176 On a Gravity DanieWs Cell of small Resistance. [June 19, 



delivering to the various parts of the bottom of the cell. The vessels 

 X and Y are furnished each of them with a shelf of thin sheet 

 copper perforated with many holes, the level of the shelf being con- 

 siderably above the level of the extremities of the siphon tubes. On 

 these shelves a large quantity of sulphate of copper in crystals is 

 placed. 



To raise the vessels X and Y up, and cause the siphons to run, two 

 boards are provided which are pushed in beneath the vessels. One of 

 these, mm, is shown beneath the vessel X in the figures. The boards 

 are made of exactly the proper thickness to cause a layer of chosen 

 depth to be run into the cell out of the vessels, or out of the cell 

 into the vessels, according as the vessels are raised up on the boards 

 or are lowered down when the boards are removed. The thickness 

 of the boards depends, of course, on the relative sectional areas of 

 the two vessels and of the cell. The depth of the layer which is 

 spread over the bottom of the cell should be not more than one- 

 eighth of an inch. The bottom of the cell must be very carefully 

 levelled in order that the layer of charged liquor may be uniformly 

 distributed, and that the half-spent liquor may be uniformly drawn 

 off. 



In setting up the cell in the first instance I commence with the cell 

 charged with weak solution of sulphate of zinc (sp. gr. 1*12), while 

 the vessels X and Y are charged with nearly saturated sulphate of 

 zinc solution. This liquor is still able to take up a very large quantity 

 of sulphate of copper; and water saturated with both sulphate of 

 zinc and sulphate of copper is much more dense than saturated solu- 

 tion of either salt alone. When the cell is to be started the two 

 reservoirs are raised into position. The thin layer of dense liquor 

 spreads itself over the bottom in a few minutes, and the cell is ready 

 for use. When the use of the cell is to cease the reservoirs are 

 lowered down, and nearly the whole of the dense liquor runs back 

 into them. This should always be done at night, or during any con- 

 siderable time when the cell is to be out of use. 



During the action of the cell sulphate of zinc is constantly being 

 formed, and it is necessary frequently to run off a portion of the con- 

 tents of the cell and fill up with pure water. I have planned for a 

 system of tubes (arranged similarly to those just described) for siphon- 

 ing off into independent vessels, and with a cell of very great dimen- 

 sions, such as I am now constructing, they will probably be necessary. 

 With the cell which I have been using for nine months, however, I 

 have been able to dispense with them. 



It only remains for me to give the dimensions of the cell now in 

 use, and of a new one in process of construction. The tray of the 

 former is 47 inches square inside measurement, and 3J inches deep. 

 The zincs are each 22 inches square, and are raised up inch from 



