of some Rocks at High Temperat- 



ures. 



51 



tube D and the annular space G respectively, as shown in the 

 figure. The tubes and space C contain pure mercury, and 



Fiar. 3. 



each couple wire is led into a separate one of the small tubes, 

 from which, by means of small pieces of platinum wire, 

 connexion may be made to the inner and outer compartments. 

 The couple wires are fixed to the key by warming the top 

 of the paraffin-wax with a hot glass rod and pressing the 

 fine wire down into the molten wax as shown. The whole 

 key is placed in a glass vessel, shown in section in the figure, 

 and the intervening space filled with cotton-wool. It will be 

 seen that in this key the various contacts are all surrounded 

 by a large mass of metal, which in turn is jacketed by a layer 

 of material of low thermal conductivity, so that the tempe- 

 rature of the mercury aud also of the lamp-holder surrounding 

 the lower junctions must be very nearly uniform throughout, 

 even when the key is comparatively near the furnace as it 

 must necessarily be. Only platinum and platinum-indium 

 wires come into contact with the mercury which does not 



E 2 



