There are many chemical reactions which are promoted or accelerated by the addition of a small quantity of some foreign substance which is not used up in the process and does not appear in the final products. Thus one of the romances of chemistry was the discovery, occasioned by the chance breaking of a thermometer in the vessel, that the presence of a small quantity of mercury greatly hastens the oxidation of naphthalene to phthalic acid, a process of great importance in the manufacture of synthetic indigo. Similarly the presence of finely divided metals accelerates many reactions, such as oxidations and hydrogenations--for example, asbestos impregnated with particles of platinum promotes the oxidation of sulphur dioxide to the trioxide in the manufacture of sulphuric acid. The researches of Baker and others, showing that certain gas reactions, which ordinarily take place rapidly, proceed very slowly indeed if the gases are thoroughly dried, point to a catalytic action of small traces of moisture. The enzymes of the human body which accelerate the chemical processes of digestion and assimilation constitute another class of catalysts, and Drs. Rideal and Taylor class under catalytic action the effect of radiant energy in promoting such combinations as that of hydrogen and chlorine, although it is perhaps rather extending the usual conception of the term to do so. These examples will indicate the wide range of the subject and help to make intelligible Ostwald's famous generalisation that "there is probably no kind of chemical reaction which cannot be influenced catalytically, and there is no substance, element, or compound which cannot act as a catalyser," which is no doubt true if very slight accelerations of reaction be taken into account. Of course a catalyst cannot affect the final state of equilibrium, but only quicken or institute (the discussion as to whether, in some cases, the catalyst initiates or merely accelerates a reaction already taking place imperceptibly slowly seems to us pointless) a reaction theoretically possible. Other, the so-called negative, catalysts hinder reactions; other substances "poison," or stop, the action of ordinarily activating materials; others again, the "promoters," increase the efficacy of the catalyst. The phenomenon is a complex one.