DR. JAMES BOTTOMLEY ON COLORIMETRY, 73 



special provision must be made to avoid needless exposure 

 to light in quantitative determinations by colorimetry, or 

 in studying the laws of the absorption of light passing 

 through coloured solutions. 



I also made some experiments with chromate of potash. 

 This I thought a stable salt suitable for experiments. 

 Nevertheless some of the results were not satisfactory when 

 one cylinder contained a solution which was several times 

 stronger than the other. For instance, a standard solution 

 was made containing 0'8 gram in 500 cub. c. of water. 

 Another solution compared with this gave the following 

 results : — 



ABC 



6-4 37 4-5865 



A repetition of the experiment gave nearly the same result, 

 namely 3"6 for the length of the column. 



It occurred to me that possibly, when potassic chromate 

 is diluted, there may be liberated a minute quantity of 

 chromic acid, which would increase its absorbent power ; 

 this might be inferred from the greatly increased absorbent 

 power imparted to the bichromate by the additional mole- 

 cule of CrOj. I therefore took the cylinder containing the 

 standard solution used in the last experiment, and divided 

 its contents into two equal columns : to one I added a few 

 drops of ammonia ; this column became slightly but per- 

 ceptibly lighter than the other, so that I have little doubt 

 some change had been effected in the constitution of the 

 dissolved salt. The hypothesis of the liberation of a little 

 chromic acid is, I think, strengthened by the fact that a 

 solution of the salt is of a deeper yellow than the undis- 

 solved salt. T think that probably a trace of carbonic acid 

 in the water had liberated a little chromic acid. 



To try what the effect of the addition of a little weak 



