712 Prof. L. T. More on the Fatigue of 



the secondary radiation excited in these latter caused serious 

 errors. To eliminate them, the brass tubes T, T' were 

 mounted in such a fashion as to stop all rays but those which 

 struck the experimental plates. 



The following preliminary steps show the reliability of the 

 arrangements : — 



1. When the electroscope was disconnected from the plates, 

 rays from the bulb, within or outside the boxes, caused no 

 appreciable motion of the leaf during a minute. 



2. With the lead shutters closed and the electroscope 

 connected to the plates, the rays produced no observable 

 ionization in the boxes. 



3. Since the Rontgen rays passed through the wire gauze 

 and the air-space between it and the metal plate, a test was 

 made to determine the secondary radiation from the gauze. 

 First, the ratio of the ionization-currents in the two boxes 

 was measured, using lead plates and iron gauze in both. 

 One of these gauze disks was then replaced by a similar one of 

 copper, and the ratio again found. As the two ratios remained 

 the same, the action of the rays on the gauze can be neglected. 

 As an additional proof, the ionization-current was measured, 

 using a lead plate ; this was then replaced by a disk of the 

 same size made of very thin aluminium- foil mounted on a fine 

 brass wire. The two observations showed that only about 

 one per cent, of the first ionization remained, and as that 

 was partly due to secondary radiation from the aluminium- 

 foil, the direct ionization of the Rontgen rays and that due to 

 the secondary radiation of the gauze must be less than one 

 per cent, and so negligible. 



The effect of the Rontgen rays was tried on iron, lead, 

 nickel, zinc, copper, and aluminium, both with old and with 

 polished surfaces in air. And, in addition to air, the gases 

 hydrogen, coal-gas, and ozone were used. The results show 

 that the secondary radiation excited by these rays depends 

 on the length of exposure, the kind of metal, its surface, 

 and the gas surrounding it ; but the changes differ in many 

 respects from the effects produced by ultra-violet light. 



In the tables the column " Exposure " shows the period the 

 trial plate was exposed continuously to the rays ; the column 

 for each metal gives the ratio of the ionization-current from 

 the trial plate to that of the standard. In each case the 

 initial reading of this ratio is expressed as 100; the per-cent. 

 change in the amount of the secondary radiation from the 

 trial plate can thus be found at once by subtracting any 

 subsequent value from the initial reading. The ionization- 



