Perkins — Molecular Weight of Radium Emanation. 465 



of the apparatus were carefully cleaned with gasoline and boiled 

 in a strong solution of potassium hydroxide. A series of iron 

 screens was also fitted into the end of C nearer the generator in 

 order to make the current as uniform as possible through the 

 inner tube. As the different parts were assembled, all the 

 bearing surfaces were given a thin coat of water glass, which 

 was also run inside around each end of the diffusion chamber. 

 After heating in the furnace at about 110° C. for several hours, 

 to set the water glass, no appreciable leak through the outer 

 jacket occurred in twelve hours. That no time should be lost 

 in pushing the collecting tubes into position, a guiding tube P 

 with a smoothly ground flaring end was fitted over the end of 

 C and worked very satisfactorily. 



The only change necessary in the diffusion apparatus, when 

 working with the emanation, was to replace the iron plugs L 

 and 1/ by small brass tubes threaded into the iron caps and 

 reaching outside the furnace, where their ends were left large 

 enough to be closed off with rubber tubing and pinchcocks. 

 The holes through these tubes were made so small that their 

 volume was negligible when compared with that of the diffusion 

 chamber. 



Electroscope. — For work with the emanation an electroscope 

 provided with a guard ring was constructed, connected with a 

 perfectly air-tight ionization chamber, made from a mercury 

 flask of nearly 3 liters capacity. The guard ring and gold leaf 

 were charged to a negative potential of 320 volts by a storage 

 battery. The motion of the gold leaf was observed by a micro- 

 scope provided with a scale of 100 equal parts. The time 

 required for the leaf to move from 20 to 80 on the scale was 

 chosen as a satisfactory means of finding the leak in divisions 

 per minute. A minute irregularity in the leaf, near the scale, 

 greatly assisted in making the readings. 



Constant pressure apparatus. — In order to collect the dif- 

 fused emanation under constant pressure, a simple device was 

 used which could be readily adjusted. A hole was made in the 

 side, near the bottom, of a common acid bottle of about 3 liters 

 capacity. This served as collecting vessel and was connected, 

 by means of rubber tubing, with an inverted balance bottle from 

 which the bottom had been removed. Enough distilled water 

 was introduced to fill the collecting bottle and leave a small 

 amount in the balance bottle. By lowering the latter as gas 

 entered the top of the collecting bottle, the two surfaces could 

 be kept at practically the same level and atmospheric pressure 

 maintained on the collected gas. 



