42 Mr. William Crookes [Feb 18, 



degree this obstructive property. Before me I have an exhausted 

 tube having two sets of poles sealed in it, one set at each end. The 

 size and distance apart of these poles are exactly the same in each 

 case. At one end of the tube I have put some thorium sulphate, at 

 the other end I have put yttrium sulphate. The exhaustion is now 

 proceeding by aid of the Sprengel pump. I attach the wires of the 

 induction coil to the poles at the thorium end, and, as you see, no 

 current will pass ; rather than pass through the tube, the spark prefers 

 to strike across the spark-gauge in air — a striking distance of 37 

 millimetres, — showing an electromotive force of 34,040 volts. Now, 

 without doing anything to affect the degree of exhaustion, I transfer 

 the wires of the induction coil from the thorium to the yttrium end, 

 and the spark passes at once. To balance the spark in air I must 

 push the wires of the gauge together, till they are only 7 millimetres 

 apart, equivalent to an electromotive force of 6440 volts : the fact of 

 whether thoria or yttria is under the poles making a difference of 

 27,600 volts in the conductivity of the tube. The explanation of 

 this eccentric action of thoria is not yet quite clear. From the great 

 difference in the phosphorescence of the two earths, it is evident that 

 the passage of electricity through these tubes is not so much dependent 

 on the degree of exhaustion as upon the phosphorogenic property of 

 the body opposite the poles. 



Other earths become very phosphorescent, and their power of 

 retaining residual phosphorescence differs greatly among themselves. 

 This property we shall presently see is one of some importance. To 

 examine this persistence of luminosity I have devised an instrument 

 similar to Becquerel's phosjphoroscope, but acting electrically instead 

 of by means of direct light. It consists of an oj^aque disc, 30 inches 

 in diameter, pierced with six openings near the edge. By means of a 

 multiplying wheel and pulley the disc can be set in rapid rotation. 

 At each revolution a stationary object behind one of the apertures is 

 alternately exposed and hidden six times. A commutator forms part 

 of the axis of the disc, and by connecting it with the wires from a 

 battery, rotation of the disc produces alternate makes and breaks in 

 the current. This primary current is then connected with the induc- 

 tion coil, from which the secondary current passes through the 

 vacuum tube containing the earth under examination. When a 

 phosphorescent body such as yttria is examined, if the wheel is 

 turned slowly no light is seen when looked at from the front, as 

 the current does not begin till the obscuration of the tube by an 

 intercepting segment, and ends before the earth comes into view. 

 "When, however, the wheel is quickly turned, the residual phos- 

 phorescence lasts long enough to bridge over the brief interval 

 between the cessation of the spark and the entry of the phosphor- 

 escent body into the field of view, and it is seen to glov/ with a faint 

 light which becomes brighter as the s])eed of the wheel increases. 



I will first put the phosphorescent earth glucina in the phos- 

 phoroscope. This phosphoresces of a bright blue colour, but the 



