Properties of Potassium-Sodium Alloy. 287 



made with various forms of apparatus, and, as the outcome o£ 

 these, it was found that a convenient mode o£ preparing a 

 suitable specimen of highly photoelectric metal was as 

 follows : — 



A tube of lead glass about 50 cms. long, and 1*5 to 2 cms. 

 internal diameter has a constriction made at one place, and on 

 one side of this a couple of platinum wires are sealed through 

 the glass which are welded to a slip of platinum foil a, b, 

 about 5 cms. long and 1 cm. wide (see fig. 1). This foil is 



Fio-. 1. 



I 



fixed close to the inside surface of the tube and opposite to it 

 two other platinum wires c d are sealed through the tube. 

 The end of this part of the tube is then drawn off and sealed 

 to a length of smaller tube with a constriction in it by which 

 the tube can ultimately be closed. A quantity of metallic 

 potassium and sodium is then cut up into small cubes under 

 naphtha, so as to give a number of clean morsels of the 

 metal. 



The operation of filling the glass tube is then as follows : — 

 The fine quill tube at one end is first bent over at an angle of 

 about 75° (see fig. 2), and by means of an indiarubber tube, 

 coal-gas, which has been dried by bubbling through strong 

 sulphuric acid, is led into it. The glass tube as a whole must 

 have previously been well cleansed and carefully dried. 

 When all the air is expelled, the fragments of sodium and 

 potassium are dropped in at the wide open end of the tube, 

 and about equal volumes of the two metals introduced. The 

 open end is then loosely stopped with a ping of aslx^tos, and 

 the glass tube heated in the blowpipe, and the open end so 

 plugged is melted and sealed. If this is done skilfully, the 

 tube can be sealed off with a rounded end by utilizing the 

 gas-pressure to blow out the softened glass. 



Fig. 2. 



^r* ^ 



When cool, the lumps of metallic potassium and sodium 

 are tilted over and should fill up about 10 to 15 cms. of the 

 length of the so sealed off tube (see fig. 2). The tube is 



