COLORIMETRICAL EXPERIMENTS. 41 



placed in similar cylinders : to one of the cylinders more 

 ammonia was added ; it appeared perceptibly darker than 

 the other. Hence it appears that the excess of ammonia 

 has some influence on the result. I presume that ammo- 

 nio-sulphate of copper has a tendency to be decomposed 

 by water, and that some change is effected even before it 

 becomes obviously marked by the formation of a turbidity ; 

 moreover it seems likely that the excess of ammonia has 

 the power to counteract this property of water and to 

 restore the original compound. Two solutions were made, 

 the bulk of each being 545 cub. c, — one containing 4000 

 of copper sulphate along with an additional 20 cub. c. of 

 ammonia, the other containing 6000 of copper sulphate 

 with 30 cub. c. of additional ammonia. The comparisons 

 were made in new cylinders graduated to millimetres. An 

 experiment with white surfaces external gave the following 

 results ; standard solution 4000 in 545 cub. c. o£ water, 

 length of column 23 centimetres : — 



ABO 



6000 i5"5 5955 



Thus the result is very near the real quantity. I also took 

 shorter lengths of the standard solution, namely 18, 13, and 

 8 centims. j the corresponding lengths of the other solution 

 were 12"4, 8*5, and 5*3 centims. Reduced to 23 centims. 

 of the standard, the lengths would be I5"8, 15, and I5'2, 

 numbers not far removed from I5'5, which was got by ob- 

 servation. I repeated the experiment with fresh solutions, 

 the bidk of the liquid being 500 cub. c, ; standard solution 

 4000 in 500 cub. c, length of column 21*2 : — 



ABC 



6000 14" 1 6000 



The number under B is the theoretical quantity, and was 



