SOME COLORIMETRICAL EXPERIMENTS. 263 



there is another difficulty to deal with : the colours to be 

 compared differ not only in intensity but also in kind. A 

 very dilute solution of bichromate gives a pale yellow with 

 a slightly greenish shade, which passes, as the amount of 

 salt is increased, into full yellow, orange-yellow, and finally 

 orange. A similar observation holds with regard to several 

 other colouring-salts. Thus, a very dilute solution of 

 permanganate of potash is pink, passing, as the strength 

 of the solution is increased, into crimson, and finally into 

 violet. I also made some experiments with a blue solution. 

 For this purpose I used an ammoniacal solution o£ sulphate 

 of copper. The tinctorial power of this salt is not so great 

 as I expected, and I made no experiments with a standard 

 solution containing less than 150. On diluting 15 cub. c. 

 of an ammoniacal solution with sufficient water to make 500 

 cub. c, I noticed after a little time that the solution was 

 somewhat turbid ; this was owing to the copper forming 

 an insoluble compound, either hydrate or a subsalt ; fur- 

 ther addition of ammonia was necessary to clarify the so- 

 lution. Standard solution 150 in 500 cub. c, of water 

 (depth 8-3) :— 



Some experiments were made with solutions of known 

 strength by sinking the disk to the depth assigned by 

 theory, to see if the given tint was attained, instead of 

 moving the disk about until the tint was estimated to be 



