412 ' Proceedings of Roy cd Society of Edinburgh. [sess. 
3h. 55m. Zinc charged negatively to 238 scale divisions from metallic 
zero : — 
- 177 sc. divs. from metallic zero after 15 secs. 
: > i > 5 5 30 ,, 
5 5 5 5 5 5 45 j } 
5 5 5 5 5 5 1 min. 
9 
5 5 5 5 5 5 5 5 
5 5 5 5 5 5 ' J 5 5 
Afterwards steady. 
The following table shows the steady potential differences in the 
'electrometer due to the conductive effect of ultra-violet light in 
our apparatus between the brass wire gauze and plates of various 
other metals. 
Insulated metal : — 
Polished zinc . 
Polished aluminium 
German silver 
Gilded brass 
Polished copper 
Oxidised copper 
The copper was oxidised by being held in a Bunsen flame. 
In the case of polished zinc, polished aluminium, polished 
copper, and oxidised copper, both positive and negative charges 
were discharged at the same rate, if we reckon the charge of the 
insulated metal from its ultra-violet-light-zero. The rates of 
reaching the ultra-violet-light-zero were not observed for gilded 
brass and german silver. 
It must again be noticed that our experiments do not tell us 
what would happen if an insulated metal, shone on by ultra-violet 
light, were surrounded by a metal of precisely the same quality of 
surface connected to sheaths. 
§ 20. So far we have mentioned only experiments in which the 
rays, whether Rontgen or ultra-violet, fell perpendicularly on the 
insulated metal. We have also made some experiments with the 
rays going parallel to the metal surfaces. 
Tor this purpose a cardboard box 46 cms. long, 19 cms. square 
(see diagram 6), lined, in the first instance, with tinfoil, connected 
to sheaths, was used. Inside this box an insulated disc of oxidised 
copper of 10 cms. diameter was supported in such a way as to 
- 0 ‘75 of a volt. 
- 0*66 
-0*19 „ 
+ 0-04 
+ 0-12 „ 
+ 102 
-149 
-132 
-124 
-113 
-111 
