260 Duane — Heat Generated by Radio-active Substances. 



In actual practice the protection of the instrument against 

 outside thermal disturbances is just as important as great sen- 

 sitiveness. In my earlier experiments I imbedded the two 

 vessels in a block, E (fig. 1), of lead (weighing 25 kilograms). 

 The vessels were held in place by a layer of paraffine, which 

 filled the space between them and the lead at the bottom. At 

 the top this space was filled with cotton wool. Two metal 

 rods, F (normal to the plane of the figure) support the block 

 of lead inside a brass box, G. These rods serve as axles 

 about which the lead can be turned, and thus the bubble of 

 air shifted to any desired position in the capillary tube. The 

 box Gr was completely enveloped in cotton wool contained in 

 a second box of zinc (not represented in the figure). This 

 system of good conducting metal screens separated by spaces 

 filled with non-conducting material furnished excellent pro- 

 tection against thermal disturbances, but was not sufficient 

 where the greatest sensitiveness was required. The whole 

 apparatus, therefore, was placed in an electrical thermostat 

 similar to the one described some years ago in this Journal.* 



In my later experiments I have replaced the cotton wool 

 with eider down, and I have added two large blocks of lead 

 on top of the box G. These blocks equalize the variations of 

 temperature coming from above. They are placed one beside 

 the other, leaving just enough space between them for the 

 tubes by which the substances to be examined are lowered into 

 the calorimeter. With these modifications I have found it 

 unnecessary to set the thermostat going, except on those days 

 when the temperature of the room undergoes wide fluctuations. 



Very often the heat due to the radio-active processes is pro- 

 duced in relatively large masses of matter. In these cases it 

 is necessary to leave the substance to be examined for a long 

 time in the upper part of the tube by which it enters the calor- 

 imeter, in order to be sure that its temperature is as nearly 

 equal to that of the calorimeter as possible. This part of the 

 tube should lie between the two large blocks of lead, and 

 should be of metal to facilitate the equalization of temperatures. 



If the generation of heat by the source is relatively large, 

 an appreciable quantity of it may be conducted down the 

 column of air into the calorimeter. In order to avoid this a 

 small quantity of eider down fastened to the end of a very 

 fine glass rod may be inserted into the tube just above the 

 calorimeter. In making an experiment the eider down is 

 removed, the substance to be examined lowered into the 

 calorimeter and the eider down quickly replaced. 



Any one of several methods may be used in measuring the 

 heat generated by the source. On lowering the source into 

 the calorimeter one can wait until a sort of thermal eqnilib- 



* Duane and Lory, this Journal, 1900. 



