1857.] exhibited by Transmitled Light. 337 



luminous bands alternating with dark spaces. The same was true 

 of coloured liquids, a solution of sulphate of indigo absorbing the 

 orange and yellow rays, and giving a spectrum consisting of a red 

 ray separated by a broad black space from the green, blue, and 

 violet. An oxy-hydrogen lime^light, covered successively by red, 

 yellow, and blue bell-glasses, produced the same effect on the 

 coloured diagrams and other objects around, as if the source of light 

 had been alternately red, yellow, and blue ; and the opacity of 

 these glasses to certain rays, and their transparency to others, was 

 further illustrated by burning spirit lamps, the wicks of which 

 had been previously sprinkled with salt, under yellow and blue 

 bell-glasses. The yellow glass appeared perfectly transparent to 

 the light which it covered, but the blue did not suffer the least 

 yellow to pass ; indeed, the soda-flame under it seemed of a pale 

 violet tipped with green. It was explained that cobalt (to which 

 the colour of the blue glass is owing) absorbs all those rays which 

 are about the dark line D of the spectrum, although it suffers those 

 a little less or a little more refrangible to pass freely ; hence a con- 

 siderable portion of the yellow light of the sun will penetrate such 

 blue glass, but the yellow of the soda-flame is absolutely stopped. 

 As a converse experiment, sulphuret of carbon lamps were ignited 

 under the yellow and blue glasses ; when the blue cover appeared 

 almost transparent and colourless, while the yellow was opaque to 

 the blue light, transmitting only some greenish rays. 



If white light be transmitted through two or more media suc- 

 cessively, each of which has a different absorbing effect upon it, 

 very unexpected results may be frequently obtained. This is true 

 of combinations of coloured glasses, or of coloured liquids. A red 

 solution of meconate of iron, for instance, appears black when seen 

 through the blue solution of an ammoniacal copper salt. If a vessel, 

 filled with the blue alcoholic solution of a cobalt salt, be immersed 

 in a pale yellow solution of chromic acid, it appears to contain a 

 deep red liquid. Green nitrate of chromium also becomes red, 

 when looked at through the same yellow solution. Similarly when 

 two coloured compounds are mixed together, which are incapable 

 of entering into chemical combination, an unexpected colour will 

 frequently result ; thus, on adding a little blue sulphindigotate of 

 potash to a solution of yellow chromate of potash, the result was 

 green, but on adding a larger amount of the blue salt it changed to 

 red. There was here no chemical change ; yet how naturally might 

 a chemist have received the unlooked-for colour as evidence of a 

 new compound ! 



This experiment introduced the subject of dichromatism. A 

 thin stratum of even a highly coloured liquid is almost destitute of 

 colour ; thus the bubbles formed on shaking acetate of iron, or 

 more familiarly, port-wine, or porter, appear white. That the 

 colour of a solution changes in intensity, becoming paler when 

 diluted, and deeper when concentrated, is known to all. This is 



