REACTION. 733 READ. substances may determine the course of a chem- ical reaction. The importance of this statement in chemical theory vriW be so much the more obvious if we consider that there is good reason for assuming that all reactions are reversible. Says Xernst: "It has been formerly often main- tained that 'reversible reactions' are exceptional, or that two different classes of reactions must be distinguished, reversible and non-reversible. But no such definite line of demarkation exists by any means, and there is no doubt that under appropriate experimental conditions it will be possible to cause any reaction whatever to take place now in one, now in the opposite direction; that is to say, in principle every reaction is reversible." But what, then, becomes of the old notion of chemical affinity? According to that notion, a chemical change takes place solely because the components of a given substance have a greater affinity for the components of another substance than for one another: two compounds AB and CD react and yield AC and BD solely because the affinities between A and C, B and D, are greater than the affinities between A and B, C and D. If this notion were correct, the op- posite reaction, viz. the transformation of AC and BD into AB and CD, would evidently be impossible. But many reversible reactions are known as a matter of fact. Therefore the old notion of chemical affinity as the sole cause of reactions must be either discarded or essentially modified. A careful study of facts leads to the following conclusion: The course of reactions and the final equilibrium to which that course leads are certainly influenced by the chemical affinities (whatever may be their ultimate na- ture) ; but those affinities are not alone in deter- mining the chemical phenomena which they influ- ence. In studying reversible reactions it Ls found that the action of masses comes in as a factor in all cases, without an exception. But it is also found that if chemically equivalent masses are .started with in all eases, the frac- tions of those masses actually entering into reaction and the fractions remaining unchanged vary from case to case. In other words, as just stated, the equilibrium finally attained depends both on the nature of the reacting substances and on the masses present to start with. Still another factor takes part in influencing the course and end of reactions, viz. the tempera- ture. Or, more exactly, the specific affinity factor at work in each ease varies with the temperature. But this variation will be con- sidered under Thermo-Chemistkt, and may be left out of account in the present discussion, in which all reactions are assumaed to take place at constant temperature. We have seen above that the total fraction found transformed when a reaction is over de- pends, among other things, on the masses present at its beginning. Similarly, it may be demon- strated bv facts that the fraction transformed in a given interval of time during the reaction depends on th? masses present at the beginning of that interval. And since the reaction itself obviouslv changes the masses present, partof the original substances gradually disappearing as such and masses of the products of the reac- tion gradually appearing in their stead, it is evident that' the magnitude of the fraction transformed in unit of time varies from instant to instant. Were this not so, the "velocity' of a chemical reaction might be defined in terms of the amounts found transformed in any finite interval of time. But since, as just explained, the velocity is variable, it can be defined only in terms of the infinitely small amounts trans- formed in infinitely short internals of time. (If this does not seem clear, see the article Cal- ctTLCS. ) It may, however, be asked: But why consider at all the velocity of reactions? The answer is. Because it is the velocity that is immediately determined by affinity and mass action; and so, conversely, it is by measuring the velocity that affinity and mass action can be studied quantitatively. In the case of a re- versible reaction, the direction of the change is the direction in which the velocitj* of reaction is the greater; the amount actually found trans- formed in any time depends on the difference between the velocities with which the two op- posite reactions take place; and when the two velocities are equal, there is equilibrium. Thus the velocities of a reaction describe its course completely. The development of the principles discussed in the precedirfg paragraphs forms the object of chemical kinetics and statics, the two sub- divisions of the modem "doctrine of affinity.* Chemical kinetics deals with chemical change; chemical statics with chemical equilibrium. At the basis of both is the law of mass action in its precise mathematical form, which may now be considered as established beyond the slightest possibility" of doubt. For it has been demon- strated in three different ways: (1) by mathe- matical deduction from the kinetic theory of gases; (2) by mathematical deduction from the laws of thermodvnamics; and (3) by extensive experimental observation. Correctly, but vague- ly, the action of masses was first luiderstood by the Frenchman Berthollet. in the beginning of the nineteenth century. In 1S67 Guldberg and Waage, two Norwegian investigators, published a work (Etudes sur les afpnites chimiques) in which the principles of chemical statics and kinetics were first stated and demonstrated in their rigidly mathematical form. But this work remained unknown for a nimiber of years, ilean- while Van't Hoff discovered the law of mass action independently in 1877; and from this date may be said to commence a new epoch in theo- retical chemistry. The principal names con- nected with the demonstration and mathematical and experimental development of the law are, besides those of Guldberg and Waage. the names of Van't Hoff, Horstmann, Gibbs, Arrhenius, and Ostwald. For further information, consult the works on theoretical and physical chemistry recommended in the article Chemistry. See also Acins: De- coMPOSiTio:* : Dissociation : Electko-Cheii- ISTBT: ESTEBS; SOLUTION; THEBMO-CHEinSTRY. READ, Charles (1819-98). A French scholar, born in Paris, where he studied law, and served first in the magistracy and then in the administration, in the department of religious education (1852-58). and in that of historical documents (1865-70). In 1852 he founded the French Protestant Historical Society. His own labors preliminary to a history of the Reforma- tion in France, which he did not complete, were mostly editorial, and include editions of Cha- mier's journal (1859), of Agrippa d'Aubigne's Tragiques (1872), and of his Enfer (1873) and
