The zinc sulphate being colorless, while the copper sulphate is of a dark blue color, the separating line of the two liquids is easily distinguishable.  This line is called the blue line and care should be taken that it does not reach the zinc and cause a deposit of copper to be placed thereon.

As has been stated, these two liquids do not mix readily, but they will eventually mingle unless the action of the cell is sufficient to use up the copper sulphate as speedily as it is dissolved.  Thus it will be seen that while the cell is free from polarization and local action, there is, nevertheless, a deteriorating effect if the cell is allowed to remain long on open circuit.  Therefore, it should be used when a constant current is required.

Prevention of Creeping:—­Much trouble has been experienced in gravity cells due to the creeping of the salts over the edge of the jar.  Frequently the upper edges of the jars are coated by dipping in hot paraffin wax in the hope of preventing this.  Sometimes oil is poured on top of the fluid in the jar to prevent the creeping of the salts and the evaporation of the electrolyte.  The following account of experiments performed by Mr. William Reid, of Chicago, throws light on the relative advantages of these and other methods of preventing creeping.

The experiment was made with gravity cells having 5-inch by 7-inch glass jars.  Four cells were made up and operated in a rather dry, warm place, although perhaps under no more severe local conditions than would be found in most telephone exchanges.  Cell No. 1 was a plain cell as ordinarily used.  Cell No. 2 had the top of the rim of the jar treated with paraffin wax by dipping the rim to about one inch in depth in melted paraffin wax.  Cell No. 3 had melted paraffin wax poured over the surface of the liquid forming a seal about 3/16 inch in thickness.  After cooling, a few small holes were bored through the seal to let gases escape.  Cell No. 4 had a layer of heavy paraffin oil nearly 1/2 inch in thickness (about 6 oz. being used) on top of the solutions.

     These cells were all run on a load of .22 to .29 amperes for
     15-1/2 hours per day for thirty days, after which the following
     results were noted: 

     (a) The plain cell, or cell No. 1, had to have 26 ounces of
     water added to it to replace that which had evaporated.  The
     creeping of zinc sulphate salts was very bad.

     (b) The waxed rim cell, or cell No. 2, evaporated 26 ounces of
     water and the creeping of zinc sulphate salts was not prevented
     by the waxed rim.  The wax proved of no value.

(c) The wax sealed cell, or cell No. 3, showed practically no evaporation and only very slight creeping of zinc sulphate salts.  The creeping of salts that took place was only around spots where the edges of the seal were loose from the jar.

     (d) The paraffin oil sealed cell, or cell No. 4, showed no
     evaporation and no creeping of salts.