TRANSACTIONS OF SECTION A. 303 
2. On the Change of Thermal Conductivity during the Liquefaction of 
a Metai. By Professor A. W. Portsr, F.R.S., and F. Simeon. 
3. Experiments on the Active Deposit of Radium. By Hi. Weuuiscu. 
SYDNEY. 
FRIDAY, AUGUST 21. 
The following Papers were read :— 
1. The Origin and Nature of the y Rays from Radium. By Professor 
Sir E. Ruruerrorp, F.R.S. 
2. athe Distribution in Space of the Stars near the North Pole. 
By Dre Bs Ws-Dysony F.L.S. 
3. The Action of the Juice of Euphorbia peplus on a Photographic 
Plate. By J. M. Perrm and H. G. Cuapmay. 
The dried milky juice of Huphorbia peplus acts on a sensitive photographic 
plate in the dark. If a photographic plate, separated by a space of 3 mm., is 
exposed for fourteen days to the dried juice spread in the form of letters on 
glass, sharp images of the letters appear as positive impressions on the plate on 
development in the ordinary way. Faint images are formed by exposures for 
such short periods as twenty-four hours, and deeper impressions, but still sharp 
and well-defined, by exposures up to thirty- -one days. The impressions on the 
plate are more marked when the separation is diminished to 1 mm., and no 
impression appears when the separation is increased to 12 mm. The images are 
characteristically well defined, though there is slight diffusion around each letter. 
If a piece of black paper, impervious to light, be inserted between the letters 
and the plate, the images appear as well defined as when the paper is absent. 
The intervention of paraffined tissue paper fails to prevent the appearance of the 
image on the plate. Images are seen when thin aluminium foil and gold leaf are 
used to separate the plate from the letters. The impression can be obtained 
through thin sheet glass. When a strong current of air is passed between the 
letters and the plate during the exposure, the image appears sharp and no 
evidence of diffusion in the direction of the current can be made out. 
On examining the dried juice with a sulphide screen no scintillation of 
particles can be seen. ° On testing the dried juice in a gold leaf electroscope 
there is no apparent increase in the rate of discharge of ionised gases. With a 
sensitive electrometer no action of the dried juice on ionised air could be 
detected. 
On heating the dried juice, the photographic action is not diminished after 
several hours’ heating to 200° C. When charred to a black mass the juice has a 
diminished action on the plate, and when incinerated to a white ash the ash 
retains a feeble action. 
This photographic action has been noted with all specimens of Huphorbia 
peplus examined by us from many localities, some at least a hundred miles apart. 
The dried juice retains its action unchanged, so that the original sample, dried 
and mounted five years ago, is as active as ever. 
The juices of many other species of Euphorbia, and of other plants with 
similar latex-bearing tubes, have no comparable action on the photographic plate. 
4. Photo-electric Effect in Selenium. By Professor O. U. Vonwiter. 
