72 DK. JAMES BOTTOMLEY ON COLORIMETRY. 



in 500 cub. c, thinking that when we work with solutions 

 which vary gradually in colour we are apt to forget the 

 initial condition. This new solution seemed quite different 

 from the old one of the same strength. The latter was 

 much darker and browner. So great was the difference 

 that 9*1 cm. of the old seemed as dark as 22*5 of the new. 

 To find whether the darkening was due to the action of 

 light or to some intrinsic cause, I divided the newly made 

 solution into two equal columns. One I left on the table 

 before the window ; the other I kept in a cylinder which 

 was closely invested with black cloth. After the lapse of 

 six hours I compared them. The one exposed was so much 

 darker that 5 cm. of the exposed solution gave a tint as 

 deep as I0"9 cm. of the unexposed. This observation was 

 made on the Saturday. On midday of the following 

 Monday, when I again compared them, the darkening had 

 evidently increased ; for 3 cm. of the exposed solution gave 

 a tint about as dark as that furnished by io"9 cm. of the 

 unexposed. 



Wishing to ascertain whether keeping in the dark would 

 reverse the action of light, on Saturday, May 24th, I took 

 a solution containing 6*4 grams in 500 cub. c, the solu- 

 tion having been prepared three days previously and 

 darkened by exposure during that interval to light. The 

 containing cylinder was closely invested with black cloth 

 and kept in a dark closet. On the morning of the following 

 Monday I thought that it appeared not quite so dark as at 

 first ; and on the evening of the same day I thought it a 

 little lighter than in the morning. After keeping it in the 

 dark for a week I found that it had become much lighter ; 

 and on June 4th, when I examined it again, it seemed 

 nearly as light as a freshly prepared solution ; there was, 

 however, a minute quantity of precipitate. 



From these results it is evident that in some cases 



