REACTION. 732 REACTION. ables us to trace the course of habituation, fatigue (q.v.), expectation, and practice (q.v.). Bibliography. Sanford, American Journal of Psychology, ii. (AYorcester, 1888-89) ; Lange, Philosophische Studien, iv. (Leipzig, 1888) ; Wundt, Physiologische Psychologic (ib., 1893) ; Jastrow, The Time Relations of Mental Phenomena (New York, 1886) ; Kulpe, Out- lines (London, 1895) ; Titehener, Outline (New York, 1899) ; id.. Experimental Psychology (ib., 1901) ; Flournoy, Observations sur quelques types de reaction simple (Geneva, 1896) ; Stern, Ueber Psychologic der individuellen Diffcrenzen (Leip- zig, 1900). See Psychological Appabatus. REACTION (in medicine). A special vital movement tending to overcome some noxious action or influence affecting the organism. The term is also applied to an effect produced by the application of a stimulus to a nerve or muscle. In surgery the term has a special sig- nificance, and is used to indicate the process of recovery from a state of collapse. Collapse, reaction, and the general effects of shock upon the system are considered under Shock. REACTION, Chemical. A term applied to the transformations of substances into other substances having more or less different prop- erties. (See Chemistry.) It is noteworthy that the mutual transformations of the allotropic modifications of one and the same element must be considered as chemical reactions. Thus, the transformation of ^-ellow phospliorus into red phosphorus is a chemical reaction. In essence the two substances are identical ; but they never- theless differ in their chemical behavior, and under the same physical conditions they possess different physical properties ; so they must be considered as two distinct chemical individuals and their transformation into each other must be considered as a chemical reaction. The general laws according to which chemical reactions take place include the law of the con- ser-ation of matter, the law of the conservation of the elements, the law of definite combining masses, the law of combining volumes, the law of mass action, and, of course, the law of the conservation of energy. The conservation of energy plays an important part in thermo-chem- istry and electro-chemisti-y (qq.v.). The other laws, with the exception of that of mass action, have been considered in the general articla Chemistry (q.v.). In the present article it remains to discuss briefly the action of masses. At first consideration the concept of mass action appears to contradict the law of definite proportions. According to the latter, while sub- stances may be mixed in any proportion what- ever, chemical combination only takes place be- tween definite relative quantities. Thus, oxygen combines directly with hydrogen only in the proportion of eight parts (by weight) of the former to one of the latter, whether a given mixture in which the reaction is caused to take place contains the two gases in this or in any other proportion. What. then, can the masses of the reacting substances have to do with the course of the reaction? A simple example may serve to illustrate the point in question. Ordi- nary alcohol and acetic acid combine in the proportion of 4fi parts of the former to 60 parts of the latter. If 46 gi-anis of the alcohol should be left in contact with 60 grams of the acid for a sufficiently long time. 30.7 grams of the alcohol would combine with 40 grams of the acid (30.7:40 = 46:60), yielding 58.7 grams of ethyl acetic ester and 12 grams of water. The rest of the alcohol (15.3 grams) and of the acid (20 gi'ams) would remain imcom- bined, side by side with the ethyl acetic ester and water, no matter how long the mixture were kept (in one experiment the mixture was ac- tually kept for seventeen years ) . Now, if in- stead of 46 grams a larger quantity of alcohol' were left in contact with 60 grams of acetic acid, more of the latter would ultimately be found to have entered into combination, and con- sequently less than 20 grams of it would ulti- mately remain free. But the proportion of al- cohol and acid combined would still be 46 parts of the former to 60 parts of the latter. This example illustrates the following principles : ( 1 ) Whatever the proportion of the reacting sub- stances present, chemical combination takes place between the same relative quantities ; ( 2 ) whatever the proportion of the reacting .sub- stances present, the possible maximum of each i may not enter into combination, a fraction of the several substances present refusing to com- bine at all, as long as they remain in contact with the products of the reaction ; ( 3 ) the amounts of the substances present determine the fraction that will enter into combination and the - fraction that will remain free. The first of these principles is nothing else than the law of definite proportions. On the other hand, the doctrine of mass action has reference to the second and third of these principles, dealing, not with the relative combining quantities, but with the extent to which combination takes place. A fact of the greatest importance for the theory of chemical transformations is that the course of many reactions can be reversed. For instance, we just said that ordinary alcohol and acetic acid partly combine to form ethyl acetic ester and water. But ethyl acetic ester and water, if allowed to remain mixed for a sufficient length of time, will react and produce free acetic acid and alcohol. In this transformation again the ester and water would partly react (in accordance with the law of definite proportions) and partly remain unchanged. Quantitative ex- periment would reveal the following facts: (1) If we should mix 88 grams of the ester with 18 grams of water (88 and 18 are, respectively, the reacting weights of the two substances ) , then 29^3 grams of the former and 6 grams of the latter would react to form 20 grams of free acetic acid and lo^a grams of alcohol, while the remaining 58 -y grams of the ester and 12 grams of water would refuse to enter into reac- tion ; (2) if after all change has ceased in our mixture, we should add to it a further quantity of either ester or water, then a further (but not complete) decomposition of ester into alcohol and acid would take place; (3) if, on the con- trary, after all change has ceased in our mixture, we should add to it, not ester or water, but either free acetic acid or alcohol, a further change would take place, resulting in the forma- tion of more ester and water. It would thus become clear that in a mixture of reacting substances with the products of their reaction, when the mixture is in a state of 'chemical equilibrium,' we may cause a reaction to take place either in one direction or in the opposite direction, by changing the relative masses of the ingredients. In other words, the masses of
