363 Miscellanies. 



there being several gradations of shade, or tint, according to the 

 greater or less degree of transparency in the different parts of the 



object. 



In this state, of course, the drawing though very beautiful, is evan- 

 escent. To fix it, all that is required is careful immersion in water, 

 when it will be found that those portions of the salt which have not 

 been acted on by the light are readily dissolved out, while those which 

 have been exposed to the light are completely fixed in the paper. 

 By this second process, the object is obtained white upon an orange 

 ground, and quite permanent. If exposed for many hours together 

 to strong sunshine, the color of the ground is apt to lose in depth, 

 but not niore so than most other coloring matters. 



The action of light on the bichromate of potash differs from that 

 upon the salts of silver. Those of the latter which are blackened by 

 lio-ht are of themselves insoluble in water, and it is difficult to imprcg- 



o 



Date paper with them in an equable manner. The blackening seems 

 to be caused by the formation of oxide of silver- In the case of the 

 bichromate of potash, again, that salt is exceedingly soluble, and pa- 

 per can be easily saturated with it. The agency of light not only 

 changes its color, but deprives it of solubility, thus rendering it fixed 

 in the paper. This action appears to me to consist in the disengage- 

 ment of free chromic acid, which is of a deep red color, and which 

 seems to combine with the paper. This is rendered more probable 

 from the circumstance that the neutral chromate exhibits no similar 



eh an ire. 



The active power of the light in this instance, resides principally 

 in the violet rays, as is the case with the blackening of the salts of 

 silver. To demonstrate this, three similar flat bottles were filled, one 

 with ammoniuret of copper which transmits the violet rays, one with 



bichromate of potassa transmitting the yellow rays, the third with 



tincture of iodine transmitting the red rays- The paper was readily 

 acted on through the first, but scarcely if at all through the second 

 and third ; although much more light passed through the bottle filled 

 with bichromate of potassa than through the one filled with ammoni- 

 uret of copper. 



The best mode of preparing paper wuth bichromate of potash is to 

 use a saturated solution of that salt; soak the paper well in it, and 

 then dry it rapidly at a brisk fire, excluding it from daylight. Paper 

 thus prepared acquires a deep orange tint on exposure to the sun. If 

 the solution be less strong or the drying less rapid the color will not 

 be so deep. 



A pleasing variety may be made by using sulphate of indigo along 

 with the bichromate of potash, the color of the object and of the pa- 



