lowered into the vessel and the lower end centered at the bottom. Finally the rubber stopper is inserted so that the upper end of the wire is centered at the top. After l/a hr the stopper is removed, and the wire is shaken free of the silver deposit, removed from the cell, washed and dried, and again carefully weighed. The contents of the cell are removed by inverting the cell (quickly) over a small (100-ml) weighed beaker. The residue is washed several times and dried, and the beaker with its contents weighed. During the washing operations, the loss of fines may be kept to a minimum by keeping the residue in a single mass. (Before the final washing a vial or similar tube with a flat bottom may be used to press the soft mass of crystals to a dense mat on the bottom of the beaker.)

Two advantages resulting from the new cell design are obvious. The concentration of the silver nitrate solution has been increased by a factor of five, and the maximum diffusion distance has been reduced by a factor probably as large as eight or ten. A third and less obvious advantage is that, as the reaction proceeds, a circulation system is set up in the cell that brings about the continuous renewal of the solution at the surface of the cathode. This circulation depends upon the fact that a solution of silver nitrate is more dense than a solution of copper nitrate of the same normality Soon after the copper wire is inserted into the cell a thin layer of solution may be noted rising along the surface of the wire. Silver-free blue solution collects at the top of the tube, and within 30 minutes the edge of the blue color has moved down and envelopes the whole solution. At this time the reaction is, for practical purposes, complete. The appearance of the two types of cells at this point is shown in the figure. The almost total lack of dark colloidal material on the surface of the silver deposit can be noted.

In addition to the merits of the new cell design in reducing markedly the production of fine colloidal material during the course of the reaction, it has an additional if not more important advantage. The new cell design permits the reaction time to be reduced from one or more days to 30 minutes. As a result, the CuaO content of the silver residue is markedly reduced6 and also, no Cu20 remains on the copper wire.

Results obtained from runs made according to the new directions are quite satisfying. The residue re- covered is decidedly more crystalline in appearance, and losses (except colloidal particles) occurring during the washing operations have been reduced practically to zero. The quantitative results of two series of runs

6 The CusO content may be reduced to zero by slightly acidi- fying the silver nitrate solution. However, the amount is so small that its effect on the value of the ratio R is negligible.

are s h o w n in Tablc 2. The runs of Series no. 3 were performed by the principle :uithor. Those of Series No. 4 were performed by fourteen high school teachers attending the XSF Summer Institute ;it Seattle Uni- versity. The results indicate that the use of t,he new method markedly reduces syst,ematic errors. Those remaining may be accounted for by the loss of very fine material during the washing operations.

Table 2. Average Results Obtained Usina New Cell

~ ~

Time, min Wt. of Cu in res., g Wt. of fines. f - Ratio, R Avg. Dev., R Series no. 5 compn-e

school teacheis. average

0 uu4 1 988 0 002

iesult-, obtained -

14 high

It is not to be expected that the precision of the results obtained by high school student,^ would be particularly better with this method than that obtained with the old method. However, it is to be expected that class averages for the ratio R would more closely approximate the value 2.00.

'Diana's silver tree ;'s made bu precipitating silver from Us solution by means ofmercuru. This interesting experiment may he performed in the following manner. .\fix together six parts of a solution of nitrate of silver, four parts of a solution of nitrate of mercuru, both completely saturated. Add a smallquantity of rain water, and put the mixture into a glass decanter, containing six parts ofamalgam, made of seven parts of mercury, hv weight, and six parts of silver leaf. In the course of some hours, there will appear small shining scales of metallic silver on the amalgam, which will increase, and shoot out i n the form of asilver tree, producing a w r y beautifulappearance."

. . . taken from the 19th Edition of "Elements of Chemistry" by J. L. Cornstock, M.D., published by liobinson, l'ratt & Co., 1836.

420 / Journal of Chemical Education