180 Mr. T. Gray on the Application of the Electrolysis 



chemical equivalent of copper, I pointed out that the current- 

 density at the surface of the cathode required to he taken into 

 consideration, hecause the plates gradually lost weight in the 

 solution. Beyond a general statement of the approximate 

 correction to be allowed for increased area of the cathode 

 plate, and a remark that it depended to sonic extent on the 

 temperature, little information was given on this somewhat 

 important part of the subject. In order to remove this defect, 

 advantage has been taken of the continual use of electrolysis 

 in the standardizing experiments already referred to; and a 

 large number of electrolytic cells with plates of widely differ- 

 ing areas have generally been used in each experiment, in order 

 to accumulate information on the subject. The effect of tem- 

 perature has also been more fully investigated. In some of 

 the special experiments on the effect of temperature, two sets 

 of cells at different temperatures were simultaneously used for 

 the purpose of finding whether the deposits were the same for 

 very small plates. The results showed that the same deposit is 

 obtained at all temperatures if the plate be small enough. 

 The smallness of the plate is, however, limited by the necessity 

 of obtaining a good deposit. In the majority of cases one set 

 of plates only was used, the object being to find the relative 

 deposit on plates of different sizes for several temperatures 

 ranging between 0° and 35° Centigrade. 



The solution used for these experiments had a density 

 varying between 1*15 and 1*18, and always contained free 

 acid. The solution was generally made to density 1*18, and 

 one per cent, of sulphuric acid added. Tins solution was then 

 used for an aggregate time of about ten hours, no additional 

 acid being added, and was then discarded. The volume of 

 solution used per square centimetre of plate-surface immersed 

 was about three cubic centimetres, and the whole of the solu- 

 tion for the set of cells was mixed between each pair of expe- 

 riments. Ordinary commercial copper sulphate was used in 

 nearly all cases, and it was dissolved in water simply taken 

 from the Glasgow water-supply, which is, however, almost 

 pure. Previous experiments showed that there was no appre- 

 ciable difference between the results obtained with solutions 

 made in this way and solutions made from pure sulphate dis- 

 solved in distilled water. In the method of preparing the 

 plates previous to immersion in the cells, practically no change 

 has been made since the previous paper was written. The 

 plates have generally been simply polished with fine, clean, 

 dry glass-paper, the loose dust wiped off with a clean silk 

 cloth, and the plates then weighed. For polishing the plates 

 a cylinder covered with clean glass-paper is fixed in the lathe 



