40 



THE CHEMISTRY OF THE SUN. 



[CHAP. 



affording an instance of selective absorption, while certain 

 kinds of blue glass will cut off all but the blue. By intro- 

 ducing both the pieces into the beam the spectrum is entirely 

 obliterated. 



When liquids have to be examined it is necessary to have 

 some sort of transparent cell to hold them. It will not be 

 necessary to buy such an apparatus ; two squares of glass witli 

 a piece of india-rubber tubing between them bent thus U, the 

 glasses being kept in contact with the tubing by two india- 



Thick. 



Thin. 



FIG. 18. Absorption of sunlight by 

 various thicknesses of a solution of 

 the salts of chromium (Gladstone). 



FIG. 19. Absorption of sunlight by 

 various thicknesses of a solution of 

 potassic permanganate (Gladstone). 



rubber bands, form a cell which is wonderfully tight, and 

 will serve our purpose excellently. This, with the inclosed 

 liquid, must be placed in front of the slit like the coloured 

 glasses. 



A little potassic permanganate, or Condy's fluid in water, thus 

 used as a light-filter will produce a deep band in the yellow part 

 of the spectrum and the adjacent regions of the orange and green. 

 Solutions of blood or magenta will give also very definite indi- 

 cations of absorption. In the former case two broad bands of 

 absorption in the green and yellow, and in the latter one in the 

 green, will be seen. 



