Duane — Heat Generated by Radio-active Substances. 259 



and the internal diameter is a little more than *5 mm . The 

 internal diameter of the two large parts is about 3 mm , and the 

 vertical parts joining the horizontal with the larger parts 

 should not have a larger diameter than the horizontal capillary 

 part. It is easier to control the movement of the bubble, 

 while placing it in the tube and reducing its size, if the capil- 

 lary tube is not joined to the ends of the larger parts, but to 

 the tops as indicated in the figure. 



The volume of each vessel is about 50 cm \ 



The interior of the vessels and of the tube must be cleaned 

 most carefully. The least dirt or grease stops the bubble, and 

 in the experiments it is well to choose the part of the tube 

 where the bubble moves most freely. 



If a source generating heat is introduced into the tube D, 

 the vapor tension is increased and the liquid pushes the bub- 

 ble toward the vessel A'. The instrument is very sensitive. 

 In my experiment I found that 1*5x10"* gram-calorie of 

 heat displaced the image of the bubble l mm on the scale. 

 This sensitiveness is due to the rapid increase of the vapor 

 tension with the temperature. Among the liquids I have 

 tried, ether seems to be the best. Ether cleans and wets the 

 surface of the glass well, it has very little viscosity and its 

 vapor tension increases rapidly with the temperature, about 

 17 mm of mercury per degree centigrade at ordinary tempera- 

 tures. Ethel chloride works well also but is much less easily 

 manipulated. 



The sensitiveness of the instrument varies a great deal with 

 the quantity of air in the vessels. If there is very little air the 

 displacement of the liquid does not change the pressure of the 

 gas much (saturated vapor tensions depending only on the tem- 

 perature), and an increase of pressure in A due to a slight pro- 

 duction of heat is opposed only by a change of level of the liquid 

 in A and A'. As ether is a light liquid, this change of level 

 opposes only a slight force to the displacement of the bubble. 

 For great sensitiveness, therefore, one must remove almost all 

 the air from the vessels, leaving only enough to form the 

 bubble. The sensitiveness depends also upon the ratio of the 

 cross-section of the capillary tube to the surface of the liquid 

 in the vessels. A decrease in the cross-section increases the 

 sensitiveness. I have found, however, that (if the liquid is 

 ether) a tube of less than *5 mm internal diameter does not work 

 well on account of the capillary forces. 



Further, the displacement of the image of the bubble is 

 increased by the lens (or reading telescope). It is not desir- 

 able, however, to multiply the displacement more than eight 

 or ten times, as the loss in sharpness of image counterbalances 

 the advantage of increased displacement. 



