Boltwood — Ratio of Radium to Uranium. 99 



it, and the joint was made still tighter by the application of a 

 small quantity of melted sealing-wax. Suspended by a small 

 hook near the top of the rod t was a soft iron wire w, which 

 extended to below the sulphur rod and terminated in a loop 

 which surrounded the rod supporting the plate p. When 

 hanging in its normal position this wire did not touch the lower 

 rod, but on bringing a small magnet near the tube T the wire 

 was deflected and established a metallic circuit between the 

 upper rod and the gold leaf. The plate p and the metal por- 

 tions of its supports were heavily gold-plated to prevent tar- 

 nishing. All of the joints and crevices were filled with sealing 

 wax except that between B and C, which was closed air-tight 

 by a rubber washer, R, between them. By unscrewing the cap 

 B the gold-leaf and its supports could be removed from the 

 case. 



The electroscope was attached by screws to an iron plate, J/, 

 which in turn was fastened to two lead ingots, Z, Z, weighing 

 about seven pounds each. The whole apparatus was therefore 

 very steady and connections could be made at the cocks with- 

 out fear of altering the adjustment. 



For charging the gold-leaf a large stick of sealing-wax was 

 used. This was rubbed lightly on the clothing and approached 

 to the top of the rod t, the wire w being brought at the same 

 time into contact with the plate p by means of a small magnet. 

 On rubbing the rod gently with the sealing-wax a sufficient 

 charge was imparted to the gold leaf to produce the desired 

 deflection. After removingthe sealing-wax stick and the mag- 

 net, the top of the rod and cap were touched with the finger. 

 This method of charging worked very well during the dry 

 weather of winter and early spring but became troublesome 

 when the air of the laboratory grew moister, and has therefore 

 been superseded by the use of a battery of small storage cells 

 giving a potential of from 300 to 400 volts. 



The fall of the gold-leaf due to leak of the electric charge 

 was observed by means of a microscope mounted on a separate 

 support in front of the electroscope. This microscope consisted 

 of a Bausch and Lomb combination of a 2-inch eye-piece and 

 a 1-J-inch objective mounted in an ordinary draw -tube. The 

 diameter of the held was about 4 mm and the eye-piece contained 

 a glass scale divided into nine divisions, which were further 

 subdivided into tenths ; each tenth of a division therefore cor- 

 responded to about 0'0M mm . 



The radium emanation given off by a known weight of min- 

 eral substance was collected in the apparatus shown in Hg. 2. 

 This was made entirely of glass and consisted of an elongated 

 bulb, A, connected by a tube with the smaller bulb B, which in 

 turn was connected with the still smaller bulb C. A weighed 



