Popular Science Monthly/Volume 75/July 1909/Josiah Willard Gibbs and his Relation to Modern Science III
|JOSIAH WILLARD GIBBS AND HIS RELATION TO MODERN SCIENCE. III|
ASSISTANT LIBRARIAN, ARMY MEDICAL LIBRARY, WASHINGTON, D. C.
Catalysis, Colloids and Chemical Purity.—When chemical change can be produced in a system by the mere presence of small quantities of another substance which itself usually remains unchanged at the end of the process, such an effect is called catalysis and the agent employed a catalytic agent. Of the varied aspects of catalytic processes we have different examples in the decomposition of substances by the presence of finely divided metals like platinum or colloidal nickel, in the rapid evolution of oxygen from potassium chlorate when a small quantity of manganese dioxide is present, in the solution of insoluble chromic chloride through the mere presence of chromous chloride, in the inversion of cane sugar by acids, in the saponification of fats and esters, in the synthesis of indigo by oxidation of naphthalin, in the standard manufacture of sulphuric acid in the leaden chambers and the later improvements of the method through the presence of platinum or ferrous oxide, in catatyptic photography without light, in the reversible physiologic and therapeutic action of the animal and vegetable ferments and enzymes, in the synthesis of nuclein during the development of the embryo, and in the pathologic effects of poisons, venoms Mid the toxins of disease. Many theories of catalytic action have been advanced, of which the earliest and most original is that of Leibig. Liebig supposes catalysis to be due to the fact that the catalytic agent has power, like that of a tuning fork, to set up sympathetic molecular vibrations in the substance acted upon, producing chemical change. This theory has been proscribed by Ostwald because, being a figment of the mind, it is neither capable of proof nor susceptible of refutation, leading the subject into a blind alley, from which further scientific advance is impossible. It has therefore remained, like Hamlet's father, "quietly inurned," as a beautiful, imaginative hypothesis which we can neither prove nor disprove. Of other theories of catalysis the most important is that of Ostwald himself, summed up in his famous definition: A catalytic agent is one which modifies the velocity of a chemical reaction without appearing in its final process. This statement introduces two new ideas, the notion of infinite swiftness and infinite slowness in chemical change and the fact that catalytic change may be brought about by a series of intermediate reactions. It will be seen that Ostwald's definition is elastic enough to include as catalytic agencies such physical forces as light, electricity, extremes of heat or cold or the action of living tissues, and from this point of view the explosion of a cartridge or a charge of dynamite by percussion, the decomposition of water by electrolysis and its synthesis by the electric spark, the effects of light in photography and in healing disease, the wonderful thermodynamic effects of Henri Moissan's electric furnace, the occasional changes of food in cold storage, are further examples or analogues of catalytic action, and this is all we know of its physical nature. As to a dynamic explanation of how catalysis takes place, we have not got beyond the familiar jest of the laboratories: "Q. What is catalysis? A. Action by contact. Q. What is action by contact? A. Catalytic action." Gibbs's treatment of the subject is interesting as affording a mathematical criterion of what catalysis is and what it is not. It will be remembered that when the entropy of an isolated chemical system, say a bar of steel, has attained a maximum or its free energy a minimum value, the final state of the substance in question has been called by Gibbs a "phase of dissipated energy," implying that it has become physically and chemically inert, so that its equilibrium will not be sensibly disturbed by the presence of other substances or by such small physical agencies as an electric spark. But when the proportion of the proximate components of the substance in connection with its pressure and temperature is such that it does not constitute a phase of dissipated energy, the contact of a very small body or physical agency may produce energetic changes in its mass which do not stop short of complete dissipation. This is catalysis, and Gibbs's definition of a catalytic agent—one capable of reducing a substance to a phase of dissipated energy without limitation as to their relative proportions—is characteristic of the mathematician. A chemical system at constant temperature has several states of equilibrium corresponding to different minima of its isothermal potentials, and on the solid diagrams of Gibbs these minima are valleys at the bottoms of sloping curves. The effect of a catalytic agent on the diagram is to obliterate the ridge between two depressions representing different states of equilibrium on the free energy surface. This means that a system disturbed by a catalytic agent may pass from a higher to a lower minimum of free energy, but never from a lower to a higher unless acted upon by external forces of considerable magnitude. When the lowest minimum of free energy, indicated by the lowest depression on the diagram, has been attained, the substance can no more leave the final phase of dissipated energy than an inert body can be made to go up a hill without the intervention of external forces. On Gibbs's showing, the phase of dissipated energy is the criterion of catalytic action, the condition for which is that the substance acted upon should not have attained such a phase, while the forces operating flow, as in other mechanical, thermal, chemical or electric happenings, from higher to lower potentials. The accuracy of this reasoning is borne out by Emil Fischer's researches in structural chemistry, which show that the intrinsic stability of chemical systems is usually such that it can not be disturbed by "intramolecular wobble," chemical change being brought about by extramolecular or catalytic influences. The mathematical treatment of catalysis gives us a deeper insight into phenomena which no one has as yet succeeded in explaining. "We have not," says Bancroft, "the first suggestion of an adequate theory of catalysis" so essential to a better understanding of chemistry and of life itself. A true theory of catalysis will enable us to solve the problem of the transmutation of the elements, of which we have already had examples in the substances derived from radium, and the recent derivation of tellurium from copper by Sir William Ramsay. The action of animal and vegetable protoplasm is probably catalytic and the chemist can now make some vegetable substances, such as indigo or alizarine, more cheaply and purely than the plants themselves do. Could we substitute inorganic catalyzers for the vegetable enzymes and ferments in all cases, we might, as Bancroft points out, duplicate everything except the plant itself. Recently Loeb has interpreted the fact that some eggs can be developed by osmotic pressure alone, while others require fertilization, by the explanation that, in the former class the nuclein synthesis, which is necessary for segmentation, is started within the nucleus as a catalytic process, one of the products of the reaction being the catalyzer itself; while eggs requiring fertilization are such that the necessary nuclein synthesis must be started by some external catalytic agency. Again catalysis is the key to the causes and treatment of infectious diseases, the toxins and antitoxins of which are probably colloidal catalytic agents. A few drops of such a colloid as cobra venom will rapidly reduce a living animal body to a definite phase of dissipated energy, as far as its vital activity (or "free energy ") is concerned, and such catalysts as colloidal metals, which Bredig has shown to act exactly like the ferments and enzymes, can themselves be "poisoned" or rendered inert by other substances, just as toxins, venoms and poisons can be neutralized by antitoxins or other antidotes. Gibbs did not discuss colloids explicitly, because substances of such indefinite or irregular formation do not admit of mathematical treatment as such, but the physics of what we know of their intimate structure is implicit in his chapters on chemical conditions obtaining at surfaces of discontinuity. Colloids are semi-solid substances, and colloidal solutions are "pseudo-solutions," being suspensions of minute, discrete particles of matter which are not true solutions, in that they obstruct the passage of light, while neither the freezing point nor the vapor tension of the solvent can be sensibly lowered. Graham thought of colloids as dynamic phases of matter, possessing internal energy, while crystalloids are static and inert. The former include reversible colloids like gelatine which, heated with warm water, will upon cooling solidify into a "gel," and redissolve upon heating into a colloidal solution or "sol"; and irreversible colloids, which, when heated with warm water, will coagulate at once into an unchangeable precipitate. Living protoplasm, as Darwin has shown in his experiments upon Drosera and other plants, acts exactly like a reversible colloid. Dead protoplasm, such as a coagulated blood clot, is an irreversible colloid consisting of a fixed network, the meshes of which contain the "sol." There is no evidence of internal structure in living protoplasm, and Hardy supposes that structure in dead protoplasm is produced by submortem or postmortem changes associated with coagulation. Whether the phase rule can be applied to colloids is still an open question bound up with the complex nature of bodies of which we know so little. But recently Siedentopf and Zsigismony have shown that colloidal metals, organic ferments and enzymes are systems in two phases of vast surface tension consisting of suspensions of ultra-microscopic particles acted upon by chemical, thermodynamic and electric potentials. Of such suspensions animal and vegetable bodies are largely made up, protoplasm being a sort of microscopic emulsion, the physiological action of which seems to be bound up with chemical, thermal, electric and osmotic changes between its semi-permeable membranes and surfaces of discontinuity and the various surface tensions and surface energies derived from the free energy of chemical or electric change. If we conceive of colloidal solutions as made up in this way, each tiniest particle being an ultramicroscopic furnace, retort or battery in itself and carrying a definite charge of electricity, we can understand how Liebig's theory of sympathetic vibrations might be applicable to colloidal catalysis at least, and how finely divided metals, serpent venoms or the excretions of microorganisms can produce the extraordinary effects they do. In close connection with the theory of catalysis is the nature of chemical purity and the fact that chemical changes rarely proceed directly to their final product, but usually pass through a series of intermediate stages. For a long time chemists have noticed that absolutely dry or pure substances will not interact directly upon each other, but the cooperation of a third substance is necessary for chemical change. Dried chlorine does not of itself act upon copper and other metals, but the presence of a little moisture will cause it to act upon them at once. A mixture of carbonic acid and oxygen is not explosive when thoroughly dry, but the slightest trace of steam will cause an explosion. The rapid solubility of zinc in sulphuric acid depends upon impurities in the former. Ebullition depends largely upon gaseous impurities in the boiling substance. Absolutely pure or distilled water has no digestive value, but, by its absorptive power, acts as an irritant or poison to the lining membrane of the stomach. Traces of moisture or other impurities have therefore a marked catalytic effect, a theory of catalysis which was first advanced as early as 1794 by Mrs. Fulhame in her "Essay on Combustion." Where water is the impurity, thermodynamic change is supposed to be due to electrolysis: the moisture being the necessary third ingredient for producing a little Voltaic circuit and the electric shock precipitating chemical action as in catalysis. The phase rule, Bancroft reminds us, has taught us to look upon an absolutely pure substance, 100 per cent, strong, as the extreme case of a two-component system, in which the concentration of the second component approaches zero as its limit. Gibbs has shown that in a system of two phases, one component of which is very small, the chemical potential of the dilute component is proportional to the logarithm of its density. As the density of the smaller component becomes less and less, its potential tends to an infinite value, which means that, at the limit, when concentration becomes evanescent, "the removal of the last traces of any impurity would demand infinite expenditure of available energy." From the view-point of mathematical chemistry there are many chemical substances that are relatively and approximately pure, but absolute purity of a chemical nature is, in Whetham's dictum, a more often a pious dream than an accomplished fact."
Ideal Gases and Gas-Mixtures.—It is in the physics of gases that the application of the molecular theory has proved most successful and the laws and equations relating to gaseous states are of considerable accuracy owing to the fact that practically all gases act alike. Although Gibbs made no explicit assumptions as to molecular dynamics, his treatment of gaseous states agrees so well with the kinetic theory that Boltzmann thought he must have had the latter constantly before his mind in framing his fundamental equations. These equations are unique in that Gibbs subjected them to an unusual test of accuracy by comparing their calculated densities of gas mixtures with convertible components with the actual measurements for nitrogen peroxide, acetic and formic acids and phosphorus perchloride by Sainte-Claire-Deville, Horstmann and others. In the case of nitrogen peroxide the difference between the observed and calculated densities scarcely exceeded.01 on the average and was not greater than.03 in any case. The agreement between the theoretical and actual values was equally striking for the other gases, and these results are among the most accurate and satisfactory in the history of physical chemistry. Interesting features of this section of Gibbs's work are his interpretation of Dalton's law as implying that "every gas is as a vacuum to every other gas," his anticipation of van't Hoff's equation in the form of Henry's law for dilute solutions of gases in liquids and his genial discussion of gas-mixtures, known in Germany as,
The Paradox of Gibbs.—If two different gases which can be separated reversibly by quicklime or other process are allowed to mix, a certain definite amount of work or available energy will be gained; but if two gases, which are in every respect identical, are allowed to mix, they could not be separated by any reversible process and there would consequently be no gain of available energy in their mixing nor any dissipation of energy (increase of entropy). But if we suppose two gases which differ only infinitesimally to mix, the first condition would still obtain and there would still be a certain gain of available energy. The question arises, what will happen if we proceed to the limit? Maxwell explained this paradox by saying that our ideas of dissipation of energy depend upon the extent of our knowledge of the subject. Could we invoke Maxwell's demon and borrow his gift of molecular vision, we should perceive that when two identical gases mix there is in reality a complete dissipation of energy, which the demon's intelligence might turn into available energy if he liked; for "it is only to a being in the intermediate stage who can lay hold of some forms of energy, while others elude his grasp, that energy appears to be passing inevitably from the available to the dissipated state." In the reasoning of energetics, the paradox is explained by saying that the more nearly alike the gases are, the slower will be the process of diffusion, so that work or available energy might indeed be gained, but only after the lapse of indefinite or infinite time, if we have such time at our disposal.
Theory of Capillarity, Liquid Films and Interfacial Phenomena.—There are two important theories of capillary action, that of Laplace, based upon the assumption that the play of molecular forces in a liquid is only possible at insensible or ultra-micrometric distances, and that of Gauss, based upon the doctrine of energy. Gibbs's exhaustive discussion of capillarity, which takes up at least one third of his memoir, is the thermodynamic or chemical completion of the purely dynamic theory of Gauss. A capillary film or interfacial layer forms a new "phase" between the two substances on either side of it, and the mathematical condition for the formation of a new chemical substance at such an interface or "surface of discontinuity" is expressible as an algebraic relation between the surface tensions of the three layers of substance and the pressure of the three phases, the surface tensions being functions of the temperature and the chemical potentials. The only stable substance which can be formed between two other phases will be the one having the least surface tension. The chemical equilibrium of solids in contact with liquids, including the delicate mathematical conditions for the formation of crystals in mother liquor, is treated dynamically as a matter of stresses and strains, and this together with the theory of interfacial formations and liquid films will embrace the possible physics of colloid substances. Gibbs gives for the first time a mathematical discussion of the mode of formation of liquid films and the conditions for their stability and his dynamic explanation of the black spots on soap films was proved quantitatively in 1887 by Reinold and Rücker's micrometric data of the relations between the thickness and surface tension of these films. The importance of liquid films in biology is obvious, and this phase of Gibbs's theory, which is capable of the widest development, has as yet received the slightest attention.
Electrochemical Thermodynamics.—One of the most important features of energetics is Gibbs's theory of the galvanic cell which shows the close interrelation existing between chemical, thermal and electric energy. The earliest pioneer in this field was Lord Kelvin, and, prior to 1878, physicists had accepted the Joule-Kelvin theory that the electromotive force of a galvanic apparatus is the mechanical equivalent of the total chemical energy liberated per unit strength in unit time. But this view, which implies that all the electric energy of a chemical cell is available, did not agree entirely with the experimental data of Boscha, Raoult and others. It was corrected and modified by Gibbs, who showed that the electromotive force of the cell is in reality its free energy per electrochemical equivalent of decomposition, from which it follows that neither solidification nor fusion of the metals at the temperature of liquefaction should cause any abrupt alteration of the electromotive force. In 1882, six years later, this important theorem was rediscovered from a different view-point by Helmholtz and brilliantly developed as to experimental confirmation. The Gibbs Helmholtz doctrine enables the physicist to trace out the variations in electromotive force due to chemical differences in different cells. In a letter to Professor Bancroft, now printed in the memorial edition, Gibbs connects the mathematical part of his theory of the electric cell with the fundamental principles of physical chemistry, the theories of van't Hoff and Arrhenius, Nernst's osmotic theory of the Voltaic cell and the equations of Ostwald and van der Waals. But perhaps his most important contributions to the theory of electricity are the two papers on electrochemical thermodynamics which he sent to the British Association in 1886 and 1888; Helmholtz, in his well-known formula for electromotive force, gives a relation such that if a cell be set up, and the reversible heat measured, the electromotive force need not be measured, but may be calculated from these data, or vice versa. In Gibbs's rendement of the perfect (or reversible) galvanic cell, both the electromotive force and the reversible heat can be predicted from his equation without the necessity of setting up any cell at all. "Production of reversible heat," says Gibbs, "is not anything incidental, superposed or separable, but belongs to the very essence of the operation." In discussing the matter in 1887, Sir Oliver Lodge raised the question whether Professor Gibbs was not regarding a galvanic cell as "too simply a heat engine" or assuming that the union of the elements in a cell primarily produces heat and secondarily propels a current. Gibbs replied that "in supposing such a case we do not exceed the liberty usually allowed in theoretical discussions" and proceeded to show, in an ingenious demonstration, that Helmholtz's equation flows as a natural consequence from his own earlier results. The accuracy of his reasoning is sustained by such developments of the subject as the "Peltier effect," in which it is demonstrated that the thermoelectric effect in systems of conductors, in which no chemical action takes place, is still proportional to the absolute temperature at any junction. In general the properties of a thermoelectric system are determined by the entropy function, and the entropy and energy in a thermoelectric network are not, as previously supposed, stored in the conductors, but, as we see in the electric transmission of motor power from a waterfall like Niagara to an engine or railway car, actually travel with the moving charge of electricity itself. In short, "entropy can be located in an electric charge."
Such are a few of the mathematical and physical consequences flowing from the single idea of entropy, and they are sufficient to define the position of Gibbs in the history of thermodynamics. In the establishment of the dynamical theory of heat, says Larmor, "The name of Carnot has a place by itself; in the completion of its earlier physical stage the names of Joule and Clausius and Kelvin stand out by common consent; it is, perhaps, not too much to say that, by the final adaptation of its ideas to all reversible natural operations, the name of Gibbs takes a place alongside theirs."
(To be Continued)
- Ostwald, "Ueber Katalyse," Leipzig, 1902.
- Loeb, Science, 1907, N. S., XXVI., 425-37.
- Darwin, "The Power of Motion in Plants," passim.
- Tr. Connect, Acad., III., 194-7.
- Larmor, "Encycl. Britan.," 10th ed., XXVIII., 169.
- Whetham, "The Recent Developments of Physical Science," Philadelphia, 1904.
- Aus vielen Stellen geht deutlich hervor, dass Gibbs auch diese molekular-theoretische Anschauung fortwährend vor Augen hatte, wenn er auch von den Gleichungen der Molekularmechanik keinen Gebrauch machte." Boltzmann, "Vorles. über Gastheorie," Leipzig, 1898, II., 211.
- Gibbs, Am. J. Sc., 1879, 3. s., XVII., 277, 371.
- Tr. Connect. Acad., II., 240.
- Ibid., 218.
- Ibid., 194-7, 225-7.
- Ibid., 227-9.
- Maxwell, "Encycl. Britan.," 9th ed., VII., 220, sub voce "Diffusion."
- Larmor, "Encycl. Britan.," 10th ed., XXVIII., 171.
- Tr. Connect. Acad., III., 391-416.
- Ibid., 403.
- Ibid., 479-81.
- Phil Tr., 1887, CLXXVII., 627, 684.
- "The quantities of the different substances combined in connection with the passage of a unit electricity are called the electrochemical equivalents of these substances." Bryan, "Thermodynamics," 164.
- Helmholtz, Sitzungsb. d. Berl Akad., 1882, 22 et seq.
- See Bancroft, J. Phys. Chem., 1903, VII., 416-427.
- "Report British Association for the Advancement of Science," 1886, 388.
- Loc. cit.
- "Report British Association for the Advancement of Science," 1888, 343-6.
- See Bryan, "Thermodynamics," Leipzig, 1907, 174, 198.
- Proc. Roy. Soc. Lond., 1905, LXXV., 292.