Condensation of the Radioactive Emanations. 569 



of its original value at —154°. This curve was obtained in 

 a steady stream of oxygen of *38 c. c. per second. For faster 

 currents the curve is displaced slightly to the right. 



When tested under the same conditions, the radium eman- 

 ation showed no such behaviour. When the solution of 

 radium chloride is retained in the gas-stream, the temperature 

 at which some of the emanation escapes condensation is only 

 slightly above the point before found at which the condensed 

 emanation commences to volatilize. Apart from the question 

 of the actual condensation-points themselves, there is, there- 

 fore, a well-marked distinction in the character of the phe- 

 nomena in the two cases. To investigate this difference, a 

 new method was devised which allowed determinations to be 

 made with the two emanations under comparable conditions. 



Experiments by the Static Method. 



The use of a steady stream of gas through the spiral in 

 which the emanation is condensed has many disadvantages, 

 some of which have been alluded to. By the use of a 

 mercury-pump, and by working in a partial vacuum, these 

 disadvantages are avoided, and the conditions of experiment 

 made more definite. PI. XIV. fig. 4 represents the arrange- 

 ment employed. 



The Geissler pump P was connected with the copper 

 spiral S and the thorium compound in the tube A, and 

 possessed a volume large in comparison with the volume of 

 the whole of the rest of the apparatus. The small bulb V 

 was first filled with hydrogen or oxygen entering at C, and 

 the thorium tube-spiral and connecting-tubes exhausted by 

 the pump to a pressure of a few millimetres of mercury. 

 The three-way tap was then reversed, the tap E being open 

 and F closed. In this way a quantity of the emanation was 

 swept out of A into the spiral ; E was closed, and the 

 emanation allowed to remain in the spiral a definite time. 

 This period varied in different experiments from 10 s to 90 s . 

 At the expiration of this interval, the pump in which the 

 mercury had been lowered was put into communication with 

 the spiral by the tap F, which was then closed, the mercury 

 raised, and the emanation expelled into H and carried on to 

 the testing-vessel by a steady current of oxygen entering the 

 tube at K, and kept continuously passing throughout the 

 experiment. The pressures employed were deduced from 

 the readings of the height of the mercury in the pump-tube 

 when the mercury was lowered, ami the relative volumes of 

 various parts of the apparatus. The various manipulations 



