222 Mr. T. Bay ley on the Colour Relations 



tentials to reach its maximum value ; but it would be more 

 valuable if it gave in addition the time-rise of the difference. 

 "We confess, however, that it is only with exceptional cells 

 that we have succeeded in getting on different occasions ex- 

 actly the same results with the same combination. Such a 

 table as we suggest, which would be a great improvement on 

 that given by Professor Clifton, could of course be constructed 

 by any one possessing an electrometer without employing any 

 special apparatus. 



In conclusion we notice, page 299, that Professor Clifton 

 sees the necessity of changing his apparatus, which could not 

 measure directly the difference of potential between two liquids 

 in contact, before he can obtain satisfactory measures of the 

 difference of potential in certain important cases. We may 

 mention that although the apparatus employed by us in our 

 investigation described in our paper of 1876 enabled us to do 

 this with considerable accuracy, still we thought it advisable, 

 in the summer of that year, to construct a new apparatus, the 

 accurate results obtainable with which will form the subject 

 of our next paper on this subject. 



We beg to remain, Gentlemen, 



Very truly yours, 



W. E. Ayrtoh. 



John Perry. 



XXXII. On the Colour Relations of Copper and its Salts, 

 By Thomas Bayley, Assoc. R.C.Sc.L* 



COPPER in solution, as is well known, imparts to the 

 liquid a blue colour. In the case of the chloride the 

 colour inclines to green, but becomes blue on dilution. Wish- 

 ing to see what relation the light transmitted by such solutions 

 bears to that reflected from the surface of the metal, I made 

 the following experiments. 



An extremely dilute solution ofcupric sulphate Laving been 

 prepared, it was placed in a glass tube closed at the end by a 

 thin plate of glass similar to those used for covering objects 

 under the microscope. The tube had a narrow side-tube near 

 the bottom ; this was fitted with a piece of caoutchouc tubing 

 and pinchtap, so that any liquid contained in the tube could 

 be drawn off. A flat plate of copper carefully polished, first 

 with trent sand and oil and then with rotten-stone, was 

 placed beneath the tube in such a manner that the diffused 



* Communicated by the Author. 



