LIGHT 



109 



Rocoella, Lecanora, Variolaria, &c., yield purple and blue colours. In this country 

 archil and cudbear, purple colours, are prepared from it. In Holland, a blue colour 

 litmus. 



Dr. Stenhouse, to -whom we are much indebted for many important inquiries con- 

 nected -with the applications of chemistry, has given the following table of the 

 lichens : 



See ARCHIL ; FRENCH PUHPLE ; LITMUS ; ORCHEIXA WEED. 



IICKNER'S BLUE. The silicate of cobalt and potash. 



XiXEBXGXTE. A hydrous carbonate of uranium and lime, named after the late 

 Baron Liebig. 



XiIEVRXTE. A silicate of iron, known as Evaite and Jenite. 



XiXCHT. (Lumiere, Fr. ; Licht, Ger.) The operation of light as an agent in 

 the arts or manufactures has scarcely yet received attention. Sufficient evidence 

 has, however, been collected to show that it is of the utmost importance in producing 

 many of the remarkable changes in bodies which are desired in some cases as tho 

 result, but which in others are to be, if possible, avoided. 



There is a very general misconception as to the power or principle to which cer- 

 tain phenomena, the result of exposure to sunshine, are to be referred. In general 

 light is regarded as the principle in action, whereas frequently it has nothing what- 

 ever to do with the change. A few words therefore in explanation are necessary. 

 The solar ray, commonly spoken of as light, contains, in addition to its luminous 

 power, calorific power, chemical power, and, in all probability, electrical power. (See 

 ACTINISM.) These phenomena can be separated one from the other, and individu- 

 ally studied. All the photographic phenomena are dependent upon the chemical 

 (actinic) power. Many of the peculiar changes which are effected in organic bodies 

 are evidently due to light, and the phenomena which depend entirely on heat are well 

 known. 



Herschel has directed attention to some of the most striking phenomena of light, 

 especially its action upon vegetable colours. As these have direct reference to the 

 permanence of dyes, they are deserving of great attention. The following quotation 

 from Sir John Herschel's paper ' On the Chemical Action of the Bays of the Solar 

 Spectrum, &c.,' will explain his views and give the character of the phenomena which 

 he has studied. He writes : 



' The evidence we have obtained by the foregoing experiments of the existence of 

 chemical actions of very different and, to a certain extent, opposite characters at the 

 opposite extremities (or rather, as we ought to express it, in the opposite regions) of 

 the spectrum, will naturally give rise to many interesting speculations and conclu- 

 sions, of which those I am about to state will probably not be regarded as among the 

 least so. We all know that colours of vegetable origin are usually considered to be 

 destroyed and whitened by the continual action of light. The process, however, is 

 too slow to be made the subject of any satisfactory series of experiments, and, in 



