of light by organisms will be seen later. The light emitted by the Welsbach mantle appears to be of a shorter wave length than that corresponding to the temperature of the Bunsen flame. It is difficult, however, to determine accurately what is the temperature of the flame. That of a body in it depends on the ratio of its powers of emission and absorption of radiation. For example, a bead of sodium phosphate on a loop of platinum wire is barely luminous in the Bunsen flame, while the platinum glows brightly.