ACTINOIMETRY. That light is capable of caus ing chemical transformations was not unknown in the eighteenth century. That silver chloride turns dark antler the action of light was known to Schultze as far back as 1727. In the latter part of the century it was recognized that sun light enables plants to decompose carbonic acid gas and set free its oxygen, and about 1810 Liebig grasped the full imRortance of this process in the economy of nature. But all photochemical knowledr.e remained qualitative until .401m W. Draper, of New York University, about 1842, in troduced the quantitative measurement of the chemical action of light, by constructing the first `actinometerf Gay-Lussac and ThEmard had observed in 1800 that a mixture of equal volumes of chlorine and hydrogen, which may be pre served indefinitely in the dark, reacts with ex plosive rapidity if exposed to intense light, and slowly if the light is weak. The product of the reaction is hydrochloric acid gas, which is freely soluble in water, while hydrogen and chlorine are soluble only sparingly. TO measure the ef fect of light in causing the reaction, Draper intro duced a mixture of precisely equal volumes of hydrogt.n and chlorine into a glass bulb (a) half filled with water and joined to a glass tube (b) likewise filled with the gaseous mixture, provided with a scale, and in its turn joined to a wide ves sel ( c) filled with water. He then exposed the upper part of a to light, whereupon part of the hydrogen and chlorine combined into hydrochloric acid, the latter was absorbed by the water. and the consequent diminution of the gaseous volume was shown by the length of the column of water entering from c into b. Draper's ac-tinometer was improved in 1862 by llunsen and Roscoe. But even the improved form of the apparatus did not permit of sufficiently rapid experimentation, and consequently the method was soon,abandoned. Instead, Bunsen and Roscoe employed an acti nometer which measured the chemical intensity of light by the time required to darken a pho tographic film to a certain standard degree.
In 1879 Eder devised a new th,thed of acti nometric measurement. if a mixture of mer
eurie chloride (corrosive sublimate) and am monimn oxalate in aqueous solution is exposed to the action of light (especially the ultraviolet rays), a chemical change takes place resulting in the precipitation of mercurous chloride (calo mel), the quantity of which measures the chemi cal intensity of the light. Account must of course be taken of the fact that the amount of precipitated not only depends on the intensity of the light, but is also influenced by the vary ing concentration of the solution during the ex periment.
In 1807 Rigollot constructed an actinometer by immersing into a solution of common salt two copper plates whose surfaces were previously oxidized in the flame of a Bunsen burner. and exposing one of the plates to the light. if the two copper plates of this apparatus are con nected by a wire, an electric current is found to tlow from the (lark plate toward the illumi nated one, the intensity of the current being pro portional to the intensity of the light, and hence being capable of serving as a measure of the latter. It may be observed that the principle of electrochemical actinometry was discovered as far hack as 1839, by Becquerel, who found that a current passed between two silver plates covered with films of chloride of silver and immersed in dilute sulphuric acid if one of the plates was exposed to the light.
i;ENERAL ParxeletEs. Passing now to the few known general principles of photochemistry, it must he observed first of all that while it was for merly believed that light of certain colors only (`actinic rays') was capable of chemical action, it is now generally admitted that chemical changes can be produced by light of any wave length whatcler, including the infra-red and the ultra iolet rays. It is further generally ad mitted that if a substance is at all incapable of absorbing light, it is also incapable of undergoing photo-chemical changes of any kind. It is often possible, however, to increase the capacity Of a substance for absorbing light, by mixing it with other substances which absorb light freely.