Page:The American Cyclopædia (1879) Volume XV.djvu/259

 SPECTRUM 247 been reached solely from the observed actions on compounds of silver ; and what is often styled " the curve of chemical force " in the spectrum we have given in fig. 7. This curve is gener- ally referred to as giving the distribution of chemical action in all cases. This is erroneous ; as long ago as 1842 Dr. J. "W. Draper showed: 1, that so far from chemical influences being restricted to the more refrangible rays, every part of the spectrum, visible and invisible, can give rise to chemical changes, or modify the molecular arrangement of bodies ; 2, that the ray effective in producing chemical or molec- ular changes in any special substance is deter- mined by the absorptive property of that sub- stance. He found that if a spectrum be re- ceived on iodide of silver formed on the sil- ver plate of the daguerreotype, and the im- Eression of the light be then developed, after it as acted for a moderate time we shall observe a stain which corresponds in character and po- sition to the blackening effect that under like circumstances would be found on any common sensitive silver paper. If, however, the action of the light be long continued, a white stain makes its appearance over all the less refran- gible regions of the spectrum. But if the daguerreotype plate during its exposure to the spectrum be also receiving diffused light of little intensity, it will be found on developing that the impression obtained differs strikingly from the preceding. Every ray that the prism can transmit, from below the extreme red to beyond the extreme violet, has been active. "10 ultra red athermic lines are present. The impression of these lines is a proof of proper spectrum action, and distinguishes it from that of diffused light, arising either from the atmos- phere or from the imperfect transparency of the prism. In a series of photographic prints accompanying a paper by Dr. Schultz Sel- lak " On the Sensitiveness to Light of Haloid Salts of Silver, and on the Connection between Optical and Chemical Absorption," may be observed the varying extent of the chemical action of the spectrum and the shifting of the place of maximum action depending on the na- ture of the chemical preparation on which the spectrum is formed. Thus, chloride of silver collodion is acted on by the portion of the spec- trum from about half way between the lines G and H up to the line N, fig. 7. Iodide of silver collodion is affected from below G nearly to the line M ; bromide of silver collodion from F to M. A mixture of silver salts formed of the iodide and bromide of collodion is sensitive to the action of the spectrum in the space from the line E to^the line M. Mixed iodide and chlo- ride of silver collodion are acted on through- out nearly the same area. The remarkable in- crease of sensitive area when the spectrum falls on the above named mixtures has long been turned to good account in practical photogra- phy. (See PHOTOGRAPHY.) The most remark- able confirmation of Draper's first proposition, as given above, is in the case of the spectral action on a surface of "West India bitumen. A glass plate is coated with this substance as follows : The bitumen is dissolved in benzine, and the solution poured on a glass plate in a dark room and drained off, leaving a film of bitumen sufficiently thin to be iridescent. This is exposed to the spectrum for five minutes, and then developed by pouring over it a mix- ture of benzine and alcohol, which will now only dissolve those portions of the film that have not been acted on by the light. The be- ginning of the impression is below the line A, its termination beyond H. Every ray in the spectrum acts. The proof is continuous except where the Fraunhofer lines fall. Dr. Draper found that the decomposition of carbonic acid gas by plants is accomplished by rays between the lines B and F, which is another instance of the chemical action of the less refrangible rays. In 1842 Sir John Herschel discovered that the yellow stain imparted by the corcTiorus Japo- nica to paper is whitened by the green, blue, indigo, and violet rays. The rose red of the ten weeks' stock is in like manner changed by the yellow, orange, and red. The rich blue tint of the viola odorata, turned green by sodi- um carbonate, is bleached by the same group of rays, that is, by those less refrangible than the yellow. The green chlorophyl of the elder leaf is changed by the extreme red. To a for- mer experimenter, Grotthus, we owe the dis- covery of the law under which these decompo- sitions of the colors of flowers take place. This law in repeated instances was verified by Her- schel, and more recently by Draper. It may be thus expressed : The rays which are effec- tive in the destruction of any given vegetable color are those which by their union produce a tint complementary to the color destroyed. Even the partial establishment of this law, already accomplished, is sufficient to prove that chemical effects are not limited to the more refrangible portions of the spectrum, but can be occasioned by any ray. The second prop- osition of Draper, that the rays which act chemically on a substance are those which are optically absorbed by it, has received am- ple independent confirmation by the recent ex- periments of Sellak in his paper cited above. Sellak found that optical and chemical ab- sorption of light exactly coincide. All colors which are sensibly absorbed (optically) by the haloid salts of silver, of a thickness of a few millimetres, produce chemical decomposition. The optical absorption of transparent plates of these substances is shown by spectral ob- servation to be confined exactly within the limits of chemiccal action. This is especially the case with mixtures of iodide and bromide of silver. Chloride of silver is colorless, iodide of silver is transparent light yellow, bromide of silver is somewhat deeper yellow, and the mixture of the last two orange yel- low. E. Becquerel in 1842 (Bibliotheque Uni- verselle de Geneve) was the first to photo- graph the Fraunhofer lines, and in doing so