170 Mr Ponton on preparing Paper 



the salt which have not been acted on by the Hght are readily 

 dissolved out, while those which have been exposed to the 

 light are completely fixed in the paper. By this second pro- 

 cess, the object is obtained white upon an orange ground, and 

 quite permanent. If exposed for many hours together to 

 strong sunshine, the colour of the ground is apt to lose in 

 depth, but not more so than most other colouring matters. 



This action of light on the bichromate of potash differs from 

 that upon the salts of silver. Those of the latter which are 

 blackened by light, are of themselves insoluble in water, and 

 it is difficult to impregnate paper with them in an equable 

 manner. The blackening seems to be caused by the forma- 

 tion of oxide of silver. In the case of the bichromate of potash, 

 again, that salt is exceedingly soluble, and paper can be easily 

 saturated with it The agency of light not only changes its 

 colour, but deprives it of solubility, thus rendering it fixed in 

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

 gagement of free chromic acid, which is of a deep red colour, 

 and which seems to combine with the paper. This is rendered 

 more probable from the circumstance that the neutral chro- 

 mate exhibits no similar change. 



The active power of the light in this instance, resides prin- 

 cipally 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 trans- 

 mits the violet rays, one with bichromate of potassa transmit- 

 ting the yellow rays, the third with tincture of iodine trans- 

 mitting 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 

 ammoniuret of copper. 



The best mode of preparing paper with bichromate of po- 

 tash 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 day light. 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 colour will not be so deep. 



A pleasing variety may be made by using sulphate of indi- 



