686 MEIDINGER'S ELEMENTS. 



two-thirds with a solution containing 140 grammes of commercial 

 Epsom salt in one litre of water. The balloon being placed mouth 

 upwards upon some hollow vessel, such as a pot or pan, in order to 

 keep it steady, is filled with solid crystals of cupric sulphate, 

 about as large as a pea or a hazel nut, and then some of the solution 

 of Epsom salt is poured in until it is quite full, the liquid also filling 

 the interstices between the crystals. The mouth of the balloon is 

 then closed by the cork carrying the tube r, and the balloon, being 

 inverted it is placed upon the cell. The narrow tube r prevents 

 the entrance of air into the balloon while it is inverted, and conse- 

 quently the discharge of any of the liquid ; it serves also to pro- 

 long the neck of the bottle, as it is desirable that the mouth should 

 be at about the middle of the copper cylinder. 



When the balloon is inverted, the liquid in it, which already con- 

 tains some magnesic sulphate, dissolves a further quantity of cupric 

 sulphate, and becomes thereby heavier than the solution of magnesic 

 sulphate contained in the glass vessel g g. Hence an opposite flow 

 of liquids takes place through the tube r : the heavier liquid which 

 contains some cupric sulphate descends, while the magnesic sulphate 

 solution ascends into the balloon. The lighter solution which has 

 thus entered again dissolves some cupric sulphate and descends in 

 its turn, making room for lighter liquid ; and it is clear that this 

 interchange of liquids will continue until there is only a completely 

 saturated solution of cupric sulphate from the bottom of the cell up 

 to the mouth of the balloon. This state of matters is w r ell shown 

 in the section of another form of Meidinger's cell, fig. 348 B, which 

 differs from a balloon element only in this, that the balloon is 

 replaced by the cylindrical vessel D, which is supported by the 

 lid E by which the cell is covered. Further, the tube r is replaced 

 by a small aperture in the bottom of the vessel D, and the copper 

 cylinder does not stand free in the cell, but in a slightly conical glass 

 vessel B, which is cemented to the bottom of the cell. As soon as 

 there is a concentrated solution of cupric sulphate below the aperture 

 of the vessel which contains the crystals, the upward and dowmvard 

 flow ceases ; but as the current decomposes some of the cupric sul- 

 phate, and tends to render the solution weaker, its strength is main- 

 tained by the descent of heavier liquid from the balloon in A or the 

 glass vessel in B. 



Copper wires are fixed to the copper and the zinc in the cell, 

 which serve as terminals or for connecting several cells ; the wire 

 which leads from the copper is insulated where it passes through 

 the liquid by a glass tube or a covering of gutta-percha. The 

 balloon has generally two grooves in the glass, on opposite sides, 



