Rate oj Evolution of Heat l>y PitMlende. 315 



vessel not being water-tight drains into the outer, from which 

 the water is siphoned off at intervals. 



By re-filling the outer vessel with ice avery ten days or BO 

 the ice in tin; inner vessel could be kept nnmelted almost 

 indefinitely. It was Found necessary to soak tin* ice in water 

 for an hour or more after planing in order to hring it to zero, 

 as it was initially much below it. Even when all possible 

 precautions were taken small temperature-differences seemed 

 to occur, possibly due to -mail variations in the purity of the 

 ice or to regelation effects, [ce Frozen From distilled water 

 not being available, commercial ice frozen From the Dublin 

 City supply had to be employed. This point i- discussed 

 later. 



( 'on pies and Galvanometer Arrangements. 



The thermo-couples employed consisted of five pairs of 

 iron and nickel silk insulated wires of diameters (hare) 

 0*12 mm. and 0*15 mm. respectively. The aseof these fine 

 wires minimized the conduction of heat along tie- couple. 

 Two couples were used. In one of them the stem inside the 

 calorimeter was cased in a glass tube and reached down 

 through tin? centre of the calorimeter to the bottom. This 

 couple was used in all the water-cooling experiments, and 

 also in the firsl pitchblende experiment. The second couple, 

 which was employed in all the later pitchblende experiments, 



laid a -lightly longer stem. The part of the stem inside tin; 



calorimeter not being enclosed in a glass tube was flexible. 

 The couple was inserted in the calorimeter before the pitch- 

 blende and pressed down so thai the Btem curved round 

 the side of the calorimeter and avoided the centre, where 

 the temperature musl be slightly higher than at the 

 surface. 



The neck of the calorimeter was plugged with cotton-wool 

 above which was a thick layer of vaseline to exclude moisture. 

 Over all was a sb eel rubber apron bound above round the 

 stem of the couple, and below round the neck of the 

 calorimeter. The outer junctions of the couple are encased 

 in :i thin glass tube and buried in the ice. Between the 

 glass tube and the calorimeter the wires are protected by a 

 thick seamless rubber ti\\>c Flexible conductors, as used in 

 electric lighting, are employed as lead-. To lessen conduction 

 oF heat down these a length of over five feet is buried in 

 the ice. for about two feet nearest the couple the leads are 

 encased in a rubber tube and lie inside the inner ice vessel. 

 The next two feet or so of leads are enclosed in a copper tube 



