198 THE ABSORPTION OF LIGHT BY TUEBID SOLUTIONS. 



grms. BaSO^ and 0*4003 grm. of lamp-black. These 

 greys were made into a paint by the addition of a little 

 water^ and pieces of cardboard covered with them. They 

 were then dried. These we may denote by Wa and W^. I 

 looked at W^ through a column 4*1 cm. long^ and en- 

 deavoured to get a similar tint with W^ under the other 

 cylinder. For the upper limit the column was 6*95 cm. 

 long, and for the lower limit 6*2 cm. long. The mean is 

 6'57 cm. I next altered the length of the column over Wp 

 to 1 1 '75. In the other cylinder for the upper limit the 

 column was 14*9 cm., and for the lower limit I4"8 ; the 

 mean is i4'85. Hence we have 



WpK"-7s=W„K'4-8s. 



By cross multiplication and elimination of W^, W^, . 



g;4- 1 j^ 14-85 _ g;6-s7g;i 1 75, 



Theory requires the sums of the indices to be equal. 



The sum on the right hand is 18-32, and on the left 

 1 8*95. The difference is, I think, not greater than what 

 might be due to errors of observation. 



