136 Prof. H. L. Callendar and Mr. H. T. Barnes. 



to crack in the platinum seal of the leg containing the zinc amalgam, 

 especially if exposed to low temperatures. We do not think that 

 they could be trusted to stand the Montreal climate. Very few of 

 our cells constructed on this pattern have survived a month or two 

 of the 0° to 30° treatment, if they contained more than a mere button 

 of amalgam. 



(2) H-Form Cells ivith Zinc Rod. — We have generally found cells 

 made with zinc rod to be more reliable. The zinc rod is cast in a 

 small glass tube of suitable size. The platinum wire, previously 

 enclosed in a capillary glass tube, is thrust into the fused zinc. 

 When cool, the glass mould is broken off, and the zinc rod cleaned 

 and amalgamated, and introduced into one leg of the H. The other 

 leg is partly filled with mercury to which connection is made by a 

 platinum wire in a glass tube after the Board of Trade method. 

 The other materials are filled in as usual. The two legs of the H 

 are then fused up at the top, the upper portions serving as mercury 

 cups. 



We prefer to use tubes of very small dimensions, and to make the 

 horizontal connecting limb as short as possible. This cell is not 

 very easy to make, owing to the double fusion on to the platinum 

 wires after the materials have been filled in, when the cell cannot be 

 inverted. If, however, the tubes are made sufficiently small, it is a 

 very convenient and sensitive form of cell. 



(3) Board of Trade Form with Crystals. — The single tube form 

 with the zinc rod cast in a similar way on to a glass capillary, is 

 equally efficient if filled with crystals, and is much easier to seal. 

 There is only one tube to seal, and it is possible to get at it evenly 

 from all sides. There is generally no difficulty in keeping the two 

 wires separate, provided that the capillaries through which they pass 

 have been drawn sufficiently thick and strong. Both the capillaries 

 may be expanded into mercury cups at the top, or the outer tube 

 itself may conveniently be used to form the mercury cup for the zinc 

 terminal. 



Portable Form. — This cell is still more easily made in a portable 

 form, in which the mercury is replaced by amalgamated platinum. 

 The cell may then be made upside down, tbe difficult seal being 

 made first, before the materials are filled in. The process of making 

 the ceil is briefly as follows. A platinum wire is sealed into a thick 

 glass capillary with a small mercury cup at one end. The wire is 

 left projecting some 2 or 3 cm. beyond tbe glass at each end. One 

 end is then hammered flat to serve as the positive element, and both 

 ends are amalgamated by heating and plunging in mercury. A 

 platinum wire capillary without a mercury cup is cast into a small 

 zinc rod, and the free end is amalgamated. A glass tube of suitable 

 size and length to form the cell is melted down in the middle till it 



