of Silver and of Copper. 401 



Table I., and are illustrated in the curves 1-12. The head- 

 ings of the Table sufficiently explain the meaning of the 

 different columns. The ordinates of the curves show the rate 

 of loss of copper in hundred thousandths of a gramme per 

 square centimetre of surface per hour. A glance at the 

 Table, or at the curves, is sufficient to show how very variable 

 the loss was, and also that it seldom exceeds the two hundredth 

 of a milligramme per square centimetre per hour. For a plate 

 of fifty square centimetres surface this would amount to a 

 quarter of a milligramme, and the copper which may be de- 

 posited on this surface in the same time is, if fifty square 

 centimetres to the ampere be chosen, about 1*2 gramme. 

 Hence if the action be nearly the same during the passage of 

 the current as when no current is passing, the error will 

 amount to about one in three thousand. The actual difference, 

 as will appear from the results discussed below, is greater 

 than this, which seems to indicate a more rapid corrosion of 

 the copper by the liquid when the current is flowing, a result 

 agreeing with those obtained by Gore. 



The rate of loss, as shown by the results given in Table I., 

 is at first greater in the dense than in the weak solutions, 

 but seems to reach a minimum between the density 1*10 

 and 1'15, a result which was confirmed by other experiments, 

 extending over shorter intervals. The loss, however, although 

 never great, is somewmat variable, and it is difficult to make 

 out anything wdth certainty, unless a very much larger num- 

 ber of experiments are made. The effect of adding acid to 

 the solution seems not to be great when the solution is dense, 

 and seems, for densities between 1*15 and 1*10, rather to 

 retard than to accelerate the action. 



The result of subsequent experiments, made with saturated 

 solutions containing different quantities of acid up to twenty 

 per cent., and electrotype copper plates, showed that the 

 effect of the addition of acid on the rate of corrosion was to 

 diminish the rate of loss under these circumstances. 



Perhaps the most interesting part of this series of results 

 is that which gives the behaviour of the plates in the solutions 

 to which no acid was added. The first action is a corrosion 

 of the plate, but after a sufficient time has elapsed the corro- 

 sion ceases and the plates rapidly gain weight by oxidation. 

 This action goes on until the plates are nearly completely 

 covered with brown oxide, after which the weight remains 

 nearly constant for a few days. Another change then takes 

 place, the plates begin again to increase in weight rapidly, 

 and on examination it is found that they have now begun to 

 form hydrated oxide. This action continues until the plates 



