OF ARTS AND SCIENCES. 345 



of the solutions used is lower, the reaction becomes less rapid, and the 

 amount of copper deposited with the silver less, until at ordinary tem- 

 peratures it is comparatively small ; and below 0° the silver comes 

 down absolutely pure, and not the least evolution of gas is observed. 

 One of the difficulties of Hampe can then be overcome by keeping 

 the beaker containing the solution in a freezing mixture ; and the 

 perfect definiteness of the reaction, which before might have been 

 questioned, is thus established. The duration of the reaction at —1° 

 is from twelve to twenty-four hours, according to the dilution of the 

 argentic nitrate. The more dilute the solution is, the longer the pre- 

 cipitation takes, and the more finely divided is the deposited silver ; 

 but when the solution is very concentrated, the reaction is completed 

 in a comparatively short time, and the silver comes down in a beau- 

 tiful compact crystalline crust which takes the form of the copper. 



The silver which was formed was collected in a Gooch crucible 

 and washed with cold water, of which less than 250 c. c. were neces- 

 sary to give a filtrate in which no trace of silver or copper could be 

 detected. 



The fact that the determinations given below agree so exactly with 

 each other is of itself proof that no silver was dissolved, — first, because 

 the precipitate in the different experiments was of very different 

 degrees of fineness ; and secondly, because the precipitate in each 

 experiment was washed with a different amount of water, the silver 

 of the last experiment having at least four times as much water passed 

 over it as that of the first. And the fact that the results are not affected 

 by either circumstance shows clearly that no perceptible amount of 

 silver could have been dissolved ; for if so, the loss must have varied 

 both with the condition of the precipitate and with the amount of the 

 wash-water. The different result obtained by Hampe was probably 

 the effect of hot water on a very finely divided precipitate. 



The silver, whether in crystalline plates or in crystalline powder, 

 formed a very convenient precipitate with which to work ; it was 

 easily transferred and easily washed, did not adhere to the glass, and 

 was in every way adapted ibr quantitative work. 



The conditions of the following experiments as regards the quantity 

 of water and the excess of argentic nitrate used above the amount 

 required to dissolve the copper, were varied as much as possible. In 

 some determinations barely enough of the argentic nitrate was used 

 to effect the solution ; while in others the excess of the silver salt 

 amounted to nearly two grams. The time allowed the reactions was 

 also varied from twenty-four to seventy-two hours. 



