Nature and Science. The Chemical Rays of the Spectrum.

Scribners monthly 5, 1873

The second memoir of Dr. John W. Draper's researches in Actino-chemistry is published simultaneously in the London and Edinburgh Philosophical Magazine and in the American Journal of Science and Art. His first memoir was on the distribution of heat in the spectrum; the present one is on the distribution of chemical force. In the former he sowed that all the rays of the spectrum possess equal heating power. In this, he shows that every ray can bring about chemical changes. Hitherto it has been supposed that the actinic force - the force that brings about chemical decompositions - is restricted to the violet of the spectrum, and in all the books treating on this subject, figures are intrduced illustrating this view. In this memoir, the error of this is shown by an analysis of several decompositions brought about by the agency of radiation. The following paragraph furnishes a summary of the evidence: -

"At this point I abstain from the adding other instances showing that chemical changes are brought about in every part of the spectrum. The list of cases here presented might be indefinitely extended, if these did not suffice. But how is it possible to restrict the chemical force of the spectrum to the region of the more refrangible rays, in face of the facts that compounds of silver, such as the iodide, which have heretofore been mainly relied upon to support that view, and in fact originated it, are now proved to be affected by every ray from the invisible ultra-red to the invisible ultra-violet? how, when it is proved that the decomposition of cabonic acid, by far the most general and most important of the chemical actions of light, is brought about, not by the more refrangible, but by the yellow rays? The delicate colors of flowers, which vary indefinitely in their tints, originate under the influences of rays of many different refrangibilities, and are bleached or destroyed by spectrum colors complementary to their own, and therefore varying indefinitely in their refrangibility. Toward the indigo ray the stems of plants incline; from the red their roots turn away. There is not a wave of light that does not leave its impress of bitumens and resins, some undulations promoting their oxidation, some, their deoxidation. These actions are not limited to decompositions: they extend to combinations. Every ray in the spectrum brings on the union of chlorine and hydrogen."

He adds, in conclusion: "The figure so generally employed in works on actino-chemistry to indicate the distribution of heat, light, and actinism, serves only to mislead. The heat curve is determined by the action of the prism, not by the properties of calorific radiation; the actinic curve does not represent any special peculiarities of the spectrum, but the habitures of certain compounds of silver."