COLORIMETRICAL EXPERIMENTS. 45 



The number under B was tte mean result of eight trials. 

 In one case the value got by experiment is too high, and 

 in the other too low. The theoretical length is 5 "65 ; and 

 5*4 is not very far from it. When I actually tried a column 

 of the theoretical length, it seemed to give the required tint 

 when the standard solution was on the left; when the 

 standard solution was on the right it seemed a little darker. 

 The mean of the two values previously obtained is 5968, 

 which is near to the real value. 



From the above experiments it seems that, when excesses 

 of ammonia are added, very fair approximations may be 

 obtained by colorimetry to the quantity of copper- sulphate 

 in solution, the white surfaces being external ; also that 

 with white surfaces internal the results are more remote. 

 In my last paper I stated that, when there was much dif- 

 ference between the standard solution and the one to be 

 compared with it, the discrepancies when internal disks 

 were used were considerable. Then I was using very small 

 quantities of colouring-matter. From these experiments, 

 where the colours were intense and the quantities of 

 colouring-matter used considerable, a similar conclusion 

 follows. The reason for this is not far to seek, and it also 

 suggests a correction that must be applied to the formula 

 when the white surfaces are inside. A white disk inside a 

 column of coloured liquid looks darker in colour than a 

 white disk outside placed a few inches below a column of 

 the same length. This may be tried by looking through 

 different columns, or through the same column, and having 

 inside a white disk of smaller diameter than the cylinder ; 

 the inside disk will then appear surrounded by a rim of 

 lighter colour. When the disk is internal, it is evident 

 that the light which illuminates it has previously passed 

 through the solution, so that we are looking not at a white 

 disk, but at a coloured disk, through a coloured solution. 



