40 



DR. JAMES BOTTOMLEY S 



results were as follows; standard solution 1600 in 500 

 cub. c, depth of disk 8-3 : — 



A B 



2400 5-6 2371- 



The number under B was the result of seven trials. 

 The value under C is a fair approximation to the real value. 

 I also tried external surfaces with these solutions ; stand- 

 ard solution 1600 in 500 cub. c, length of column 8*4 : — 



A B C 



2400 6*2 2161 



Somewhat further from the correct value than we might 

 expect. I also tried the theoretical length of column ; 

 with external disks it appeared a Httle too light. The 

 results obtained with the last standard solution are incon- 

 sistent with the results obtained with the first. In the 

 latter case the results are too low^ while previously they 

 were too high. Errors of observation arising from imperfect 

 perception of colour, from imperfection of instruments and 

 unfavourable conditions of light (for many of the experi- 

 ments were made during the winter months, sometimes on 

 gloomy unfavourable days) would, no doubt, contribute to 

 this result ; yet, allowing for all these, there seemed to be 

 some other cause. In my last paper I mentioned that an 

 ammoniacal solution of copper, when largely diluted, 

 became turbid, and that to carry out the experiment an 

 additional quantity of ammonia was necessary. This small 

 quantity was added at hazard, as I did not think it would 

 have any influence on the result. It seemed to me after- 

 wards to be a point worth examining in connexion with 

 the above experiments. Two solutions were made, each 

 containing 5 cub. c. of the previously mentioned copper- 

 solution with 245 cub. c. of water. The solutions were 



